Enhancing District Heating System Efficiency: A Review of Return Temperature Reduction Strategies

Featured Application

The findings of this review provide a comprehensive understanding of return temperature reduction strategies in district heating systems, with a focus on system-level components. These insights can guide the design, operation, and optimization of district heating networks, enhancing energy efficiency and reducing operational costs. This review also serves as a valuable resource for developing smart control systems and cascading applications, thereby supporting the transition to low-temperature district heating schemes and facilitating sustainable energy goals.

Abstract

This review paper provides a comprehensive examination of current strategies and technical considerations for reducing return temperatures in district heating (DH) systems, aiming to enhance the utilization of available thermal energy. Return temperature, a parameter indirectly influenced by various system-level factors, cannot be adjusted directly but requires careful management throughout the design, commissioning, operation, and control phases. This paper explores several key factors affecting return temperature, including DH network, heat storage, and control strategies as well as the return temperature effect on the heat source. This paper also considers the influence of non-technical aspects, such as pricing strategies and maintenance practices, on system performance. The discussion extends to the complex interplay between low return temperatures and temperature differences, and between operational temperature schemes and economic considerations. Concluding remarks emphasize the importance of adopting a holistic approach that integrates technical, operational, and economic factors to improve DH system efficiency. This review highlights the need for comprehensive system-level optimization, effective management of system components, and consideration of unique heat production characteristics. By addressing these aspects, this study provides a framework for advancing DH system performance through optimized return temperature management.

1. Introduction

District heating (DH), also known as Community Heating, is a large-scale central heating system that distributes centrally generated heat through a pipe network to various buildings (residential, commercial, and industrial) for space heating, domestic hot water production, and process heat. The “EU Heating and Cooling Strategy” aims to enhance the use of renewable sources and recover energy from industrial waste, with DH technology being the most effective, dependable, and sustainable method to achieve these goals. DH systems can flexibly utilize any thermal energy source due to their use of sensible heat as the heat transport medium. The advantages of DH systems include environmental benefits from centralized emission management, cost-effective operation due to economies of scale, and efficient utilization of waste heat that would otherwise be lost [1,2,3,4,5].

1.1. Background

The piping network of a DH system comprises interconnected pipeline zones, including a transmission pipeline (used when the heat source is distant [6,7] or multiple districts require heat supply [4]), a distribution network, building connection service pipes, and building indoor piping systems. Each segment consists of two pipes: the supply line, which carries the heated medium from the heat source to end-users, and the return line, which carries the cooled medium back to the heat source. Together, these zones form a closed hydronic loop where various parts, systems, and parameters influence each other. Ensuring the highest overall efficiency of the system requires careful evaluation of the interactive thermal and hydraulic effects of these sub-parts [8,9,10,11,12,13,14].

In a DH system, thermal and hydraulic interactions can vary widely. The ‘Valve Authority’ illustrates how components in a hydronic closed loop operate harmoniously, as it measures the ratio of an automatic control valve’s hydraulic resistance to the overall resistance in the piping loop [15,16,17,18]. Variables such as differential pressure, hydrostatic pressure, and supply temperature can alter the operational characteristics of thermostatic radiator valves [18,19,20]. Differential pressure control valves ensure a constant pressure difference for the secondary loop regardless of the primary side’s pressure, thus stabilizing secondary-side equipment operation and maintaining hydraulic balance on the primary side [14,17,21,22,23].

In addition to the aforementioned interrelationships, temperature significantly affects the functioning of DH system components. The historical evolution of DH systems reveals a trend of decreasing temperature levels. The phases of this development are: (i) steam in the first generation (1G) systems, (ii) super-heated pressurized water above 100 °C in the second generation (2G), (iii) pressurized water below 100 °C in the third generation (3G), (iv) water with supply temperatures as low as 55 °C and return temperatures around 25 °C in the fourth generation (4G), and (v) the advanced heating and cooling supply via fifth-generation (5G) technology. More detailed information on these developments can be found in references [24,25,26]. This overall reduction in operating temperatures over DH generations has facilitated easier access to various energy sources [27].

1.2. Problem Statement

Beyond the general reduction in both supply and return temperatures, significant interest has focused on decreasing the return temperature to improve the cooling of the heating water and increase the temperature difference (ΔT) in DH systems [28,29]. While ‘Low ΔT Syndrome’ typically describes issues in district cooling systems with undesirable low return temperatures, it also applies to the problem of undesirably high return temperatures in DH systems [1,30].

1.3. Significance

It is valuable to present quantitative measurements that highlight the benefits and efficiency improvements achieved through such return temperature reductions for DH systems. By examining specific data points and empirical studies, we can more clearly understand the positive impact on system performance, energy savings, and overall operational efficiency.

Zinko et al., at the International Energy Agency (IEA) report, [11] present the economic savings achieved in a combined heat and power plant at a capacity of 12 MW_e and 40 MW_th with a reduction of 10 °C at the return temperature by DH. The obtained savings are given in detail with 71,785 EUR/year as a saving in the heat loss, 27,446 EUR/year as a saving in the pumping cost, and 101,236 EUR/year for the excess production of electricity achieved (assumed price rates in this case study are given as 23 EUR/MWh_NCV for fuel, 110 EUR/MWh_el for electricity, and 40 EUR/MWh_th for heat).

Frederiksen and Werner [1] provide a representative cost reduction gradient of approximately 0.16 EUR/MWh_th·°C based on an analysis of 27 Swedish DH systems. For example, a DH system producing 500 GWh_th/year could achieve savings of 800 kEUR with a 10 °C reduction in return temperature, given a currency exchange rate of SEK/EUR = 0.106. Additionally, considering the benefits of flue-gas condensation, a return temperature reduction of 5 °C can increase the heat utilization rate by 1–5% compared to normal conditions [30]. According to [30], the average annual return temperature in Swedish DH systems was 47 °C, which could be reduced to 32 °C with the available technological know-how, as detailed in a licentiate thesis from 1998 [31].

Zinko et al. [11] emphasize the issue of high return temperatures in many existing DH systems, based on the authors’ experiences. The report estimates a potential saving of approximately USD 100 million for Sweden for each degree reduction in return temperature, as per a personal communication with Dr. Sven Werner in 1999. An illustrative case study within the same IEA report describes a district with a 135 km distribution network supplying 1650 detached houses, with an annual heat demand of 327 GWh_th and a heat loss of 52 GWh_th. The study indicates that a 5 °C reduction in return temperature can lead to a decrease in heat loss by 2080 MWh_th, a 14% reduction in flow rate, and a 36% reduction in pumping electricity consumption (assuming pump electricity consumption is proportional to the cube of the flow rate). Additionally, the case study notes that a 10% increase in temperature difference results in a 10% reduction in flow for a DN800 pipe over 5 km. This increased temperature difference allows for pipe dimension reduction to DN700, yielding a saving of 440 EUR/m (with an exchange rate of EUR/USD = 0.8 in 2005).

1.4. Literature Review

DH systems have been widely studied, with several review papers addressing their technological evolution, market trends, and environmental benefits. However, while previous reviews have provided valuable insights into various aspects of DH, none have specifically focused on a system-level overview with an emphasis on return temperature reduction. This review aims to fill that gap by synthesizing existing research on system-level components and operational strategies that influence return temperatures, thereby improving DH network efficiency.

Werner [32] presents a global overview of DH (and district cooling), structured around market dynamics, technical advancements, supply characteristics, environmental impacts, institutional frameworks, and future outlooks. The review highlights key challenges, including the limited uptake of DH in buildings, country-specific variations in implementation rates, and a general lack of recognition of DH potential for carbon dioxide reduction. These findings emphasize the importance of optimizing system-level components, such as thermal storage and hydraulic balancing, to maximize the environmental and economic benefits of DH systems.

Lake et al. [33] provide a historical perspective on district energy systems, examining their development and current applications. The authors underscore the role of government policies and incentives in shaping the efficiency and adoption of DH systems. They also discuss the environmental advantages of DH, including reduced greenhouse gas emissions and enhanced energy security. However, the review focuses on broader aspects of DH and does not delve into the specific system-level measures required for return temperature reduction.

Mazhar et al. [34] explore the evolution of DH systems through four generations, emphasizing the integration of low-temperature renewable heat sources and decentralized heating technologies. The study highlights operational optimizations and policy support as critical factors for the transition to fourth-generation DH systems. Although the review provides an excellent overview of these developments, it lacks a detailed discussion of system-level strategies, such as thermal balancing and advanced control methods, for reducing return temperatures.

Galatoulas et al. [35] focus on fourth-generation DH technologies, including the incorporation of renewable energy sources like solar and geothermal systems. The study highlights the importance of thermal storage for balancing supply and demand and reviews early implementations of smart DH and cooling systems. Despite its comprehensive coverage of modern technologies, the review predominantly addresses supply-side innovations, leaving system-level considerations, such as hydraulic balancing and network design, less explored.

1.5. Aim, Objectives, and Scope

This review paper aims to elucidate the current state of knowledge regarding techniques for return temperature reduction in DH systems and the associated technical considerations. The need for this review is underscored by the significant implications for both new and existing DH systems, highlighting the importance of return temperature reduction in system design and implementation.

The objective of this paper is to provide a comprehensive analysis of the relevant literature organized around the theme of return temperature reduction. This review is uniquely structured into several key sections: DH network, heat storage, control strategies, heat source, system-level optimization, and application methodologies. This paper represents the first survey of its kind, integrating a detailed review of contemporary practices with a thorough synthesis of methodologies across various stages of implementation.

The goal is to provide a holistic overview of the current state of knowledge, offering a detailed synthesis of existing methods and their interactions. By considering these elements within a unified framework, this review aims to demonstrate how the integration of individual contributions leads to a cumulative effect on overall return temperature reduction. A holistic approach, as an additional focus, recognizes the interdependence of key system components, ensuring that improvements in one area complement advancements in others rather than being considered in isolation. By adopting this system-wide perspective, this review highlights how a coordinated optimization of DH networks, heat storage, control strategies, and heat sources enhances overall system efficiency, facilitating long-term sustainability and improved energy performance.

1.6. Review Approach

The term “low return temperature” was explored using academic research databases, focusing on journal articles, conference papers, review papers, theses, and editorials published in English up to the year 2018. Although the primary search was limited to English-language sources, some non-English papers were included based on citations within relevant English publications; these citations are noted accordingly in the text. Additionally, references that discuss pertinent studies without directly addressing their impact on return temperature were considered to broaden the scope of this review (see Figure 1). It is worth mentioning that a specific article, [36], could not be located. However, credible non-English articles directly related to “return temperature reduction” were included based on their citations in relevant references and a careful review of their abstracts or summaries. Notable examples of such articles are [37,38,39,40,41,42,43].

2. Thematic Review and Analysis

This section presents a state-of-the-art review, focusing on various techniques and strategies aimed at lowering return temperatures in DH systems (see Figure 2). This review is organized into five thematic areas: DH networks, heat storage systems, control strategies, heat sources, and system-level optimization as well as implementation strategies and novel concepts. These themes are based on the most frequently referenced methods and approaches, supported by the relevant literature.

The potential causes of high return temperatures (can also be termed as ‘low ∆T syndrome’ in DH) and their appropriate remedies are discussed within these themes. Each section meticulously examines specific aspects and their corresponding impact on return temperatures, with a distinct focus in the heat sources section on how return temperatures influence heat generation efficiency, as extensively documented in the pertinent literature.

Return temperature is not a direct operational parameter but a distinctive signature influenced by input factors such as heat load and supply temperature, as well as the design and proper operation of equipment. Thus, return temperature is often used as an indicator of the performance of a DH system and its components, such as building substations [11,17,29,44,45,46].

2.1. The Distribution Network

This section examines the influence of hydraulic balancing on DH return temperatures, as well as the role of bypass short circuits installed at various points within the DH network for different purposes, along with potential alternatives proposed in the literature.

2.1.1. Hydraulic Balancing

A main cause of high return temperatures in DH networks is the immature cooling of DH water due to excessively high flow rates exceeding design specifications. The primary goal of hydraulic balancing is to ensure each building in the DH system receives the necessary flow, achieved by installing differential pressure control valves to maintain a steady supply–return pressure difference. This ensures adequate flow rates to all buildings, preventing an increase in supply temperature by the DH operator to avoid thermal discomfort. Hydraulic imbalance can occur at various levels, from radiator units in individual homes to flats in multi-family buildings, and can also distort components’ characteristics, such as the hydraulic response of thermostatic radiator valves to detected indoor temperatures [14,22,23,47,48].

Trüschel [17] emphasizes that hydraulic balance has a greater impact on return temperature than control valve characteristics. However, partially closed or throttled control valves at some consumer sites affect the operation of other components and the overall DH return temperature. Łukawski [49] demonstrates that a single closed thermostatic radiator valve can raise the DH return temperature by approximately 0.8 to 1 °C during cold periods in an 80/60 °C system, with detailed annual analysis provided in Figure 6.7 of the reference [49].

Zhang et al. [48] highlight the drawbacks of lacking hydraulic balance, where the farthest radiator, without pressure and flow controls, becomes the dominant factor in system operation, leading to excessive flow rates. To ensure adequate flow to this distant room, the main flow rate is increased for the entire building, resulting in a flow rate 3.2-fold higher than necessary and rates in closer rooms 7–8-fold higher. Conversely, implementing hydraulic balancing with differential pressure control and pre-set thermostatic radiator valves in a system designed for 75/50/18 °C (supply/return/indoor) reduces the return temperature from 63 °C to 52.1 °C, achieving a 16.2% reduction in heat consumption and a 76.3% reduction in pump electricity consumption.

Jangsten [50] describes the impacts of hydraulic balancing on the operating temperatures of buildings along with data for structures without balancing, all of which are claimed to be situated in the same area of the DH network (Figure 3).

2.1.2. Bypass Applications

Bypass flows (also known as ‘recirculation’), when not properly managed or due to faulty operations, can significantly elevate return temperatures by transferring heat carrier water from the supply line to the return line without utilizing its thermal energy.

Various types of bypasses in DH networks include [23,51,52] (with additional types and their purposes outlined in [52,53]):

• ‘Thermostatic bypasses’, which redirect supply water to the return line during low-heat demand to prevent excessive cooling.
• ‘Minimum flow bypasses’, which ensure a minimum flow rate when there is no demand.
• ‘Flow control bypasses’, which are equipped at constant-speed pump outlets to return excess flow to the return line and adjust the DH flow rate.
• ‘Admixing bypasses’, which are installed at building substations to lower the supply temperature and prevent scalding.

Averfalk and Werner [52] estimate that bypass flows constitute 10% to 20% of the total annual flow. Evidence of their impact on return temperature is detailed in [44,54], where the deliberate opening of flush bypasses in a new housing complex of 200 apartments increased the flow rate from 10–20 m³/h to 60 m³/h, causing the supply–return temperature difference to drop from 25–30 °C to less than 5 °C. Subsequently, closing these bypasses restored the temperature difference to 30–35 °C despite lower heat demand during the test period [44].

• Thermostatic Bypasses

Thermostatic bypasses, installed at substations or DH network branch ends, prevent supply water from cooling excessively during summer and low-load periods. Without them, supply quality may fall short of thermal comfort needs and cause long wait times for domestic hot water. The bypass redirects supply water to the return line to maintain warmth when the temperature drops below a set threshold (a process known as ‘standby’ or ‘idling’ for substations [52,55]), helping avoid inadequate supply quality and prolonged hot water wait times [56,57,58,59]. For instance, in the Lystrup low-temperature DH system (55/25/20 °C), thermostatic bypasses at consumer substations are set to 40 °C, while intermediate substations are set to 35 °C and the flow rates are adjusted to 3 L/min with a ±2.5 °C dead-band [58,60].

Li et al. [61] demonstrate the variation in thermostatic bypass flow (installed only at branch ends) and its impact on DH return temperature across heat load periods (Figure 4). They also note that, according to some DH practices, the bypass set temperature should be 10 °C lower than the supply temperature.

Figure 5 shows various alternatives to bypass use, with (a) and (b) representing current bypass practices and (c)–(e) depicting alternative solutions. Substations equipped with storage tanks for domestic hot water (e) typically require minimal or no bypass flow. Continuous charging of these tanks during off-demand periods maintains network activity, preventing supply water cooling and stagnation [57,58,59,61,62].

Yang et al. [63] show that storage options, achieving a return temperature of approximately 27.6 °C, are superior to heat exchanger equipment with thermostatic bypass, which results in a return temperature of approximately 38.4 °C (measured in May). An alternative layout with an additional booster heater (e.g., electric heater and micro-heat pump) near the hot water tap can quickly meet hygiene requirements. In a subsequent experiment conducted in July, buildings with only heat exchangers achieved a return temperature of approximately 22.3 °C, a significant improvement from the 38.4 °C observed in May.

Yang et al. [64] explore using booster heaters with micro-storage tanks as an alternative to thermostatic bypasses. A micro-storage tank helps avoid excessive electricity use by the booster heater when the hot water temperature is low at the start of tapping. The local booster heater maintains the water at a comfortable temperature until the hot DH water arrives. The study finds that with a micro-storage tank, the return temperature can be as low as 16 °C with an ultra-low-temperature supply of 35 °C. In comparison: (i) a return temperature of 22 °C is achieved with a low-temperature supply of 50 °C and a heat exchanger alone together with the comfort bathroom concept; (ii) a return temperature of 27 °C is noted with a heat exchanger and thermostatic bypass at a supply temperature of 65 °C; and (iii) a return temperature of 25 °C is observed with a storage substation at a supply temperature of 65 °C [64,65].

Vaillant Rebollar et al. [59] performed a simulation-based analysis comparing various substation designs and bypass solutions under two bypass application strategies. The cases analyzed include (i) a substation with an external bypass line and instantaneous heat exchanger (Figure 5a), (ii) the same configuration with an internal bypass line (Figure 5b), (iii) a substation with a storage tank and instantaneous heat exchanger without bypass (Figure 5e), and (iv) a substation with only a storage tank with built-in coils circulating DH water. For the instantaneous heat exchanger, the average return temperature during on-demand operation is 23 °C, ranging from 21 °C to 31 °C, while in the charging mode, the average return temperature is 32 °C.

Figure 6 presents the simulation results, showing that the conventional strategy using branch-end bypasses with a set temperature of 35 °C (supply temperature at 60 °C) achieves the lowest return temperature when storage is used. Another strategy, involving bypass lines to all DH network substations and maintaining a modest flow when the supply temperature falls below 50 °C, is also considered. This internal bypass approach, as depicted in Figure 5b, further reduces return temperatures compared to external bypasses. However, the return temperature for domestic hot water storage tanks with built-in coils is considered excessive due to hygiene requirements that necessitate maintaining storage temperatures above 55 °C to prevent legionella bacteria growth [59].

Brand et al. [57] propose the ‘comfort bathroom’ concept (Figure 5c) as an alternative to thermostatic bypasses, using bathroom floor heating instead. This approach reduces the average DH return temperature from 27.7 °C to 23.8 °C, decreasing heat loss from the return line by 35% and total DH heat loss by 13%. Yang et al. [66] note, however, that due to flow restrictions, bathroom floor heating cannot fully replace thermostatic bypasses, limiting the extent of the return temperature reduction.

Another alternative is the “summer recirculation line,” a three-pipe system that redirects cooled supply water to a dedicated recirculation line before returning it to the heat source (Figure 5d) [52,61]. Simulations by Li et al. [61] show that with flue-gas condensing units, the return line temperature is 22 °C, while the recirculation line can reach 44 °C or 53.5 °C depending on whether an instantaneous heat exchanger or storage equipment is used. In a two-pipe network with bypass units, return temperatures are 35.5 °C or 45.6 °C, respectively [61].

In constant-flow DH systems utilizing constant-speed pumps, thermostatic bypasses function similarly to thermostatic control valves in variable-flow systems by regulating indoor temperatures. When the desired indoor temperature is achieved, the heating flow is redirected through 3- or 4-port valves directly to the return pipeline to prevent overheating, while the total DH flow remains constant due to the constant-speed pump [23,67].

• Hydraulic Bypasses

Hydraulic bypass units are critical components in DH systems, serving to regulate flow and prevent damage or inefficiencies. They are categorized into two main types: ‘minimum flow bypass’ and ‘flow control bypass’.

Minimum flow bypass units maintain a minimum flow rate in the DH network when control valves are closed, thereby preventing excessive temperature rise or potential damage to pumps. This functionality is essential to ensure continuous operation and protect system components from overheating or failure [23,51,68].

Crane [54] illustrates that minimum flow bypasses, operating at a flow rate of 4.2 m³/h with a DH supply temperature of 80 °C, result in increased return temperatures throughout the heating season (August to December). Specifically, return temperatures rise from 52 °C during peak load to 67 °C at minimal demand. When bypasses are inactive, the return temperature remains around 45 °C during the same periods [44,54].

To address high return temperatures associated with minimum flow bypasses, the use of variable-speed pumps is recommended. These pumps can adjust the DH flow to the minimum required and shut off when the network is not under load. It is also suggested to avoid overestimating peak load and ensure redundancy can handle the entire peak load. Additionally, employing multiple smaller pumps, such as three operating at half the peak load capacity, rather than two larger pumps, and using small ‘jockey’ pumps alongside peak flow pumps, can further optimize the system [23,54,69].

Flow control bypass units use a recirculation circuit between the pump outlet and inlet to manage excess flow in systems with constant-speed pumps. These bypasses adjust the overall DH flow by redirecting surplus flow [23,51,68]. An alternative to this method is installing a control valve at the pump outlet, which adjusts the system curve and pump operation. However, while this method can alter the flow, it often results in high return temperatures at the heat source [23,51,68]. To mitigate excessive return temperatures and hydraulic issues, variable-speed pumps are recommended [23].

Certain heat generation units, such as conventional boilers, require a minimum return temperature to prevent condensation at the exhaust gas, which can damage the heat recovery exchanger and boiler shell. Typically, flow control bypasses are used to raise the return temperature above the condensation point by mixing it with the supply medium before entering the boiler [53,70,71]. Newer boilers address this issue by using corrosion-resistant materials [70,71].

In building applications, DH systems with constant-speed pumps often use a bypass with a 3-way control valve installed between the inlet and outlet of the interior heating units for flow adjustment. This setup, unlike flow control at the pump outlet, maintains indoor temperatures similarly to thermostatic radiator valves. By bypassing the flow when the indoor temperature is at the set level, this method increases the return temperature to match the supply medium’s temperature [23,72,73].

• Admixing Bypasses

An admixing bypass line reroutes return medium to the supply medium in direct substation setups to lower the inlet temperature to the interior heating system. This approach prevents excessively high temperatures that could scald users and helps maintain the operational condition for the thermostatic radiator valves. It is rewarding to note that increasing the supply temperature leads to return temperature reduction for a given radiator size. However, this mixing reduces the thermal potential of the supply medium at the building inlet, leading to exergy loss. Additionally, high supply temperatures contribute to exergy loss during heat generation and increased heat loss from the DH network [1,28,74].

Gong and Werner [74] demonstrate that exergy loss in DH systems occurs due to the mixing of supply and return mediums, as well as heat transfer losses at the substation heat exchanger and radiator. Specifically, there is a 50% loss over the fuel exergy content during heat production, a 12% loss due to heat loss over the DH supply, and a 66% loss from the delivered heat to the building. To maximize the thermal potential of DH supply water and achieve lower return temperatures and flow rates, admixing bypasses should be avoided [1,75].

2.1.3. Cascading Applications

In DH systems, cascading involves the sequential use of heat by multiple units or subbranches, where each unit or cluster utilizes the thermal energy from the return medium of the preceding unit. This arrangement can occur at any level—substation, network, or both—enhancing the discharge of supplied heat potential and lowering the overall return temperature by further utilizing the residual thermal energy in subsequent units operating at lower temperatures (see Figure 2 for a network cascading example). This approach maximizes the thermal potential of the supplied heat by ensuring that the return medium, instead of being directly recirculated to the heat source, is further utilized in another heat demand sector before being discharged. Cascading is particularly effective in low-temperature DH systems, where multiple stages of heat extraction align with varying temperature requirements across different consumer types. For successful implementation, cascading requires precise hydraulic balancing and advanced control mechanisms to ensure stable temperature gradients and avoid unintended thermal mixing. Smart temperature control valves and differential pressure regulation are essential for maintaining optimal heat distribution and preventing excessive cooling at the final stages of the cascade [1,11,76].

Renovations enhancing a building energy efficiency and upgrades to substations in high-temperature DH systems form the condition with varying operational temperatures across sub-networks. This allows for a cascaded network configuration, where the return line from a higher-temperature sub-network can supply a lower-temperature sub-network within the same DH system [77,78].

Gudmundsson et al. [78] illustrate a network cascading application where a high-temperature DH network (Høje Taastrup) supplies a low-temperature DH network (Sønderby). In this setup, the high-temperature supply and return mediums are mixed at an “area-substation” designed for cascading, adjusting the temperature for the low-temperature network. For instance, with a 90 °C supply and a 52 °C return from the high-temperature network, the low-temperature network receives a supply, resulting a final return temperature at 26 °C. This arrangement allows the high-temperature DH to meet 80% of Sønderby’s heat demand, with the remaining 20% covered by mixed supply. The authors suggest that such cascading applications can enhance heat utilization and significantly lower return temperatures in existing DH systems [30,60,78].

Mayer [79] presents a cascading concept in large-scale DH systems with multiple sub-networks. The primary sub-network operates at 90/70 °C, serving high-demand consumers such as hospitals, baths, and industries. The secondary sub-network, which receives its supply from the return of the first sub-network, operates at 70/50 °C. This sub-network serves the majority of consumers and includes traditional radiators, air handling units, and booster heaters for domestic hot water. The tertiary sub-network, supplied by the return from the second sub-network, functions as a modern low-temperature network with temperatures of 50/40 °C and relies on additional water heaters for domestic hot water. It is also noted that peak load stations and local heat pumps should be considered at the connections between sub-networks. The primary advantage of network cascading is the reduction in network heat loss by lowering operational temperatures in the sub-networks when feasible.

The ‘URBANcascade’ project by the Austrian Institute of Technology [80] aims to enhance DH systems through network cascading. Key aspects of the project include reducing the supply temperature required by buildings and improving cooling to lower return temperatures; categorizing buildings based on their temperature and flow requirements; assessing sub-networks according to these categories; and evaluating heat transmission and generation, with a focus on local heat pumps and micro-grids. The project identifies temperature levels of 90 °C, 50 °C, and 25 °C, demonstrating the ability of various sub-networks to support different return temperature levels across successive networks and consumer substations.

Köfinger et al. [81] conducted a comparative analysis of cascading strategies for supplying low-temperature consumers through various substation configurations. Two strategies were evaluated: (i) connecting the substation intake to the DH return line with an admixing bypass to increase the inlet temperature for domestic hot water units, and (ii) connecting one substation intake to the DH return line for space heating and another to the DH supply line for domestic hot water. Both strategies resulted in reduced return temperatures compared to a reference case with no low-temperature consumers. Winter simulations revealed return temperatures of approximately 46 °C and 42 °C for the first and second strategies, respectively, compared to 50 °C in the reference case. The study also compared excess flow requirements, noting an 18% increase in average flow rate for traditional substation connections, with cascading strategies showing increases of 18% and 8%.

Kilkis [82] analyzed a network cascading application for a low-temperature DH system in Nevşehir, Türkiye, using a low-grade geothermal well with a brine temperature of 65 °C and a flow capacity of 360 t/h, with a design outdoor temperature of −15 °C and an overall heat demand of 11.5 kWth per apartment. The study highlighted several constraints without network cascading:

• Designing home radiators without over-dimensioning necessitates a peak plant with a larger capacity to raise the supply temperature from 60 °C to 90 °C.
• DH return temperatures exceeded 60 °C during cold spells below −7.5 °C, preventing effective supply from the geothermal well.
• Radiators would need to be oversized by 160% to maintain a 60/30 °C operation scheme without a peak plant.
• Economic optimization is achieved by using radiant panels alongside peak plant temperature boosting.

To address these constraints, a cascading approach is proposed: a primary cascade level supplies homes with radiators, while a secondary level provides DH return to homes with radiant panels. The design includes

• A peak plant boosting the DH supply temperature to 75 °C.
• Radiator-equipped homes, oversized by 46%, cooling to a return temperature of 50 °C.
• Radiant panel-equipped homes, receiving a return temperature of approximately 35 °C from the return pipeline, which is then redirected.

Expanding the proportion of homes with radiant panels from 40% to 60% can increase the DH capacity, accommodating up to 1900 apartments and increasing geothermal well capacity from 12.6 MWth to 13.4 MWth, while reducing peak plant capacity from 4.3 MWth to 3.4 MWth [82].

Castro Flores et al. [83] investigated the hydraulic interface between high-temperature DH networks and low-temperature sub-networks, focusing on area-substations configured with heat exchangers to separate the two systems. Direct connection methods, such as mixing shunts and 3-pipe connections, are briefly mentioned [84] (could not find this citation). Two indirect area-substation configurations were explored:

• The first option utilizes a heat exchanger with inlets from both the DH supply and return pipelines to boost temperature when return temperatures are low, maintaining the required supply temperature for the low-temperature sub-network.
• The second option involves a two-stage setup with series-connected heat exchangers. The lower heat exchanger pre-heats the return water from the low-temperature sub-network, while the upper heat exchanger further raises the supply temperature to the desired level using an additional inlet from the DH supply pipeline. Mixing occurs at the DH side when the upper heat exchanger is operational.

Data indicate that the DH return water can meet 20–50% of the heat demand for the low-temperature sub-network. The performance of the substation used by the low-temperature sub-network is influenced by the DH return temperature, which reflects the operations of consumer building substations. Future research should focus on evaluating the relationship between higher return temperatures in the high-temperature network and lower temperatures in the low-temperature sub-network. Additionally, the use of a two-stage area-substation setup is recommended when DH return temperatures are high [83].

The return medium in current DH systems can be repurposed for various applications, such as heating agricultural greenhouses, aquaculture tanks, and road snow melting, utilizing different network cascading techniques [85]. For instance, Langendries [75] demonstrates a two-stage cascading system at Saint-Ghislain, Belgium, where return medium from a geothermal-based DH heats a 4000 m² greenhouse before pre-heating sludge for biogas production. Additionally, Basciotti et al. [86] highlight that varying return temperatures in large-scale DH networks give opportunities for integrating local heat sources, like heat pumps and solar thermal plants, as part of cascading applications.

2.2. Heat Storage Systems

In DH systems, heat storage is employed to shift excess production to periods of high demand. This includes medium-scale storage at the network level, small-scale storage at substations, and large-scale seasonal storage. This section focuses on medium-scale heat storage systems at the heat source and their impact on return temperature.

Parsloe [23] observed the storage requirements for two different temperature schemes:

• For an 80/60 °C temperature scheme, a peak flow rate of 2.4 L/s necessitates a storage volume of 2880 liters for a 20 min discharge duration.
• For an 80/40 °C temperature scheme, a peak flow rate of 1.2 L/s requires 1440 liters of storage capacity for the same 20 min discharge duration.

Behnaz Rezaie et al. [87] demonstrated that thermal storage capacity increases with decreasing DH return temperature. Specifically, storage capacity rises linearly from 290 MWh_th at a return temperature of 55.4 °C to 500 MWh_th at 40 °C.

2.3. Control Strategies

This section examines various control mechanisms employed in DH systems, emphasizing network operations. Both physical and operational control measures are crucial for maintaining low return temperatures in DH systems. The chosen control strategy affects the design, functionality, and overall performance of the system [28,56].

Küçüka [28] examined various control techniques focusing on indoor temperature maintenance, substation types (direct and indirect), and substation control (details given in

Table 1. Control strategies as appointed for the cases studied in [28].

Cases	Substation Type	Control Type	Control
			Control at Indoor Heating System	Control at Substation	Radiator Inlet Temperature
Case A	Direct	Variable Flow	Thermostatic Radiator Valve	-	Same as the DH network (85 °C)
Case B	Direct	Constant Flow	-	Supply temperature adjustment by admixing bypass and 3-way valve	Set to weather compensation
Case C	Indirect	Variable Flow	Thermostatic Radiator Valve	-	Fixed at 80 °C
Case D	Indirect	Constant Flow	-	Supply temperature adjustment by flow control at DH side of heat exchanger (no flow adjustment at the indoor heating system)	Set to weather compensation

). The study pertains to a DH system in Balçova, İzmir, Türkiye, with a supply temperature of 85 °C, a degree hour of 39,470 °C·h, and an overall peak heat demand of 73 MW_th under the design boundary conditions of 20/0 °C (indoor/outdoor temperatures). The results, detailed in Figure 7, show that indirect substation configurations (Cases C and D) lead to higher return temperatures and geothermal fluid consumption—approximately 8 °C and 94 m³/h more during high heat load periods compared to direct substation configurations (Cases A and B). Furthermore, variable-flow control (Cases A and C) is shown to be more efficient than constant-flow control (Cases B and D) during low-load periods.

The following sub-sections address control tactics in DH systems, focusing on flow control strategies, supply temperature considerations, and low-flow operation. Multiple control strategies can be assessed concurrently in practice. Lauenburg [29] identifies three heat output control strategies: (i) flow regulation with a constant DH supply temperature, (ii) supply temperature adjusted via the weather compensation curve with a steady flow, and (iii) simultaneous flow regulation with supply temperature based on the weather compensation curve.

2.3.1. Flow Control

This section examines flow control techniques for reducing return temperatures in DH systems, specifically variable-flow and constant-flow strategies. As illustrated in Figure 7, the variable-flow strategy is more effective in achieving lower return temperatures compared to the constant-flow strategy, a finding supported by various studies [23,47,75].

• The Constant-Flow Strategy

Constant-flow operation has been prevalent due to its simplicity in maintaining hydraulic balance and the cost benefits of using a constant-speed pump. This pump is typically set to handle peak heat loads at its most efficient speed, providing a consistent flow rate year-round. Heat output is generally controlled by adjusting the supply temperature according to the outdoor temperature, using a weather compensation curve. Bypasses that divert supply water to the return pipeline when the desired indoor temperature is reached are also used to regulate heat output. However, constant-flow systems are criticized for high return temperatures, poor heat production control, and excessive heat loss [47,48,49,73,75,88,89].

Applications to modify DH flow with constant-speed pumps, not considered in variable-flow methods, include adjusting the network system curve via a throttling valve at the pump outlet, and installing a bypass line between the pump inlet and outlet to redirect excess flow to the return pipeline [23,51,68].

• The Variable-Flow Strategy

The variable-flow strategy, an alternative to the constant-flow method, employs a variable-speed pump and decentralized self-regulating flow control valves at consumer sites (e.g., thermostatic radiator valves). This approach enhances flexibility and responsiveness to fluctuations in heat demand, thereby reducing overconsumption and improving thermal comfort. This allows for the optimization of return temperatures according to load conditions, achieving the lowest feasible return temperature [17,23,48,73,90].

Thermostatic radiator valves can be used to maintain variable flow in DH systems with constant-speed pumps, though this requires minimum flow bypasses, which can increase return temperatures [73,75]. Additionally, central flow control, where pump speed is adjusted based on outside temperature, is another proposed control option [75].

Łukawski [49] highlights the benefits of transitioning from a constant-speed to a variable-speed indoor heating pump. The variable-speed pump adjusts its speed to handle low-flow conditions, whereas the constant-speed pump operates at up to 30% of its nominal flow, with flow regulation achieved through throttling at the control valve. During low-load periods, the variable-speed pump results in lower DH return temperatures compared to the constant-speed pump, with reductions of up to 2.8 °C and averages of 1.1 °C observed for the 80/60 °C temperature scheme, and reductions of up to 1.9 °C and averages of 0.9 °C for the 55/45 °C scheme.

2.3.2. Supply Temperature Control

Supply temperature control aims to maintain an optimal and consistent temperature in the pipe network to efficiently meet building heat demands. This set supply temperature significantly impacts system performance and return temperature. Traditional DH systems either use a fixed supply temperature year-round or a variable supply temperature adjusted according to outdoor conditions via a ‘weather compensation control’ technique, also known as ‘temperature reset control’ or ‘external temperature compensator’. Figure 8 illustrates various forms of these weather compensation curves, which typically focus on adjusting the supply temperature in response to outdoor conditions (these curves normally do not directly account for variations in return temperature).

Similarly, substation controllers utilize an outdoor temperature reset approach by adjusting the inlet temperature to the internal heating system based on external conditions. This adjustment is achieved by controlling the flow through the substation heat exchanger in indirect configurations or by mixing the return medium with the supply medium in direct configurations.

Liao et al. [91] found that weather compensation curves outperform fixed supply temperature methods in terms of performance. However, improper commissioning of weather compensation curves can result in worse performance than fixed supply temperatures [71,91], the main reason being attributed to the lack of controller feedback on indoor temperature [92]. Additionally, Arena and Faakye [93] highlight a lack of information on optimizing weather compensation curves for achieving low return temperatures in residential hydronic systems.

Figure 8 reveals significant variation among weather compensation curves. This observation is supported by Tunzi et al. [94], who found that two distinct optimization strategies carried out for the same district produced different temperature curves.

Liao et al. [91,95] propose an adaptive control method, termed the “Inferential Control Scheme,” as an alternative to weather compensation curve controllers. This method adjusts the supply temperature based on real-time measurements of space heat load. Unlike weather compensation controllers, which can lead to overheating and high return temperatures, especially with outdated thermostatic radiator valves, the Inferential Control Scheme utilizes feedback to precisely meet heat demand without causing overheating or underheating.

Parsloe [23] proposes a smart “delta T” controller that integrates return temperature monitoring with weather compensation, applicable at both the building and DH levels. This controller is designed to minimize the return temperature while also optimizing the supply temperature when feasible.

Figure 8. Illustrations of various weather compensation curves sourced from the literature: Andersen et al. [96], Gustafsson et al. [88], Tunzi et al. (Sc: A and Sc: B denote scenarios for the same district) [94], Zinko et al. (for Skogås DH in Sweden and Cheongju DH in Korea) [11], and Karlsson and Ragnarsson (for indirect substations with an optimized supply temperature of 80 °C aimed at minimizing return temperature) [97]. Dashed lines indicate expected or resultant return temperatures, while solid lines represent supply temperatures, with both types of lines depicted in the same color.

Figure 8. Illustrations of various weather compensation curves sourced from the literature: Andersen et al. [96], Gustafsson et al. [88], Tunzi et al. (Sc: A and Sc: B denote scenarios for the same district) [94], Zinko et al. (for Skogås DH in Sweden and Cheongju DH in Korea) [11], and Karlsson and Ragnarsson (for indirect substations with an optimized supply temperature of 80 °C aimed at minimizing return temperature) [97]. Dashed lines indicate expected or resultant return temperatures, while solid lines represent supply temperatures, with both types of lines depicted in the same color.

2.3.3. Low-Flow Operation

Low-flow conditions, often resulting from reduced return temperatures and increased temperature differentials at the end-user site, can be managed using the ‘Kiruna Method’. This method involves either increasing the supply temperature while decreasing the flow rate or expanding the size of room heaters to maintain consistent heat output [13,17,23,29,54,98,99]. Johansson [100] provides data on temperature balancing for a low-flow operation with an 80/30 °C temperature scheme. Maintaining low-flow operation requires careful management of the pump minimum flow rate and the efficiency of control valves, such as substation control valves and thermostatic radiator valves.

Parsloe [23] identifies a potential issue with 2-way control valves, such as thermostatic radiator valves, which may fail to achieve the necessary minimal valve opening (referred to as ‘rangeability’) during low-demand conditions. This limitation can result in higher-than-expected return temperature levels. In contrast, the low-flow thermostatic valves discussed by Frederiksen and Wollerstrand [101] (citing [102]) are engineered for reliable performance under low-flow conditions.

Trüschel [17] emphasizes that low-flow strategies can effectively achieve low return temperatures, provided that adequate hydraulic balancing and proper component design are maintained. In this approach, radiators must rapidly adapt to changes in flow rates and utilize internal heat gains. However, unlike high-flow systems, low-flow systems are more susceptible to local network deviations that can elevate return temperatures. To mitigate these issues, Trüschel recommends careful commissioning of valve settings, selecting a pump with a steep characteristic curve, and maintaining a low differential pressure. These measures are intended to minimize the adverse effects caused by open valves within the local network [17].

Ward et al. [103] highlight an issue with ultra-low-flow operation, where the unintended mixing of supply medium with radiator water at the radiator inlet can lead to a reduction in radiator heat output.

2.4. Heat Source

The impact of low return temperatures on heat sources varies depending on the type of heat source and the overall system design. Research consistently shows that a lower return temperature, combined with reduced flow rates, generally benefits performance near the heat source. However, the relationship between return temperature and heat generation efficiency can differ significantly across various types of heat generation units and their specific connection configurations. Therefore, it is crucial to examine return temperature issues from the perspective of the heat source. This section reviews the relevant literature, including studies on residential heating systems, to provide a comprehensive understanding of how different heat sources respond to low return temperatures.

Parsloe [23] identifies several limitations imposed by DH supply and return temperatures on heat production units. For gas condensing boilers, achieving condensation requires a return temperature below 55 °C, with optimal condensation occurring at around 35 °C. Higher return temperatures can prevent condensation and potentially cause corrosion. In contrast, biomass boilers have a minimum return temperature of 60 °C to prevent corrosion caused by condensation. Cogeneration units face issues due to large temperature differences between supply and return and low flow rates, which can result in asymmetrical cooling. Heat pumps, on the other hand, exhibit reduced efficiency and output at higher supply temperatures. Parsloe’s research suggests that these constraints should be considered in the design of the primary loop interacting with the heat production unit, while secondary loops serving the DH network can benefit from reduced return temperatures [23].

2.4.1. Exhaust-Gas Condensation

Exhaust-gas condensation units are highly sensitive to DH return temperatures, as they recover more heat at lower temperatures [1,11]. Frederiksen and Werner [1] note that some DH systems deliberately increase the supply temperature to achieve a lower return temperature, thereby enhancing heat recovery in condensing units. Zinko et al. [11] illustrate (in Figure 6.2-2 in the reference [11]) the effect of lowering DH return temperatures on heat production from flue-gas condensation units using various fuels, based on internal communication with Fagersta Energetics AB in 2000. They find that reduced return temperatures improve heat recovery for all fuel types, with fuels high in moisture, such as wood chips and peat, showing greater increases in condenser heat output compared to natural gas and LPG. Additionally, the use of an air humidifier further enhances condenser heat output. For instance, with wood chips, an air humidifier increases heat output by 32% compared to 27% without it at a return temperature of 35 °C.

2.4.2. Combined Heat and Power

This section provides a detailed examination of three cogeneration types, also known as combined heat and power systems: back-pressure, extraction-condensing, and a combination of both within the same cycle (see [104,105] for comprehensive details). Maintaining a low temperature at condensers, which heat the DH water, results in reduced pressures, increasing the steam flow through the turbine, thereby enhancing electricity output [45,106].

• Back-Pressure

Ommen et al. [107] examine the impact of various temperature schemes—70/35, 90/40, and 105/50 °C—on the power efficiency of a back-pressure cogeneration unit. Their findings indicate that changes in the return temperature have a minimal effect on power generation efficiency, with less than a 1% variation for a 10 °C change. Conversely, a 10 °C change in the supply temperature significantly affects efficiency: a reduction can increase efficiency by up to 2.5%, whereas an increase can reduce it by 3–4%.

Im et al. [104,108] present experimental data spanning from January to August, revealing a linear relationship between the DH return temperature and the electricity output from the steam turbine. Their findings indicate that the electrical output increases from 67.1 MW_el to 67.5 MW_el as the return temperature decreases from 59.8 °C to 55.1 °C.

Figure 9 illustrates the impact of the DH return temperature on combined heat and power plant operations. Higher return temperatures require increased supply temperatures to maintain constant heat output, leading to a substantial rise in DH mass flow rate. Specifically, the flow rate increases from 745 kg/s to 1020 kg/s as the return temperature rises from 45 °C to 65 °C, necessitating a supply temperature increase from 99 °C to 107 °C [108].

Habka and Ajib [109] evaluate the impact of return temperature on a geothermal-sourced organic Rankine cycle cogeneration unit with both parallel and serial configurations. In the serial configuration, geothermal water first heats the cogeneration unit evaporator, then the outlet medium is directed to the DH heat generation unit. The study, based on the Glewe, Germany facility, finds that decreasing the return temperature from 55 °C to 40 °C linearly boosts electricity output from 1.5 kW_el to 6.8 kW_el, and enhances overall exergetic efficiency from 0.58 to 0.67 under a DH heat demand of 150 kW_th and a supply temperature of 75 °C. Additionally, with a DH heat demand of 150 kW_th and a return temperature of 50 °C, increasing the DH supply temperature from 60 °C to 85 °C does not affect net power generation (constant at 3.55 kW_el), but further increasing it to 90 °C significantly reduces electricity production by 1.77 kW_el.

In another study, Habka and Ajib [110] investigated a serial connection where the outflow from the evaporator is directly transmitted to the DH heat exchanger, without considering temperature boosts. Their control logic involves setting the evaporator outlet temperature based on DH supply and return temperatures, and subsequently adjusting the evaporation pressure for the organic Rankine cycle. Under a DH heat demand of 150 kW_th and a supply temperature of 75 °C, they observed a linear increase in electricity production from 2.2 kW_el to 8.7 kW_el and an increase in overall exergetic efficiency from 0.56 to 0.66 as the return temperature decreased from 55 °C to 40 °C. Similarly, changes in supply temperature from 60 °C to 85 °C did not affect net electricity generation, which remained at 4.68 kW_el, while further increases to 90 °C resulted in a significant reduction of 2.5 kW_el.

• Extraction-Condensing

Dalla Rosa et al. [111,112] show power loss in extraction-condensing cogeneration units relative to DH heat load and operational temperatures. Their study, represented by the ‘z-factor’ (Equation (1) and Figure 10), finds that DH supply temperature significantly impacts electricity loss more than DH return temperature. However, unlike back-pressure cogeneration cycles, reducing the return temperature can yield substantial electricity savings under fixed supply temperatures in existing DH systems.

$$z-factor={\dot{W}}_{loss}/{\dot{Q}}_{produced} \left[{MWh}_{el}/{MWh}_{th}\right]$$

(1)

The same relation for the z-factor (as a function of the supply and return temperature) can be seen for a nuclear plant at a capacity of 1 GW_el, as cited from [79] (the same z-factor can also be found for a nuclear plant at a capacity of1.3 GW_el in this reference), as shown in Figure 11.

Zinko et al. [11] reports an increase of 5 kWh_el/MWh_th for a 2-stage heat extraction cogeneration plant if the return temperature is reduced by a degree of 10 °C.

• Extraction and Back-Pressure

Johansson et al. [106] investigated the impact of return temperature on a cogeneration unit comprising: a back-pressure condenser used as a pre-heater for DH water, and a secondary condenser, fed by extraction from the low-pressure turbine, utilized to elevate DH water to the designated supply temperature.

The electricity-to-heat ratio, defined as the cogeneration electricity output minus the electricity consumption of the DH and cogeneration cycle pumps, relative to the heat delivered to the DH, was assessed across varying DH heat loads and supply temperatures as functions of the return temperature. The return temperature shows minimal impact on the electricity-to-heat ratio across all DH heat loads and supply temperatures. For example, lowering the return temperature from 54 °C to 38 °C with a heat load of 43 MW_th results in a minor increase in the ratio, with a difference of 0.013. Conversely, the DH supply temperature significantly affects the ratio; reducing it from 98.5 °C to 88.5 °C at the same heat load of 43 MW_th results in a notable increase in the ratio, with a difference of 0.05 [106].

• Fuel-Cell micro-Cogeneration

Windeknecht and Tzscheutschler [113] simulated a fuel-cell micro-cogeneration unit with a capacity of 1.5 kW_el and 0.85 kW_th, employing a control scheme that maintains a fixed electricity output rate while allowing the thermal output to vary with the return temperature. The study illustrates how the return temperature affects the heat output of the solid oxide fuel cell, as depicted in Figure 12.

2.4.3. Heat-Only Boilers

Cockroft et al. [115] illustrate the impact of varying return temperatures on the performance of different residential boiler types, with and without condensing features, as shown in Figure 13. The data demonstrate that incorporating condensing technology significantly enhances boiler efficiency overall. Specifically, the efficiency of the boiler improves as the return temperature decreases.

Girts Vigants et al. [117] report that the condensing boiler unit contributes 11.8% to the total heat supply over the entire heating season in the municipal DH system in Ludza, Latvia, with the remainder provided by a woodchip boiler with a nominal capacity of 8 MW_th. They also demonstrate the impact of return temperature on the condensing boiler efficiency, showing an increase from 8% at a return temperature of 58.3 °C to 19% at 43.3 °C. The dew point temperature for the flue gases is specified as 65 °C.

2.4.4. Heat Pumps

The role of heat pumps in advancing renewable energy-based electricity generation is substantial. They can be integrated into DH networks at various levels and with different source-sink configurations. Enhancing heat pump COP can be achieved by reducing compressor operation, which is facilitated by lowering the condenser temperature or raising the evaporator temperature [107,118,119,120].

Zinko et al. [11] note that for DH heat pumps with a single compression stage, the DH supply temperature has a more significant impact than the return temperature. Maivel and Kurnitski [121] assessed the effect of return temperature on the seasonal performance factor of a 5 kW_th residential heat pump system, considering temperature drops across radiator units. They found that the lowest return temperature results in the highest seasonal performance factor, measured at 3.72 [-]. Additionally, a direct connection configuration between the heat pump and the radiator unit was identified as optimal for achieving the lowest return temperature [121].

2.4.5. Solar Collector

Paulus [122] examines the performance of domestic solar collectors in relation to the DH return temperature and provides an economic analysis of various substation layouts, considering prosumer connections. The study focuses on a feed order of from-return-to-return for consumer connections rather than from return to supply. The efficiency of solar collectors and system costs are influenced by substation features such as local heat usage and pre-heating of domestic hot water. The effect of return temperature on collector performance varies with the substation configuration. Specifically, for a single-glazed collector, performance increases linearly from 226.5 kWh_th/m²·y to 624.9 kWh_th/m²·y as the return temperature decreases from 90 °C to 40 °C. For a double-glazed collector, performance rises linearly from 379.4 kWh_th/m²·y to 729.5 kWh_th/m²·y under the same conditions. Although all solar collector types show improved performance with lower return temperatures, the extent of this effect differs. Double-glazed collectors exhibit a smaller performance change compared to single-glazed ones. The study also notes that while local use arrangements yield high performance, they incur additional costs [122].

2.4.6. Industrial Excess Heat

To recover industrial excess heat, the process medium must initially be cooled using the DH return temperature. Subsequently, the recovered heat from the process medium is utilized to elevate the DH medium to the required supply temperature. A reduction in the DH return temperature by 5 °C has been reported to enhance heat recovery efficiency, potentially increasing it by 10–15% [11].

2.5. System-Level Optimization

This section elucidates the necessity for system-level optimization, focusing on the determination of an optimal DH operation temperature scheme. Specifically, it addresses the goal of establishing a supply temperature that achieves the lowest feasible return temperature while maintaining reasonable performance levels. Additionally, this section presents the relevant literature that supports various considerations and arguments essential for effective optimization.

2.5.1. Operation Temperature

The supply temperature in DH systems is determined by the DH operator, based on factors such as weather compensation curves or fixed supply temperature schemes. The return temperature reflects the cooling capacity of consumer units in response to the specified supply temperature, as well as control and cascade considerations within the system components. Consequently, the supply/return temperature configuration varies significantly across DH systems, as each system design process must account for specific operational temperature parameters [1,54,123].

Lauenburg [29] highlights the challenge of determining the optimal supply temperature, which requires site-specific analysis considering the type of heat production units and the anticipated return temperature from other DH system components. CIBSE [69] recommends optimizing temperature to account for lifespan costs and environmental impact, based on annual analysis. Crane [54] generally advises using a weather compensation curve to minimize DH heat loss, especially at low loads, though exceptions may apply if pump consumption and return temperatures are deemed excessive. This underscores the need for system-level temperature optimization and accurate commissioning of weather compensation curves.

Zinko et al. [11] underscore the necessity of system-level temperature optimization by addressing the trade-off between lowering supply temperatures, which reduces DH heat loss, and potential increases in pipe diameters that could lead to greater heat loss. High supply temperatures are expected to yield low return temperatures, assuming only heat emitter performance. Therefore, optimizing supply temperature requires considering DH heat loss, the efficiency of heat production units, and the operation of end-user components such as substations and indoor units.

Frederiksen and Werner [1] indicate the importance of site-specific optimization with examples. For instance, one example involves selecting a high supply temperature to achieve a low return temperature that facilitates flue-gas condensation at the heat source. Conversely, in another scenario involving a combined heat and power plant, a low supply temperature is chosen to enhance the power-to-heat ratio in the cogeneration cycle or to improve the coefficient of performance in a large heat pump plant.

Ommen et al. [26] illustrate the optimal supply temperature levels for different heat production units within the same district. For an extraction-type cogeneration unit, the ideal supply temperature is identified as 66 °C. Conversely, for a central large-scale heat pump, a supply temperature of 63 °C is recommended. Considering both types of heat production units, the final return temperature is observed to range between 22 °C and 27 °C, with the higher return temperature associated with increased pump electricity consumption compared to the cogeneration unit.

In a case study of the Gävle DH system, Cuadrado [45] examined the impact of return temperature on various system components, including the Johannes cogeneration plant. This plant’s heat production involves three stages: flue-gas condensation, back-pressure from the low-pressure turbine, and extraction from the mid of the high-pressure turbine. The study observed significant reductions in pump consumption and DH network heat loss, alongside enhanced recovery from the flue-gas condensation unit, as depicted in Figure 14.

2.5.2. Optimization Dilemmas

In the context of system-level optimization, addressing the operational temperature is critical; however, it is equally important to consider other factors that contribute to the overall efficiency and effectiveness of the DH system. While optimizing the supply temperature to achieve a lower return temperature is a primary objective, several additional considerations must be integrated into the optimization process.

Kilkis [82] underscores the necessity of optimization by addressing the conflict between radiator oversizing and the temperature-boosting activities of peak plants, each of which influences the return temperature differently. Furthermore, Ovchinnikov et al. [124] emphasize the impact of boundary conditions on the optimal design of space heating systems, noting that the relative effectiveness of different configurations may vary depending on the specific boundary conditions.

Yang et al. [125] highlight that the additional electricity required by instantaneous heaters for domestic hot water production underscores the need for system-level optimization. This optimization should assess whether the benefits of improving the DH operation temperature with these heaters can be outweighed by advancements in the DH network and heat source. Similarly, Johansson et al. [126] identify another optimization challenge related to add-on fans, noting that their power consumption might exceed the surplus electricity generated from reduced return temperatures at the DH heat production plant. Consequently, it is recommended to design an on-off fan controller that deactivates the fan when consumption surpasses excess generation. For a comprehensive analysis of add-on fan operations, see [126,127].

Ljunggren [128] indicates that optimization is necessary for cases of radiator or space heating heat exchanger oversizing. The reduction in the heat transfer coefficient due to oversizing can negate the benefits of increased heat transfer surface area, thus underscoring the need for careful optimization.

2.6. Implementation Strategies

In implementing return temperature reduction strategies at end-user sites, it is crucial to address both the technical and non-technical aspects of deployment. Special attention must be given to the implementation techniques, which encompass raising consumer awareness and establishing a pricing structure that incentivizes performance improvements.

2.6.1. Tariff Structures

The tariff and pricing system plays a critical role in optimizing heat usage within DH systems. Historically, efforts have focused on promoting the use of heat meters over the strategy as with the annual fees based on the amount of heated space, which do not account for actual consumption [129]. The examination of tariff plans and historical expenses in DH systems is detailed in references [129,130,131]. This section aims to analyze and develop formulas for tariff structures designed to support lower return temperatures, which are essential for enhancing the overall energy efficiency of DH systems.

It is essential to emphasize the experimental measurements reported by Karlsson and Ragnarsson [96], which compare the temperature drop recorded by customer installations under two different tariff structures at various supply temperature levels (see Figure 15). The authors observe that the volumetric flow rate tariff structure offers superior performance compared to the maximum flow restriction tariff structure, as it provides more effective cooling at end-user sites. Detailed information on these and other tariff structures is available in [129].

Chatenay et al. [129] highlight a limitation of conventional heat meters, noting that they lack the capability to assess cooling provided by consumers’ heating units. This deficiency poses a risk, particularly when a consumer site is equipped with inadequately sized heating units, which may lead to exceptionally high flow rates.

• Motivation Tariff

Many DH systems incentivize customers to reduce their return temperatures by implementing pricing schemes that offer bonuses for achieving lower return temperatures and/or impose penalties for exceeding specified thresholds. This approach, known as a ‘motivation tariff,’ is also referred to as a ‘cooling tariff’ or ‘incentive tariff’.

The Roskilde DH system, which supplies heat to 6500 customers, implements a motivational tariff, as detailed by VEKS [132]. The implementation of this tariff involved several steps:

• Determination of Reference Levels: The reference return temperature level was initially set by measuring the average return temperature of 28.5 °C during the 2001/2002 period.
• Threshold Establishment: Based on these measurements, thresholds were established with a natural band of 5 °C. A bonus was awarded if the supply–return temperature difference exceeded 35 °C, while a penalty was imposed if it fell below 30 °C.
• Consumer Engagement: Efforts were made to promote consumer awareness of this new tariff and to educate local heating and plumbing engineers about return temperature reduction techniques.
• Initial Results: In the first year of implementation (2004), 1000 consumers received bonuses for meeting the desired return temperature criteria, while 4500 others received penalties. This incentive structure effectively motivated changes in consumer behavior.

However, issues were identified, including poor cooling in some homes and faults with heating systems. Due to technological constraints that limited the ability to improve cooling, 48 consumers were exempted from the motivational tariff [132].

The Danish heat transmission company TVIS implemented an incentive tariff based on an anticipated annual savings of EUR 564,000 per degree Celsius reduction in return temperature (assuming an exchange rate of DKK/EUR = 0.106) in 2015. The tariff set a threshold at 42.9 °C, with pricing for both bonuses and penalties calculated at EUR 0.098 per GJ per °C. Middelfart Fjernvarme, a DH operator affiliated with TVIS, adopted this incentive structure, projecting savings of EUR 51,000 per degree Celsius reduction in heat purchase costs and EUR 14,700 per degree Celsius in DH network operational costs, translating to a cost reduction of up to 20% and a surcharge increase of up to 205%. Additionally, a dynamic threshold for the return temperature was established, adjusting according to the DH supply temperature, as illustrated in Figure 16 [94,133,134].

Jangsten et al. [135] report that Göteborg Energi AB employs a similar tariff structure. Danes et al. [92] further detail that a comparable tariff is used in the Copenhagen area, where the reference threshold for the average return temperature is set at 33 °C, with a natural band of 10 °C (ranging from 28 °C to 38 °C). The pricing for bonuses and penalties is set at EUR 0.71 per MWh_th per °C, based on an exchange rate of DKK/EUR = 0.106.

In Sweden, the incentive tariff is determined using a formula that considers both the flow rate and heat consumption during a monthly metering period. The approach involves establishing a reference temperature level based on annual theoretical analysis. Each consumer substation is then evaluated based on recorded heat and flow consumption to determine the applicable reward or penalty. The Malmö DH system in Sweden is noted as a success story with this tariff. Prior to its implementation in the early 1990s, the supply–return temperature differential was below 30 °C between November and March. Over the next 10 to 12 years, the incentive tariff improved this differential to 43 °C. The Malmö DH company did not gain financially from penalties; rather, the true benefit came from the reduced supply–return temperature differential. However, the tariff’s effectiveness was diminished due to inadequate consumer education, highlighting the critical role of customer awareness in maximizing the benefits of such incentive schemes [46].

• Alternative Pricing Models

When selecting pricing models as alternatives to incentive tariff systems, several strategies warrant consideration.

One approach involves basing the pricing model on the internal energy of the delivered volume, specifically the thermal capacity of the DH water, irrespective of how it is utilized or cooled at the consumer site. In this model, the cost of DH water is determined by the quantity consumed, with higher prices associated with higher supply temperatures. The onus of managing thermal capacity usage falls on the consumer (Robbe Salenbien, personal communication in an internal meeting on 7 June 2017).

Chatenay et al. [129] propose a novel heat meter unit that measures mass flow, supply temperature, and optionally outdoor temperature. This unit calculates the expected return temperature based on outdoor conditions, offering a method analogous to the previously described approach.

Additionally, Küçüka [136] suggests a pricing strategy based on flow measurement to incentivize reductions in flow rate, potentially leading to improved cooling at consumer installations. This cost-effective approach is particularly applicable in DH systems with high linear density, such as those examples commonly found in Iceland [129].

• Tariff in Network Cascading

Attention must also be given to the tariffing of cascading DH networks, particularly as the trend of connecting low-temperature consumers to the return pipelines of high-temperature DH systems becomes more prevalent (see Section 2.1.3).

Köfinger et al. [81] report that some Austrian DH systems currently employ a tariff structure that accommodates cascading. This tariff consists of two separate charges: one for the volume of return medium consumed and another for the supply medium used, which is infrequently required in small quantities when the return medium alone cannot satisfy the heat demand. The authors caution that in certain rare scenarios, excessively low return temperatures may lead to over-consumption of the supply medium [85].

2.6.2. Ownership Border

The delineation of ownership boundaries for substations in DH systems refers to the point at which responsibility for maintenance and repair shifts from the utility to the consumer. Typically, in DH systems, ownership is segmented: consumers own the building substations, while the DH operator retains ownership of the distribution piping network. The establishment of this ownership boundary is crucial for achieving low return temperatures, as it affects the maintenance of substations.

Parsloe [23] underscores the importance of ongoing monitoring of the supply–return temperature at DH substations, and VEKS [132] recommends annual inspections of consumer substations.

Markowicz [46] presents a comparison of cooling capacities for substations based on ownership status (see Figures 9 and 13 in this reference). The data indicate that substations owned by the DH operator have an average yearly cooling capacity of 38–40 °C, whereas consumer-owned substations exhibit poorer cooling performance with a supply–return temperature difference of 30.7 °C. Notably, the temperature range for company-owned substations spans from 5 °C to 60 °C, with lower temperatures achieved in smaller units. Chatenay [129] highlights the necessity for a low-flow penalty, which is currently absent from heat meters. Additionally, “group substations”, which are co-owned by consumers and the company, demonstrate cooling capacities ranging from 30 to 50 °C [46].

2.7. Novel Concepts

This section focuses on cutting-edge unique ideas that will help reduce return temperature.

2.7.1. Pressure-Independent Thermostatic Radiator Valve

Hydraulic imbalance is a significant factor contributing to insufficient cooling and high return temperatures in variable-flow DH systems, as discussed in Section 2.1.1. Pressure-independent thermostatic radiator valves represent an advanced solution to this issue. These valves integrate two functions within a single unit: one stage regulates the differential pressure, while the other functions as a thermostatic radiator valve. This design ensures accurate flow control, maintaining the necessary flow within the heated space despite variations in operating conditions (particularly dynamic pressure changes in the DH network, as detailed by Trüschel [17]). The use of these valves simplifies network re-balancing during renovations and commissioning due to their broad operational range. The primary benefit of these advanced valves is their ability to achieve low return temperatures by effectively managing the required flow to the heating terminals. Additionally, they reduce the risk of human error during operations such as balancing and commissioning, which are commonly associated with high return temperature problems [137,138,139].

2.7.2. Decentralized Pumps

A transformative innovation in DH systems involves the deployment of small-scale, demand-driven decentralized pumps at end-user sites, replacing traditional large-scale central pump stations. These small pumps operate on an on-demand basis, adjusting the required flow by regulating the pump speed, thereby eliminating the need for flow control valves. This approach offers several benefits, including the removal of thermostatic radiator valves and differential pressure valves (or pressure-independent thermostatic radiator valves). In conventional DH systems, the central pump speed is set to accommodate the most distant consumers, leading to excessive throttling of flow by nearby consumers due to high pressure. The new demand-driven consumer pumps effectively mitigate this issue by preventing flow loss from throttling and maintaining better control over the flow. Improving flow regulation to match exact heat demand is crucial for reducing high return temperatures, making the decentralized pump concept a promising solution in this regard [29,140,141,142,143].

Lauenburg [29], referencing Olsson [144], advocates for a configuration that includes direct connections between the main and sub-networks, coupled with demand-driven decentralized consumer pumps, to achieve low return temperatures. For indirect substations, the configuration involves two demand-driven pumps positioned on either side of the heat exchanger [143].

Kuosa et al. [143] present a simulation comparing traditional branched (‘tree-like’ or ‘Y-type’) DH networks with a novel ring network incorporating decentralized demand-driven consumer pumps. The results indicate that the ring network, utilizing decentralized pumping, achieves lower return temperatures compared to the traditional DH system, particularly when the outdoor temperature ranges between −5 °C and +5 °C. Specifically, the average return temperatures are 25 °C and 20.4 °C for the branched and ring network configurations, respectively, with decentralized pumps. In contrast, the traditional control system, employing a central pump station, yields return temperatures of 26 °C and 24.2 °C for the same network types.

2.7.3. Advanced Thermo-Hydraulic Fluids

While not a novel approach, the use of additives in the heat carrier medium or other make-up fluids to enhance the overall efficiency of DH systems merits attention, particularly in the context of reducing return temperatures. A prominent example involves drag-reducing additives, which aim to decrease friction losses in DH distribution piping. These additives can potentially yield significant savings in pumping energy and allow for the reduction in pipe diameters. However, it is important to note that such additives may adversely affect the heat transfer properties of the heat carrier medium, depending on their specific type. Studies have indicated that while there may be a minor reduction in heat loss within the DH network, there can be a significant decrease in heat transfer efficiency through heat exchangers due to these additives [145]. Additionally, references suggest the potential of advanced fluids, such as high-temperature phase-change slurries, in improving heat transfer, provided that the DH system and its components are redesigned accordingly [145,146,147,148,149,150,151].

2.7.4. Heat Pump Sourced by Return Medium

Langendries [75] mentions the concept of equipping large-scale heat pumps which make use of the DH return medium as a source hence further reducing of the return temperature, as applied in the EAB DH system located in Berlin, Germany [75,152].

2.7.5. Monitoring and Control

Monitoring of the consumer units is possible in recent DH systems with prevailing use of smart heat meters and well-built communication IT structure. Despite use of this concept for the heat metering, the application of this structure is suggested to be extended in continuous evaluation of the performance of the consumers’ substations. Any deviation in the monitored data than the expected (theoretically defined) level of the return temperature during the operation can be due to the poor commissioning and/or the faulty operation. Moreover, the system-level operational optimization becomes possible as to the stored data by this continuous monitoring [23,153].

3. Discussion

To enhance the efficiency of DH systems and support the integration of variable renewable energy sources, maintaining low return temperatures is increasingly critical. Return temperature, influenced by factors such as heat load, supply temperature, flow rate, and system design, cannot be directly adjusted but is a key indicator of system performance. This review aims to consolidate current knowledge on return temperature reduction techniques, focusing on the most cited methods and evidence from the literature.

Hydraulic imbalance is a significant cause of elevated return and supply temperatures in DH systems (see Figure 3). This imbalance arises from uneven hydraulic pressure distribution, leading to high flow rates near the pump station and low flow rates farther away. Low differential pressure can cause thermal discomfort and necessitate higher supply temperatures, while high differential pressure can lead to overheating and disrupt control equipment.

Bypass flows, which redirect the heat carrier medium from the supply line to the return line without transferring its thermal energy, also contribute to elevated return temperatures. While various types of bypasses are employed in DH networks, not all are redundant, particularly when considering the current technological advancements in DH systems.

To maintain thermal comfort for household hot water usage and reduce wait times, thermostatic bypass units are used to prevent excessive cooling of the supply medium during off-peak periods and summer. As illustrated in Figure 4, rising bypass flow rates correlate with increasing return temperatures during low demand periods. The figure also highlights that during periods of lower heat demand (e.g., at 3281 h and 0.26 kW_th load duration), return temperatures increase steadily. This corresponds to the situation that return temperature tends to increase under partial-load conditions, primarily due to low flow rates reducing the efficiency of heat extraction at substations.

Figure 5 shows various traditional and modern thermal bypass applications, each affecting return temperature reduction differently. Historically and traditionally, bypass applications have served to sustain a minimal flow rate through the network, most notably to prevent the supply line from cooling excessively during low-demand periods. In contemporary DH systems aiming for optimal efficiency, several alternative strategies can obviate or significantly reduce the need for such bypass flows without undermining system operation or user comfort.

One of these approaches is the comfort bathroom concept. Rather than employing continuous bypass flow to keep the supply line at a favorable temperature, a minimal but consistent supply to the bathroom heating unit is maintained during low-demand times (including summer in colder regions). By ensuring that sufficient thermal energy remains in the supply line, the medium does not have the opportunity to cool down extensively, thus eliminating the demand for a dedicated bypass circuit. This concept effectively repurposes what might otherwise be wasted bypass flow into a beneficial partial load for maintaining user comfort.

A further, albeit distinct, solution is the application of recirculation strategies that recycle surplus heat within the supply side instead of re-introducing it into the return line. These strategies help to reduce the incremental rise in return temperature by ensuring that any underutilized heat is redirected for further use or staging rather than contributing to a higher return temperature profile in the main DH loop. Consequently, the network benefits from more consistent and lower return temperatures.

An additional method to replace or minimize bypass flow involves integrating heat storage solutions into the system design. Instead of counting on continuous bypass operations to retain stable supply temperatures, storage units can absorb surplus heat and release it on demand. This measure not only curtails unnecessary bypass flows but also confers operational flexibility and a more dynamic response to fluctuating heat loads. During charging, the heat storage tank draws a flow rate from the supply line analogous to a bypass, yet it does so more purposefully: rather than merely circulating the medium back into the return, it stores thermal energy for subsequent utilization. This setup offers a more energy-efficient alternative, as it moderates return temperatures and significantly lessens avoidable heat losses.

Figure 6 provides a concise comparative illustration of how bypass configuration and substation design influence average return temperatures. Substations that rely on a dedicated domestic hot water storage unit, rather than an instantaneous heat exchanger, show a distinct advantage in driving down return temperatures. This outcome highlights that the thermal buffering effect of the storage tank not only meets hot water demand but also helps temper the flow returning to the DH network.

In parallel, the data suggest that internal bypass approaches wherein a modest flow is maintained at the substation side, particularly when supply temperatures decrease below 50 °C further minimize return temperatures compared to branch-end bypass loops. In contrast, branch-end (external) bypasses configured with a set temperature of 35 °C typically produce higher return temperatures.

Nevertheless, systems designed around a storage tank that uses a built-in heat-exchanger coil necessitate water setpoints exceeding 55 °C to meet hygienic requirements, specifically for preventing legionella growth. These higher setpoint temperatures curtail some of the energy efficiency benefits from operating at lower return temperatures, since the need to maintain elevated tank temperatures unavoidably leads to an overall increase in return temperature.

Hydraulic bypass valves are categorized into minimum flow bypasses and flow control bypasses. Minimum flow bypass valves tend to increase return temperatures throughout the heating season, with a more significant rise during low-load periods. To address the high return temperature issue caused by these valves, variable-speed pumps are recommended. These pumps can maintain the necessary DH flow while being capable of shutting off when the network is not under load. Similarly, flow control bypass units benefit from the use of variable-speed pumps.

Admixing bypasses are problematic as they lower the supply temperature just before it enters the radiator unit, leading to decreased performance and efficiency. This reduction in supply temperature results in a higher return temperature, reduces efficiency at the heat source, and increases network heat loss. To mitigate these issues, supply temperature adjustments should be made at the heat source rather than at the end-user site.

The temperature management scheme, with a focus on return temperature effects, positively influences the functionality and capacity of heat storage units. However, research in this area remains limited.

Cascading applications allow multiple subbranches within a DH network to utilize heat sequentially. Each unit or group of similar units can use the thermal energy from the return medium of preceding units when available. By further employing the residual thermal energy from a previous unit in another unit operating at a lower temperature, the overall heat potential is maximized, and the total DH return temperature is reduced. A cascaded network is feasible when the return line from a high-temperature sub-network can supply a lower-temperature sub-network, especially if the latter benefits from building renovations. For instance, low-energy buildings near high-temperature DH systems, whether newly constructed or renovated, can be efficiently supplied through network cascading. This approach enhances heat utilization and significantly lowers the return temperature in the existing DH system. Various cascading techniques are employed in current DH systems, including using the return medium for applications such as heating agricultural greenhouses, aquaculture fish tanks, and road snow melting.

The choice of control technique for any component within a DH system significantly influences both the design and functionality of those components, as well as the overall system performance. Figure 7 demonstrates that the return temperatures achieved in a specific network are contingent upon the control strategies implemented at various levels and for different types of substation units.

Historically, constant-flow control has been favored due to its simplicity in maintaining hydraulic balance and the cost-effectiveness of using inexpensive constant-speed pumps. However, this approach is associated with drawbacks, including high return temperatures, inefficient heat production control, and increased heat losses. In contrast, the variable-flow strategy, which employs variable-speed pumps and/or decentralized self-regulating flow control valves at end-user sites, offers a more effective alternative. Variable-flow control adjusts the flow to match varying load conditions, mitigating the need for bypass units and reducing return temperatures compared to the constant-flow method.

Traditional DH systems typically operate with either a variable supply temperature that adjusts based on weather conditions or a constant supply temperature maintained throughout the year. While weather compensation control is generally more advantageous compared to a constant temperature scheme, it is crucial to note that improper calibration of the weather compensation curve can result in performance inferior to that of a constant supply temperature approach. Identifying the optimal weather compensation curve from the numerous alternatives available in the literature (see Figure 8) remains a challenge for achieving effective operation and minimizing return temperatures. Additionally, weather compensation controllers lack feedback on whether the desired indoor temperature is achieved. Adaptive control strategies could address this issue by preventing overheating, which contributes to elevated return temperatures.

Maintaining low-flow operation in DH systems can be achieved by increasing the supply temperature while reducing the flow rate or by enlarging room heaters. However, this approach requires meticulous management of the pump’s minimum flow rate and the efficient functioning of control valves. If control valves cannot be adjusted to their minimum opening at low-demand conditions, return temperatures may rise more than expected. Although low-flow systems offer rapid response to changing flow rates and are advantageous when internal heat gains are significant, they are more susceptible to local network issues compared to high-flow systems. To mitigate these challenges, it is advisable to select a pump with a steep characteristic curve, avoid excessively high differential pressure, and carefully calibrate valve settings rather than making adjustments at the DH level.

Robust flow control mechanisms, coupled with refined supply-temperature regulation, are integral to sustaining lower return temperatures (see Figure 7 and Figure 8). Traditional constant-flow systems often raise return temperatures through fixed-speed pumping and bypass measures, whereas variable-flow methodologies employing variable-speed pumps, decentralized self-regulating valves, and active supply-temperature controls align the network more closely with real-time demand. Approaches such as the Kiruna Method further curtail return temperatures by carefully coordinating flow reduction with appropriate temperature differentials, provided that control valves and pump characteristics are aligned to these low-flow conditions. While weather compensation curves offer a basic means to match supply temperature to outdoor conditions, their performance can be undermined if commissioning or calibration is inadequate. Consequently, advanced control paradigms such as inferential control or delta T feedback schemes present promising avenues for improving system responsiveness and maintaining occupant comfort.

Achieving more refined system-level optimization in large-scale DH networks necessitates both advanced control techniques and tightly coordinated operations across all network components to accommodate fluctuating heat demand and supply. Adopting variable-flow strategies in place of static flow rates is especially pivotal: beyond merely reducing pumping energy, dynamically adjusting flow rates in response to load profiles (while ensuring control valves can operate effectively at low flows) contributes to consistently lower return temperatures. Moreover, while weather compensation curves are widely employed, their efficacy hinges on proper commissioning and real-time indoor feedback. Recent studies recommend more adaptive approaches, such as inferential or delta T-based control, which better match supply temperatures to the instantaneous needs of both buildings and the broader network, thereby minimizing risks of overheating at end-user installations.

In large-scale systems, an additional challenge arises when local inefficiencies, such as imbalanced flow or improperly tuned valves, lead to widespread return temperature elevations. Precise coordination—through well-calibrated thermostatic and control valves, together with suitably specified pump characteristics—helps mitigate these “hotspots”, thereby enhancing system-wide efficiency. For networks confronted with large load swings, the strategic use of heat storage solutions can further buffer peaks and reduce unnecessary bypass flow, using any surplus effectively instead of reintroducing it into the return line at high temperatures. Through a combination of advanced control schemes, rigorous hydraulic balancing, and dynamic storage usage, large-scale DH networks can maintain both operational reliability and robust return temperature reduction, even under highly variable load conditions.

Research indicates that a lower return temperature combined with a reduced flow rate has a beneficial effect near the heat source. From a network-centric perspective, incorporating renewable energy sources (e.g., solar thermal fields, biomass, and geothermal wells) into DH systems does not inherently guarantee lower return temperatures. While renewables reduce fossil fuel dependency and mitigate carbon emissions, they primarily influence supply-side generation rather than return-line thermodynamics. Instead, the return temperature is governed by internal system design and operation, encompassing hydraulic configurations, substation technologies (e.g., instantaneous versus storage-based heat exchangers), control mechanisms such as weather compensation or delta T feedback, and end-user practices. Consequently, the decisive levers for minimizing return temperatures lie in rigorous hydraulic balancing, advanced flow control schemes, and the precise calibration of consumer-side valves and thermostats. The renewable energy installations, albeit vital for decarbonizing the system, play a relatively peripheral role in actively reducing return temperatures; the system’s ability to manage flow rates, distribution temperatures, and end-user interactions ultimately determines whether low-return-temperature targets are met.

Exhaust-gas condensation units are particularly sensitive to the DH return temperature, as they achieve higher heat recovery with lower return temperatures. Consequently, some DH systems intentionally increase the supply temperature to achieve a lower return temperature, thereby enhancing heat recovery at their condensing units.

When considering cogeneration units, it is important to recognize that maintaining a low temperature at the condensers, which heat the DH water, reduces pressure and enhances steam flow through the turbine, thereby increasing electricity output. Notably, changes in DH return temperature lead to a substantial increase in DH pump load with only minor variations in efficiency (see Figure 9). The temperature scheme of DH operation significantly influences electricity loss in an extraction-condensing cogeneration unit relative to DH heat load (see Figure 10). Furthermore, the supply temperature has a more pronounced effect on the electricity-to-heat ratio compared to the return temperature, depending on the cogeneration cycle configuration.

For heat-only boilers, regardless of their condensing capacity, the return temperature significantly impacts boiler efficiency, as illustrated in Figure 13. The same figure also highlights that incorporating condensing technology in the boiler unit markedly enhances overall efficiency.

Heat pump units are gaining popularity, with evidence indicating that lower return temperatures improve seasonal performance factors. However, it is noted that the impact of DH supply temperature on heat pump efficiency is more pronounced than that of the return temperature.

Solar collector performance improves with decreasing return temperatures. For prosumers, the effect of return temperature variations depends on the chosen substation layout.

In the recovery of industrial excess heat, where an industrial process medium is cooled while a DH medium is heated, lowering the DH return temperature enhances the industrial cycle’s heat recovery capacity.

The heat supplier determines the supply temperature, while the characteristic return temperature of the DH system is influenced by the cooling capacity of consumer units at the specified supply temperature and the control considerations for all system components. This underscores the necessity for site-specific temperature optimization, which must account for all factors affecting lifecycle costs and environmental impacts, alongside an annual review. The performance of the system is significantly influenced by its components, and changes in return temperature markedly affect the system’s characteristics, each with distinct implications (see Figure 14). Optimization problems vary widely and can be formulated with different objectives and constraints. The effectiveness of a particular layout may differ based on the specific boundary conditions of the site.

When addressing the load on the peak plant, the set supply temperature is a critical factor. An optimization problem can be formulated to determine the appropriate radiator dimensions to reduce the return temperature, as both supply temperature and radiator dimensions have distinct impacts on return temperature. Additionally, another optimization task involves evaluating whether enhancements to the DH operational temperature achieved by instantaneous heaters can be offset by improvements to the DH network and heat supply. Although radiator add-on fans significantly improve return temperature, an optimization issue arises regarding whether their electricity consumption is justified by the additional electricity generated from the heat source operation due to the reduced return temperature.

To effectively reduce return temperature in DH systems, it is essential to consider both technical and non-technical factors. Given that heat demand drives DH systems, increasing consumer awareness is crucial, as it significantly influences demand levels. Implementing a pricing structure that incentivizes return temperature reduction can encourage performance improvements at end-user sites (see Figure 15).

A common approach involves establishing a motivation tariff with bonuses for excessive cooling and/or penalties for insufficient cooling. Successful examples from Denmark and Sweden are detailed in Section 2.6, outlining the steps taken and results achieved. Another strategy includes charging consumers based on the internal energy of the supplied medium or its volume, which gives end users control over their use of thermal capacity.

The ownership of end-user substations is also important, as these require regular maintenance. Annual inspections of substation cooling performance are necessary, either if substations are owned by the DH company or if mandated by law.

Pressure-independent thermostatic radiator valves offer a solution for maintaining hydraulic balance, thereby mitigating thermal discomfort and inadequate cooling at end-user stations. In contrast, decentralizing demand-driven pumps at end-user locations provides an alternative to traditional DH system configurations. This approach eliminates the need for a central primary pump station and hydraulic control valves, thereby effectively maintaining the required flow rate at all end-user stations according to demand and generally reducing excessive return temperatures.

Although not commonly emphasized, the addition of additives to the heat carrier medium or other make-up fluids could be considered to reduce return temperatures. Another innovative approach involves configuring a large-scale, centralized heat pump unit that utilizes DH return medium as a heat source. Modern DH systems, equipped with smart heat meters and robust IT communication infrastructure, are capable of monitoring end-user stations. It is recommended to extend this application for continuous evaluation of substation performance (cooling capacity), even though it is primarily used for heat metering.

A comprehensive overview of the key technical, operational, and economic considerations that influence return temperature reduction in DH systems is shown in

Table 2. Technical, operational, and economic factors essential for return temperature reduction in DH systems.

Category	Consideration	Description
Technical	Hydraulic Balancing	Ensures even heat distribution and prevents return temperature spikes.
	Cascading Applications	Utilizes residual heat effectively before rejection, reducing return temperature.
	Supply Temperature Control	Adjusting supply temperature dynamically for optimal return temperature control.
	Smart Control Systems	Advanced real-time control mechanisms optimize system performance and reduce inefficiencies.
	Heat Storage Systems	Thermal storage solutions to stabilize temperature variations and improve efficiency.
Operational	Bypass Regulation	Minimizing unnecessary bypass flows that elevate return temperatures and impact efficiency.
	Adaptive Flow Control	Flow adjustments based on dynamic heat demand variations for better temperature regulation.
	Demand-Driven Pumping	Decentralized pumping solutions that enhance energy efficiency and return temperature reduction.
	Consumer Behavior and Awareness	Encouraging behavioral changes, maintenance practices, and awareness campaigns for efficient energy use.
	System Integration Challenges	Addressing system-wide integration of new technologies while maintaining efficiency objectives.
Economic	Investment Feasibility	Evaluating cost–benefit trade-offs of return temperature reduction measures for economic viability.
	Tariff Structures and Incentive Mechanisms	Implementing pricing strategies that incentivize efficient heat utilization and penalize inefficiencies.
	Cost Savings from Efficiency Measures	Reducing heat loss and operational costs through better system design and optimization strategies.
	Economic Impact of Lower Return Temperatures	Lower return temperatures improve fuel efficiency and enhance economic performance of DH systems.

. These factors collectively contribute to optimizing system efficiency, minimizing energy losses, and enhancing the economic viability of DH networks.

A holistic approach in DH efficiency improvement necessitates the simultaneous consideration of multiple interdependent system components rather than optimizing each in isolation. Correct operation of the DH network, particularly its design and control strategies, plays a pivotal role in ensuring system-wide efficiency, with a major cumulative impact on overall performance. This interconnection extends to heat source efficiency, system-level optimization, and the integration of novel concepts and implementation strategies, all of which are necessary to achieve long-term sustainability and enhanced energy performance.

The DH network itself serves as the backbone of an efficient system, with hydraulic balancing emerging as the most critical factor in maintaining operational stability. The proper application of bypass mechanisms must be carefully regulated, as excessive or incorrect use can significantly elevate return temperatures. Instead, cascading should be encouraged when temperature conditions allow, ensuring that residual heat is effectively utilized before being rejected from the system.

Network design and operation, while influenced by control strategies at both the local and system levels, reciprocally affect the effectiveness of these very control mechanisms. A well-structured network design facilitates advanced control implementation, while an inadequately balanced network imposes limitations on control strategies, leading to inefficiencies in heat distribution and return temperature management. This reciprocal relationship underlines the necessity of considering network and control strategies as a unified entity within the broader system architecture.

Implementation strategies further extend the scope of a holistic approach by integrating consumer engagement and future-oriented innovations alongside system design, operation, control, and heat source management. Encouraging consumers to adopt efficient heat utilization practices, combined with policy-driven motivation tariffs, supports the effectiveness of technical measures. Meanwhile, novel concepts, including advanced monitoring, smart substations, and decentralized energy integration, pave the way for next-generation DH systems that are more adaptive and resilient.

System-level optimization provides an indirect yet comprehensive perspective on the holistic nature of DH efficiency improvement. A key dilemma emerges: How does network design influence operation, and how does operation dictate future design adaptations? Addressing this question requires a forward-looking strategy that accounts for network renovations, whether aimed at transitioning to lower operational temperatures or upgrading infrastructure to integrate renewable energy sources and industrial waste heat into existing heat supply configurations. The optimal control strategy must balance multiple considerations, including minimizing bypass usage, implementing precise hydraulic balancing, adopting adaptive supply temperature control, and deploying smart control mechanisms. Additionally, implementation strategies must incentivize end-users to align their consumption behavior with system-level efficiency goals, reinforcing a cumulative and self-reinforcing improvement cycle.

The selection of return temperature reduction strategies in DH systems requires careful consideration of their respective benefits and limitations, particularly in balancing technical feasibility, economic constraints, and operational efficiency. While some methods, such as hydraulic balancing and bypass minimization, provide immediate improvements in network stability and temperature control, others, like cascading applications and thermal storage integration, require significant infrastructure adaptations and investment.

Table 3. Trade-offs between different return temperature reduction methods.

Methods	Benefits	Limitations
Hydraulic Balancing	Improves overall network stability, reduces temperature imbalances.	May require retrofitting in existing networks with older pipe infrastructure.
Bypass Minimization and Smart Flow Control	Directly reduces unnecessary heat losses and avoids return temperature spikes.	Requires advanced monitoring and control, potential operational complexity.
Cascading Applications	Maximizes residual heat utilization, enhances efficiency.	Requires well-structured temperature zoning, may not be easily implemented in legacy networks.
Supply Temperature Optimization	Reduces heat losses and improves efficiency at the source.	In older networks, reduced supply temperature may result in inadequate heating performance.
Heat Storage Integration	Stabilizes network fluctuations, enhances flexibility.	High upfront investment, space requirements.

summarizes the key trade-offs between different return temperature reduction methods, providing a comparative assessment of their advantages and constraints to guide decision making in both existing and newly designed DH networks.

The effectiveness of return temperature reduction strategies in DH systems depends significantly on whether the network is an existing infrastructure or a newly designed system. While both categories require efficiency improvements, the approaches differ due to variations in technical constraints, operational flexibility, and the feasibility of system modernization and upgrades.

Existing DH networks face several challenges, including aging infrastructure, limited flexibility for large-scale retrofits, and consumer adaptation barriers. Given these constraints, strategies such as hydraulic balancing, bypass minimization, and adaptive flow control are the most practical solutions. Hydraulic balancing ensures that heat is distributed evenly across the network, preventing temperature imbalances that can lead to high return temperatures. Similarly, minimizing unnecessary bypass flows and implementing smart flow control mechanisms can help optimize return temperatures without requiring extensive physical modifications to the network. However, achieving significant improvements in existing networks necessitates a gradual integration of advanced control mechanisms, allowing for a staged upgrade of system components while minimizing disruptions to current operations.

In contrast, new DH networks offer a unique opportunity to incorporate low-temperature system designs and optimize efficiency measures from the outset. These systems benefit from greater flexibility in integrating cascading applications, smart thermal storage, and decentralized demand-driven pumping. By designing networks with structured temperature zoning, cascading applications can be fully implemented, allowing residual heat to be effectively utilized before being rejected. Furthermore, incorporating smart thermal storage solutions enhances system stability and enables dynamic load balancing, reducing peak demand pressures on the network. Decentralized pumping technologies further contribute to operational efficiency by enabling localized flow adjustments that match demand in real time. The focus in new DH networks is on building fully integrated, low-temperature systems that maximize renewable energy use, minimize heat losses, and enhance system-wide adaptability.

Ultimately, while existing networks require incremental modernization through targeted upgrades, new DH networks can be designed holistically to incorporate cutting-edge technologies and innovative operational strategies. The long-term sustainability of DH systems will depend on the ability to align technical improvements with economic feasibility, ensuring that both existing and newly established networks achieve optimal return temperature reduction and enhanced energy efficiency.

4. Conclusions

This review study aims to provide a comprehensive overview of current methodologies and technical considerations for reducing return temperatures in DH systems. Maintaining low return temperatures is increasingly vital for enhancing overall system efficiency. However, return temperature is not a parameter that can be directly controlled; it is influenced indirectly through various system parameters considered during the design, commissioning, operation, and control phases.

This review highlights key aspects that impact return temperature, including technical considerations within the DH network, cascading applications, innovative concepts, and control strategies. The section on heat sources examines the impact of return temperature on heat generation efficiency. Additionally, non-technical factors, such as pricing strategies and the delineation of ownership boundaries, are also addressed. System-level optimization, integrating both technical and non-technical factors, constitutes the final observation in this review.

Several general conclusions emerge from this review, highlighting areas that have not yet been extensively explored.

Hydraulic imbalance across the network significantly contributes to elevated supply and return temperatures in DH systems. Bypass flows, which often exacerbate high return temperatures, are not universally redundant.

Cascading provides a notable advantage by enhancing the utilization of available heat and reducing the overall return temperature. Cascaded networks are particularly effective in systems operating under varied temperature schemes, optimizing heat utilization and substantially lowering return temperatures.

The variable-flow control strategy, which adjusts flow using variable-speed pumps and decentralized self-regulating flow control valves at end-user sites, offers a more effective alternative to constant-flow systems. Similarly, a variable supply temperature system based on weather compensation generally performs better than a constant temperature scheme, though proper commissioning of the weather compensation curve is essential for site-specific optimization.

The interplay between return temperature and reduced flow rates benefits various heat sources, including cogeneration units, heat-only boilers, heat pumps, solar collectors, and industrial excess heat recovery systems.

System-level optimization is crucial, involving site-specific temperature adjustments and annual analyses to account for lifecycle costs and environmental impacts. Effective implementation strategies should focus on incentive tariffs to motivate end-users to reduce return temperatures or develop pricing structures based on the internal energy of the supplied medium. Additionally, substation maintenance must include annual inspections, either mandated by legislation or conducted by the DH company.

Pressure-independent thermostatic radiator valves can help maintain hydraulic balance while mitigating thermal discomfort and high return temperatures. Demand-driven pumps, installed decentrally at end-user substations, present a viable alternative to traditional DH systems. Furthermore, smart heat metering systems facilitate ongoing monitoring of end-user substations and should be given due consideration.

This review highlights the necessity of a holistic approach to optimizing DH efficiency, emphasizing the interdependence of network design, operational strategies, control mechanisms, heat source utilization, and consumer engagement. Achieving sustained improvements in return temperature reduction requires a system-level perspective where hydraulic balancing is prioritized, bypass applications are minimized, and cascading is effectively implemented when temperature conditions allow. The reciprocal relationship between network operation and control strategies further underscores the importance of integrated planning, ensuring that system-wide adaptations enhance rather than constrain efficiency. Smart control mechanisms, dynamic supply temperature adjustments, and targeted implementation strategies are critical in fostering both technical optimization and consumer-driven efficiency improvements. Ultimately, addressing these elements collectively enables DH systems to transition towards more resilient, cost-effective, and environmentally sustainable heating solutions, ensuring long-term operational efficiency and adaptability to future energy demands.

One key research gap identified in this review is the lack of comparative studies assessing the effectiveness of return temperature reduction strategies across different DH networks. Future investigations should aim to quantify the relative impact of strategies such as hydraulic balancing, cascading, bypass minimization, and smart control systems under controlled conditions. Developing a standardized methodology for effectiveness assessment would provide valuable insights for DH operators and policy makers, facilitating more informed decision making for network optimization.

5. Future Directions

As previously noted, return temperature is a characteristic influenced by various established parameters rather than being directly adjustable. Therefore, it is essential to address system-level concerns throughout the design, commissioning, operation, and control phases, with a particular emphasis on reducing return temperature levels. Future research should focus on several key areas to advance this field.

An integrated design approach is crucial, as each topic reviewed affects return temperature differently. Prioritizing low return temperatures within the system boundaries should be a central goal. This approach should be supported by system-level simulations that emphasize developing comprehensive thermo-hydraulic models for all system components. Evaluating temperature levels accurately requires an integrated approach encompassing system design, operation, and control.

The ongoing debate regarding whether to prioritize low return temperatures or temperature differences for optimal system performance is complex and context-dependent. The optimal strategy may vary based on the specific characteristics of the DH system. Similarly, consideration must be given to whether to focus on maintaining low return temperatures or reducing the operational temperature scheme, such as lowering the supply temperature. This trade-off often involves balancing energy efficiency with economic considerations.

Extended off-peak periods also need to be considered in system-level studies. Optimizing system performance during these periods, particularly in terms of maintaining low return temperatures, is critical. Future research should emphasize smart adaptive control systems to address challenges arising from poor commissioning of weather compensation, which can lead to excessive return temperatures and thermal discomfort. Demand-based strategies that focus on system performance in relation to demand characteristics may offer promising solutions.

Site-specific system-level optimization should focus on operational temperature levels, involving all system components and control units. An optimal system should ideally achieve low temperatures, whether for supply, return, or both. Additionally, the impact of system temperature levels on the efficiency of heat production must be considered, taking into account the unique characteristics of various heat production facilities. This raises the question of whether to prioritize low supply temperatures, low return temperatures, or both.

The development of fault detection algorithms based on return temperature levels is crucial. Analyzing return temperatures can help operators detect potential issues within the heat distribution network. This review highlights the need for a comprehensive approach to optimizing return temperatures in DH systems, integrating technical, operational, and economic factors to enhance overall system efficiency.

Although the technical merits of various return temperature reduction strategies have been explored, a structured economic analysis comparing their respective costs and benefits remains underdeveloped. Current research offers only fragmented, case-specific cost estimates without a standardized framework for evaluating the financial feasibility of advanced hydraulic balancing, smart control systems, and cascading applications. Future work should therefore prioritize developing systematic cost–benefit analyses that quantify the trade-offs between increased capital outlays and the resulting long-term gains in operational efficiency, cost savings, and emissions mitigation. Field demonstrations (either through pilot upgrades of existing networks or newly installed systems) would provide valuable real-world evidence on return-on-investment projections and the sensitivity of outcomes to local conditions. Additionally, a more nuanced exploration of policy frameworks, incentive structures, and organizational barriers, particularly at the utility or building-owner level, could help clarify the pathways for large-scale adoption of innovative control schemes and heat storage solutions. Such expanded investigations, firmly grounded in both economic metrics and stakeholder challenges, would significantly strengthen the knowledge base for achieving robust, cost-effective DH modernization.

We acknowledge that adaptive and inferential control methods may hold significant promise for optimizing both energy savings and system reliability in DH networks. To build a more robust evidence base, future studies could focus on pilot implementations or real-world demonstrations that track performance under variable operating conditions over multiple heating seasons. Such research would clarify not only technical aspects (e.g., stability and ramp-up times) but also the cost–benefit trade-offs, providing operators, policy makers, and designers with actionable insights for wide-scale adoption.

Future research would benefit from systematically examining how these lower return temperatures interact with different heat generation technologies. In particular, deeper studies of cogeneration systems and heat pumps under variable thermal and load conditions could clarify the degree to which an improved temperature differential enhances operational efficiency, capacity factor, and reliability. Detailed cost–benefit analyses, possibly supported by pilot projects in larger-scale DH networks, would further establish the direct correlation between network-side optimization and generation-side performance. Ultimately, such investigations could provide planners and operators with a stronger evidence base for designing systems that maximize overall energy savings while sustaining or improving supply reliability.

This manuscript underwent language refinement with the assistance of ChatGPT (Version 3.5) to enhance readability and clarity, while ensuring that all content remains original and solely the work of the authors.