A Wastewater Heat Recovery System as a Solution to Improve the Energy Efficiency of Buildings and Reduce Greenhouse Gas Emissions: Technical, Financial, and Environmental Aspects

Abstract

Greywater can be a valuable energy source in buildings. Its advantages over other renewable energy resources include its daily availability, regardless of weather conditions. Consequently, wastewater heat exchangers are increasingly used in domestic hot water preparation systems. This article presents the results of tests on three DHW installation variants, including two integrated with various drain water heat recovery exchangers. A horizontal DWHR exchanger (a prototype of a new exchanger design) reduced the energy demand for hot water preparation by up to 29.6%, while a commercially available vertical DWHR unit (“tube-in-tube”) reduced this demand by up to 64.7%. This reduction was primarily influenced by the flow rate from the shower head and the mixed water temperature. Furthermore, a Life Cycle Cost analysis showed that, despite the additional costs associated with implementing DWHR exchangers, the traditional water heating method was the least cost-effective solution in all calculation cases. Furthermore, the tested wastewater heat exchangers significantly reduced CO₂ emissions compared to traditional water heating. This indicates the great potential of wastewater heat recovery systems in decarbonizing the building sector.

1. Introduction

The construction sector is characterized by one of the highest water and energy demands worldwide [1,2,3] and accounts for approximately 40% of global carbon dioxide emissions [4]. To achieve the goals of the Paris Agreement [5] and Sustainable Development Goals (SDGs) [6], improving the energy efficiency of buildings and decarbonizing the construction sector is crucial. This is particularly important for residential buildings, which consume over 26% of end-use energy [7]. As much as 78% of the energy in these buildings is used for space heating and domestic hot water (DHW) preparation [8]. For many years, the energy efficiency of DHW systems has been neglected and overlooked due to their insignificant share in total energy use for buildings [9]. Advancements in building technologies have improved the energy performance of buildings, thereby reducing the energy demand for space heating and cooling while increasing the proportion of energy for DHW in the total energy balance of buildings. In 2023, the share in European countries was 15.1%, although in some countries, it was significantly higher [10]. For example, in Denmark, the Netherlands, and Slovakia, water heating accounts for 24.2%, 21.3%, and 20.7% of the total energy demand of buildings, respectively [10]. Data presented in [11,12] indicate that this share can reach up to 50%.

The energy efficiency of domestic hot water (DHW) systems can be improved by replacing outdated heat sources, reducing heat losses, using water-saving taps, and implementing renewable energy sources (RESs) [13,14,15]. RES technologies based on solar and ground energy have been used in buildings for water heating for years. However, in countries with temperate climates and changing seasons, these solutions are often weather-dependent. Another RES worth considering is greywater, which was officially recognized by the EU as a renewable energy technology in 2018 [16].

In recent years, the perception of greywater as a useless waste product has shifted. It is increasingly employed as an alternative water source in buildings [17,18], as an energy source [19], and within hybrid systems [20]. However, it should be emphasized that only 10–20% of greywater’s potential as an energy source is utilized [21] and, in most buildings, wastewater heat is irretrievably wasted. The greatest advantage of greywater for energy use lies in its temperature, particularly that generated during bathing. Wastewater discharged from showers has a temperature of 30 °C to 40 °C [22,23], which is why this wastewater is most often the energy source in heat recovery systems. Hao et al. [24] pointed out that 90% of heat from greywater can be recovered.

Various heat exchangers for greywater heat recovery are available on the market [17,19]. The two basic types are vertical heat exchangers and horizontal heat exchangers [25], which, in residential buildings, are most often installed at the greywater outlet from showers. Regardless of their design and installation method, these heat exchangers operate based on the countercurrent flow of water and wastewater. These devices differ not only in their heat recovery efficiency, but also in their construction and dimensions. Vertical DWHR units are characterized by significant height, reaching up to two meters. This can create challenges for installation in existing buildings where the plumbing system does not incorporate a wastewater heat recovery system. This is particularly problematic in single-story buildings without basements. Vertical heat exchangers can be installed in existing buildings; however, this often requires substantial construction work and increases the costs of implementation. In such situations, horizontal heat exchangers are a better solution. Due to their smaller dimensions (primarily height), they are easier to integrate into existing building installations. This is particularly true for DWHRs built into the shower tray or integrated with linear shower drainage. The compact dimensions of horizontal shower heat exchangers represent a key advantage. However, these solutions are typically characterized by lower heat recovery efficiency than vertical DWHR units [26,27].

The amount of heat recovered from wastewater and the associated CO₂ emissions reduction depend on many factors, including the type of exchanger, mixed water temperature, cold water temperature, and water flow rate [28]. Several methods are available to assess the energy and environmental efficiency of drain water heat recovery exchangers. Most studies rely on laboratory testing, which is labor-intensive and costly, while others employ numerical modeling and machine learning methods.

Table 1. Cases studies found in the literature.

Reference	Type of DWHR	Methodology	Main Conclusions
[29]	DWHR storage device	Numerical simulations, CFD analysis, and experimental studies	The DWHR storage unit had the capacity to recover from 34% to 60% of the energy available in the greywater
[30]	Vertical spiral DWHR exchangers integrated with solar domestic water heaters	Experimental studies and numerical simulations	The average effectiveness was approximately 55%
[31]	Linear horizontal DWHR exchanger (prototype)	Experimental studies and machine learning methods	The effectiveness of the DWHR was 19.37–36.94%
[32]	Liner horizontal DWHR unit (prototype)	Experimental studies	The effectiveness of the DWHR exchanger was 25.54–33.44%
[33]	Horizontal plate shower exchanger (prototype)	Experimental studies	The efficiency of the new prototype in real operating conditions reached up to 62%
[34]	Vertical falling-film drain water heat recovery heat exchanger	Transient System Simulation Tool (TRNSYS) software	The effectiveness of the DWHR reached up to 60%
[35]	Vertical “tube-in-tube” drain water heat exchanger	Experimental studies	A reduction was achieved in the energy consumption for domestic hot water preparation of approximately 45.7% to 60.8%
[36]	Vertical falling-film drain water heat recovery heat exchanger	Laboratory studies, NTU method	The effectiveness of the fully wet surface varied between 29 and 46%, and that for the partially wet surface between 21 and 42%
[37]	Horizontal, plate heat exchanger	Laboratory studies, NTU method	The least favorable test results were ε = 18.6%, NTU = 0.24; the most beneficial results were ε = 50.0%, NTU = 0.99

presents selected case studies found in the literature.

One of the first studies on heat recovery from wastewater examined a wastewater tank heat exchanger [38]. The authors showed that the average annual energy saving for water heating was 30%, which represented approximately 10% of the total household energy consumption. In another study [39], a new design approach consisting of a systematic calculation procedure was presented. Ramadan et al. [39] pointed out that the DWHR exchanger’s effectiveness increases with the boiler outlet temperature and decreases quasi-linearly as the cold inlet temperature increases. Heat recovery efficiency is also influenced by the water flow rate, as indicated by studies conducted in Iran [40], the UK [41], and Canada [42]. Others emphasize that the efficiency of a shower heat exchanger is influenced by its installation method [43]. The efficiency of a horizontally mounted exchanger was 16.13%, while that of the same exchanger installed vertically increased to 24.69% [43]. The influence of the angle on the DWHR efficiency was also confirmed by research conducted by Manouchechri et al. [44].

The current state of scientific knowledge shows that researchers have focused primarily on determining the energy efficiency of drain water heat recovery exchangers. Less frequently, research has focused on the financial and environmental assessment of wastewater heat recovery systems, which is a relevant decision-making criterion. The literature contains studies relating to the financial [35,45,46] and environmental [20,47,48] analysis of DWHR units. However, to the authors’ knowledge, there is no comprehensive study covering the technical, financial, and environmental aspects of implementing DWHR exchangers in single-family buildings. It should also be emphasized that the vast majority of published studies have focused on a single specific wastewater heat recovery solution. Considering the varying technical requirements and significant differences in heat recovery efficiency between vertical and horizontal exchangers, comprehensive testing was conducted on a commercially available vertical DWHR (tube-in-tube) and an innovative horizontal DWHR solution developed by the authors. This enabled a comparative analysis of the tested exchangers and identification of the key factors influencing their energy, financial, and environmental efficiency. An integrated approach to heat recovery systems demonstrates their benefits and expands the possibilities of their implementation in buildings. Therefore, the research results, in addition to their scientific aspects, also have practical and promotional implications. Considering the existing research gap in the comprehensive and comparative evaluation of various drain water heat recovery solutions and the importance of improving the energy efficiency of buildings, research was conducted with the following objectives:

• Conducting experimental studies on a newly designed horizontal drain water heat exchanger and a commercially available vertical DWHR unit;
• Determining the energy efficiency of the tested exchangers and identifying the parameters influencing this efficiency;
• Conducting a Life Cycle Cost analysis for several variants of domestic hot water installations, including configurations taking into account the tested DWHR exchangers;
• Determining the CO₂ emissions of the analyzed DHW installation variants and assessing their environmental impact.

2. Materials and Methods

The research was divided into several main stages, as shown in Figure 1. The first step involved a review of the literature and an analysis of commercially available wastewater heat recovery exchanger solutions. Based on this review, a new horizontal drain water heat recovery exchanger solution was developed and a prototype was built. Laboratory tests were then carried out on both exchangers to determine their energy efficiency and compare this with that of the most commonly used vertical DWHR. The results of these tests served as the basis for a financial analysis of different domestic hot water system configurations, including a traditional solution and variants incorporating wastewater shower heat exchangers. The subsequent stage of the research focused on assessing the environmental impact of implementing DWHR exchangers. To this end, CO₂ emissions, one of the most harmful greenhouse gases, were determined. The final stage involved a discussion of the obtained research results, formulating final conclusions, and defining future research directions.

2.1. Characteristics of Analyzed DWHR Exchangers

As part of the research presented in this article, a comprehensive analysis was conducted of two different types of DWHR exchangers. The first is a commercially available vertical “pipe-in-pipe” heat exchanger, the Showersave QB1-16, Q-Blue [49], made of copper. Warm greywater flows gravitationally through a smaller diameter pipe, while cold water flows in the opposite direction through the space between the pipes. The second heat exchanger is a new, different design protected by a patent [50]. Inside the shower tray, at its bottom, a heat exchanger consisting of copper tubes is mounted, through which cold water flows to the heater. Wastewater generated during bathing flows through the shower tray chamber washes the heat exchanger, and after heat exchange, is discharged into the sewer system. Detailed technical specifications and diagrams of the tested wastewater heat recovery exchangers are shown in Figure 2 and

Table 2. Technical parameters of analyzed drain water heat recovery exchangers.

Type of DWHR Unit	Parameter	Value	Unit
Vertical DWHR—Showersave QB1-16	Length	1.68	m
	Diameter (external)	50	mm
	Diameter (internal)	45	mm
Horizontal DWHR—prototype	Chamber length	0.8	m
	Chamber width	0.8	m
	Chamber height	0.1	m
	Pipe diameter	8.16	mm
	Total length of pipes	16.72	m

.

2.2. Laboratory Tests of Analyzed Drain Water Heat Recovery Units

The financial and environmental analysis of the tested drain water heat recovery exchangers were based on laboratory tests conducted at the Laboratory of Water and Wastewater Flow Control Techniques at the Rzeszów University of Technology. These tests were performed on a research stand that allowed for obtaining conditions close to real ones. The tested hot water preparation system was designed to reflect the most commonly used wastewater heat recovery configuration, in which water preheated in the DWHR unit is supplied to both the water heater and the shower mixing valve. In this solution, the water flow rate through the exchanger is the same as the flow rate of the discharged greywater, allowing for the highest efficiency of wastewater heat recovery [51]. Figure 3 shows a diagram of the laboratory stand, while Figure 4 shows a photo of the completed stand.

The domestic hot water system was equipped with a range of devices and measuring fixtures enabling testing over a wide range of input parameter variations.

Table 3. Main equipment of the research stand.

Device	Purpose of Use	Type of Device, Manufacturer	Technical Data	Number of Pieces
Water heater	Hot water preparation	KDE-27. BONUS.PL, Kospel, Poland	Maximum power 27 kW, 400 V, smooth temperature control 30 °C to 60 °C	1
Thermostatic mixing valve	Mixing cold and hot water and ensuring a set constant temperature	ATM 343, Afriso, Germany	DN15, temperature control 35 ÷ 60 °C, max. 90 °C	1
Flow meter	Measurement of water flow, m3/h	Sharky 473, Diehl Metering, Germany	Ultrasonic flow meter with nominal flow 0.6 m3/h, DN 15, IP 54, 5–130 °C, DC 3–5.5 V	3
Data recorder	Data recording every 1 s	MultiCon CMC-144, Simex, Poland	Number of outputs: max. 18 analog (4–20 mA); max. 72 SSR; 36 relays (1 A/250 V) or 18 relays 5 A/250 V. Number of inputs: max. 15 universal inputs; max. 72 analog inputs (0/4–20 mA or 0/1–5 V or 0/2–10 V); max. 72 digital inputs; max. 36 TC inputs; max. 18 RTD inputs; max. 18 counter/flowmeter inputs, 19–50 V; DC 16–35 V AC or 85–260 V AC/DC	1
Temperature sensor	Measurement of water and greywater temperature, °C	TOPE-L0384–Pt500, Simex, Poland	Resistive sensor made of platinum, measurement accuracy class AA (from ±0.10 (0 °C) to ±0.27 (100 °C)), 0–150 °C	5

presents detailed data on the devices and measuring equipment, while

Table 4. Input variables adopted for research.

Variable Research Parameters	Value	Unit
Temperature of mixed water at outlet of shower head, Tsh	34, 38, 42, 46	°C
Shower length, lsh	5, 10, 15	min
Water flow rate from shower head, qsh	5.0, 7.5, 10.0	L/min

presents the parameter values subject to change during the tests, which were conducted over a two-month period (February–March 2024). Bath water temperature values were determined based on available research in this area and the authors’ own experience [52,53,54].

2.3. Energy Efficiency of Domestic Hot Water Preparation System

To determine the impact of implementing a drain water heat recovery system on the energy efficiency of domestic hot water production, energy demand was calculated using Equations (1) or (2), depending on the installation variant. Each variant considered a variable number of shower users corresponding to a single-family home. According to statistical data, the average household size in Poland is 3.9 people [55]. Therefore, the study assumed a variable number of residents M of 3, 4, and 5. This allowed for a broader examination of the financial and environmental impact of implementing a DWHR system. Three DHW installation variants were considered:

• Variant 1—a water heater supplied directly with cold water from the water supply network (Equation (1));
• Variant 2—a water heater supplied with preheated water in a vertical DWHR exchanger (Equation (2));
• Variant 3—a water heater supplied with preheated water in a horizontal DWHR exchanger (Equation (2)).

$${\mathrm{E}}_{\mathrm{D}\mathrm{H}\mathrm{W}1}={\displaystyle \frac{{365·\mathrm{M}·\mathrm{q}}_{\mathrm{s}\mathrm{h}}·{\mathrm{l}}_{\mathrm{s}\mathrm{h}}·{\mathrm{c}}_{\mathrm{w}}·{\mathsf{\rho }}_{\mathrm{w}}{·(\mathrm{T}}_{\mathrm{h}}-{\mathrm{T}}_{\mathrm{c}})}{\mathsf{\eta }·3.6·{10}^6}}$$

(1)

$${\mathrm{E}}_{\mathrm{D}\mathrm{H}\mathrm{W}2,3}={\displaystyle \frac{{365·\mathrm{M}·\mathrm{q}}_{\mathrm{s}\mathrm{h}}·{\mathrm{l}}_{\mathrm{s}\mathrm{h}}·{\mathrm{c}}_{\mathrm{w}}·{\mathsf{\rho }}_{\mathrm{w}}{·(\mathrm{T}}_{\mathrm{h}}-{\mathrm{T}}_{\mathrm{p}\mathrm{r}\mathrm{e}})}{\mathsf{\eta }·3.6·{10}^6}}$$

(2)

where E_DHW1 is the annual amount of energy needed to prepare domestic hot water in Variant 1, kWh; E_DHW2,3 is the annual amount of energy needed to prepare domestic hot water in Variant 2 or Variant 3, kWh; M is the number of residents; q_sh is the water flow rate from the shower head, m³/s; l_sh is the daily shower length, s; c_w is the specific heat capacity of water, J/(kg·K); ρ_w is the density of water, kg/m³; T_h is the temperature of hot water, °C; T_c is the temperature of the cold water supplied to the water heater, °C; T_pre is the temperature of preheated water, °C; and η is the efficiency of the water heater (η = 0.95).

Based on the determined annual energy demand for each variant and the savings achieved thanks to the use of shower heat exchangers, the degree of reduction in energy consumption was calculated from Equation (3).

$$\phi ={\displaystyle \frac{{\mathrm{E}}_{\mathrm{D}\mathrm{H}\mathrm{W}1}-{\mathrm{E}}_{\mathrm{D}\mathrm{H}\mathrm{W}2,3}}{{\mathrm{E}}_{\mathrm{D}\mathrm{H}\mathrm{W}1}}}·100\%$$

(3)

2.4. Financial Analysis of Domestic Hot Water Preparation System

The financial aspect of using DWHR units in domestic hot water preparation systems was examined based on Life Cycle Cost analysis. This method, being a useful decision-making tool, is used in many economic sectors [56]. In addition to initial investment costs, it also takes into account operating costs incurred throughout the life of the product, technology, or building [57,58]. Selecting a solution solely based on capital expenditures can lead to poor financial decisions, as operating costs are often significantly higher than purchase costs. This is particularly important when analyzing high-capital investments, such as those in the construction sector [59,60]. This applies to both new buildings and modernized buildings [61,62]. Applying the LCC methodology at the design stage of a building or modernization project contributes to the development of sustainable buildings with reduced environmental impact [63]. The value of the LCC indicator for different variants of domestic hot water production was calculated from Relationship (4).

$${\mathrm{L}\mathrm{C}\mathrm{C}}_{\mathrm{D}\mathrm{H}\mathrm{W}}={\mathrm{I}\mathrm{N}\mathrm{V}}_{\mathrm{i}}+{\sum }_{\mathrm{t}=1}^{\mathrm{T}}{\displaystyle \frac{{\mathrm{O}\mathrm{M}\mathrm{C}}_{\mathrm{i}}}{{\left(1+\mathrm{r}\right)}^{\mathrm{t}}}}$$

(4)

where INV_i is the capital expenditures of variant I, €; OMC_i is the operating costs of the variant I, €; T is the total lifetime, years; r is the discount rate, –; and t is the year of use.

Incorporating the discount rate into the financial analysis reflects the changing value of money over time, making it possible to determine the actual future benefits of a given investment [64]. The discount rate is a key element in cost–benefit analysis, especially when these differ significantly over time [65]. The choice of total lifetime also significantly influences the final results of the financial assessment. According to the European Commission’s recommendations for investments related to energy technologies, this period should be 15–25 years [66]. The planned operational life of the facility and the durability of the materials used should also be taken into account.

In the financial analysis of the considered domestic hot water preparation options, the total LCCs included the INV_i capital expenditures incurred for each option (the cost of purchasing and installing DWHR exchangers, along with the required installations) and the OMC_i operating costs resulting from the energy consumption for water heating (

Table 5. Data used for LCC analysis.

Parameter	Value
The cost of purchasing and installing a vertical DWHR heat exchanger with a connection set INVV	801 €
The cost of purchasing and installing a horizontal DWHR heat exchanger INVH	338 €
The cost of installing additional sewage and water supply installation in Variant 2 INVDUL2	350 €
The cost of installing additional sewage and water supply installation in Variant 3 INVDUL3	170 €
Energy unit cost ce	0.26 €/m3
Discount rate r	6%
Annual increase in energy prices ie	2%
Total lifetime T	15 years

). The study did not account for the costs of installing the entire water and sewage system, as these would be identical across all variants. Instead, the focus was solely on the investment costs associated with implementing a wastewater heat recovery system.

Due to the uncertainty of some input data, LCC analysis should always include a sensitivity analysis [60]. Its main goal is to identify and assess the parameters and boundary conditions that most significantly affect the Life Cycle Costs of the analyzed investments, and thereby influence the decision-making process. In assessing the life cycle of buildings and their installation equipment, it is pivotal to examine changes in parameters such as the discount rate, construction costs, operating costs resulting from the consumption of a given utility, and unit prices, as well as the total lifetime [67,68,69]. Taking the aforementioned into account, this study conducted a sensitivity analysis based on two scenarios of changes in the main factors of the financial model of the analyzed investment:

• Scenario A: annual increase in operating costs resulting from energy consumption in the DHW system amounting to 4% (current inflation rate in Poland [70])—Scenario A_4% and 10% (according to EC, [71])—Scenario A_10%;
• Scenario B: discount rate r of 8% (Scenario B_8%) and 10% (Scenario B_10%).

2.5. Environmental Analysis—Greenhouse Gas Emissions

The environmental impact of implementing the studied DWHR exchangers was assessed based on pollutant emissions associated with energy consumption for domestic hot water preparation. Among these, CO₂ emissions are particularly detrimental to the environment and human health, accounting for up to 97% of all greenhouse gas emissions in the construction sector [72]. Accordingly, the study concentrated on determining carbon dioxide emissions, calculated using Equation (5). The specific CO₂ emissions indicator for Poland was adopted based on [73], and is 662 gCO₂/kWh.

$${\mathrm{E}}_{{\mathrm{C}\mathrm{O}}_2}=\mathrm{e}·{\mathrm{E}}_{\mathrm{D}\mathrm{H}\mathrm{W}}$$

(5)

where E_CO2 is the CO₂ emissions, g; and e is the carbon intensity, g/kWh.

3. Results

3.1. Assesment of the Effectiveness of the Drain Water Heat Recovery Exchangers

Laboratory tests for a wide range of variable input parameters allowed us to obtain thousands of temperature data points. The constant hot water temperature set on the heater was T_h = 55 °C ± 1 °C, while the cold water temperature T_c supplied to the system from the network ranged from 11.08 °C to 12.88 °C (horizontal DWHR exchanger) and from 11.34 °C to 14.88 °C (vertical DWHR exchanger). To evaluate the actual preheating capabilities of cold water in shower heat exchangers, the temperature difference between the cold water temperature T_c and the preheated water temperature T_pre was determined. The results of these analyses are presented in Figure 5. In the case of the vertical exchanger, an increase in cold water temperature of up to approximately 22 °C was observed (T_sh = 46 °C, q_sh = 5.0 L/min). Significantly smaller temperature increases were observed for the horizontal heat exchanger, where the largest increase was only 10.05 °C (T_sh = 46 °C, q_sh = 5.0 L/min).

Temperatures recorded at the test stand were used to determine the efficiency of heat recovery from wastewater for two different DWHR exchangers, as well as to calculate the annual energy consumption for household water heating, using Equations (1) and (2). The E_DHW parameter was determined for different installation variants (Variant 1, Variant 2, and Variant 3) and for different numbers of occupants M and shower durations l_sh. The results of these tests are shown in Figure 6, Figure 7 and Figure 8. The data presented in the figures clearly indicate that the vertical heat exchanger enables significantly greater heat recovery from wastewater discharged from the shower, thereby reducing the amount of energy required to heat the water to the desired temperature. Energy consumption in each of the variants considered is influenced by the water flow rate from the shower head q_sh, the water temperature T_sh, the number of occupants M, and the shower duration l_sh.

The conventional installation with an electric water heater (Variant 1) is characterized by the highest E_DHW energy consumption in each of the analyzed calculation cases. For a 5 min bath, the annual energy demand E_DHW in this variant ranges from 720.75 kWh (34 °C, 5.0 L/min) to 2097.48 kWh (46 °C, 10.0 L/min) (three people), from 961.05 kWh (34 °C, 5.0 L/min) to 2796.64 kWh (46 °C, 10.0 L/min) (four people), and from 1201.26 kWh (34 °C, 5.0 L/min) to 3495.80 kWh (46 °C, 10.0 L/min) (five people). Extending the bathing time l_sh to 10 and 15 min results in a two-fold (Figure 7) and three-fold (Figure 8) increase in the annual energy consumption required to produce domestic hot water. The highest E_DHW value of 10,487.40 kWh is obtained for the case when the installation is used by five people and the remaining calculation parameters are q_sh = 10.0 L/min, l_sh = 15 min and T_sh = 46 °C. Considering this one specific calculation case, the implementation of the wastewater heat recovery system causes a decrease in E_DHW to 4213.74 kWh (vertical DWHR exchanger) and to 8040.33 kWh (horizontal DWHR exchanger).

The determined energy demand for water heating was used to calculate the reduction factor φ from Equation (3), which simultaneously defines the energy efficiency of the tested heat exchangers. The test results in this regard are shown in

Table 6. Energy demand reduction factor.

Type of DWHR Exchanger	Temperature of Mixed Water Tsh, °C	Water Flow Rate from Shower Head qsh, L/min
		5.0	7.5	10.0
		Energy Demand Reduction Factor φ, %
Vertical	34	54.9	52.8	49.5
	38	60.7	57.2	51.4
	42	64.0	58.8	53.7
	46	64.7	58.5	57.4
Horizontal	34	25.1	23.4	21.7
	38	26.9	23.6	22.0
	42	27.1	24.4	22.8
	46	29.6	26.0	23.3

. Based on these results, it can be concluded that the higher the q_sh, the lower the energy demand reduction factor. This occurs because less energy is recovered from wastewater as it flows through the DWHR exchangers and, consequently, there is higher energy demand for domestic hot water production from the power grid. When q_sh = 5.0 L/min, the use of a wastewater heat recovery system allows for reductions from 54.9% (34 °C) to 64.7% (46 °C) and from 25.1% (34 °C) to 29.6% (46 °C) for vertical and horizontal DWHR exchangers, respectively. An increase in the water flow rate from the shower head to 10.0 L/min causes a several-percent decrease in wastewater heat recovery efficiency. This is particularly noticeable for the vertical DWHR exchanger, where the φ value decreases by 5.4% to 10.3% depending on the mixed water temperature. In the case of the horizontal heat exchanger, the effect of T_sh temperature on its energy efficiency is lower. The energy demand reduction factor decreases by 3.4% to 6.3% with an increasing water flow rate.

3.2. Life Cycle Cost Analysis of Domestic Hot Water Preparation System

Laboratory test results and the estimated energy demand for the considered domestic hot water system variants were used to conduct a financial LCC analysis. LCC values were calculated for the data described in Section 2.4, and are shown in

Table 7. Results of Life Cycle Cost analysis for l_sh = 5 min.

Number of Residents	Variant of DHW Installation	Mixed Water Temperature Tsh. °C	Water Flow Rate from Shower Head qsh. L/min
			5.0	7.5	10.0
			LCC. €
3	Variant 1	34	2075.8	3017.3	4004.4
		38	2445.5	3576.9	4724.9
		42	2804.0	4079.8	5429.5
		46	3177.6	4612.7	6040.7
	Variant 2	34	2072.0	2519.7	2915.3
		38	2096.8	2639.5	3181.6
		42	2155.5	2806.5	3454.2
		46	2260.2	2953.1	3578.1
	Variant 3	34	2062.9	2818.1	3588.1
		38	2296.1	3240.2	4151.0
		42	2553.3	3590.1	4617.5
		46	2745.4	3923.0	5061.3
4	Variant 1	34	2767.7	4023.0	5339.2
		38	3260.7	4769.2	6299.8
		42	3738.6	5439.7	7239.3
		46	4236.9	6150.3	8054.3
	Variant 2	34	2379.0	2975.9	3503.4
		38	2412.0	3135.6	3858.5
		42	2490.3	3358.3	4222.0
		46	2630.0	3553.8	4387.1
	Variant 3	34	2581.2	3588.1	4687.3
		38	2892.1	4151.0	5420.7
		42	3235.1	4617.5	6093.2
		46	3491.2	5061.3	6683.0
5	Variant 1	34	3459.6	5028.8	6674.0
		38	4075.9	5961.5	7874.8
		42	4673.3	6799.7	9049.1
		46	5296.1	7687.9	10,067.9
	Variant 2	34	2686.0	3432.1	4091.5
		38	2727.3	3631.8	4535.3
		42	2825.1	3910.1	4989.7
		46	2999.7	4154.6	5196.2
	Variant 3	34	3099.5	4358.2	5732.1
		38	3488.1	5061.7	6648.9
		42	3916.8	5644.9	7489.5
		46	4237.0	6199.6	8226.7

,

Table 8. Results of Life Cycle Cost analysis for l_sh = 10 min.

Number of Residents	Variant of DHW Installation	Mixed Water Temperature Tsh. °C	Water Flow Rate from Shower Head qsh. L/min
			5.0	7.5	10.0
			LCC. €
3	Variant 1	34	4151.5	6034.6	8008.8
		38	4891.0	7153.8	9449.7
		42	5608.0	8159.6	10,858.9
		46	6355.3	9225.4	12,081.5
	Variant 2	34	2992.9	3888.4	4679.7
		38	3042.5	4127.9	5212.2
		42	3160.0	4462.0	5757.4
		46	3369.5	4755.3	6005.2
	Variant 3	34	3617.8	5128.2	6776.9
		38	4084.1	5972.5	7877.1
		42	4598.6	6672.2	8885.8
		46	4982.8	7337.9	9770.4
4	Variant 1	34	5535.4	8046.1	10,678.4
		38	6521.4	9538.4	12,599.6
		42	7477.3	10,879.5	14,478.6
		46	8473.7	12,300.6	16,108.6
	Variant 2	34	3606.9	4800.8	5855.9
		38	3673.0	5120.2	6566.0
		42	3829.6	5565.6	7292.9
		46	4108.9	5956.7	7623.3
	Variant 3	34	4654.3	6668.3	8866.5
		38	5276.2	7793.9	10,333.4
		42	5962.2	8727.0	11,678.4
		46	6474.4	9614.6	12,857.9
5	Variant 1	34	6919.2	10,057.6	13,348.1
		38	8151.7	11,923.0	15,749.5
		42	9346.6	13,599.3	18,098.2
		46	10,592.1	15,375.7	20,135.8
	Variant 2	34	4220.9	5713.3	7032.1
		38	4303.5	6112.5	7919.7
		42	4499.3	6669.3	8828.4
		46	4848.4	7158.1	9241.4
	Variant 3	34	5690.9	8208.3	10,956.1
		38	6468.2	9615.4	12,789.8
		42	7325.7	10,781.7	14,471.0
		46	7965.9	11,891.2	15,945.4

and

Table 9. Results of Life Cycle Cost analysis for l_sh = 15 min.

Number of Residents	Variant of DHW Installation	Mixed Water Temperature Tsh. °C	Water Flow Rate from Shower Head qsh. L/min
			5.0	7.5	10.0
			LCC. €
3	Variant 1	34	6227.3	9051.8	12,013.2
		38	7336.5	10,730.7	14,174.6
		42	8411.9	12,239.4	16,288.4
		46	9532.9	13,838.1	18,122.2
	Variant 2	34	3913.9	5257.0	6444.0
		38	3988.3	5616.4	7242.8
		42	4164.5	6117.4	8060.7
		46	4478.7	6557.4	8432.3
	Variant 3	34	5172.6	7438.3	9911.3
		38	5872.2	8704.7	11,561.6
		42	6643.9	9754.3	13,074.7
		46	7220.1	10,752.9	14,401.7
4	Variant 1	34	8303.1	12,069.1	16,017.7
		38	9782.1	14,307.6	18,899.4
		42	11,215.9	16,319.2	21,717.9
		46	12,710.6	18,450.9	24,162.9
	Variant 2	34	4834.9	6625.7	8208.3
		38	4934.0	7104.8	9273.4
		42	5168.9	7772.9	10,363.9
		46	5587.9	8359.5	10,859.4
	Variant 3	34	6727.5	9748.4	13,045.8
		38	7660.2	11,436.9	15,246.2
		42	8689.2	12,836.5	17,263.6
		46	9457.5	14,167.9	19,032.9
5	Variant 1	34	10,378.8	15,086.4	20,022.1
		38	12,227.6	17,884.4	23,624.3
		42	14,019.9	20,399.0	27,147.3
		46	15,888.2	23,063.6	30,203.7
	Variant 2	34	5755.9	7994.4	9972.6
		38	5879.8	8593.3	11,304.0
		42	6173.4	9428.4	12,667.1
		46	6697.1	10,161.7	13,286.6
	Variant 3	34	8282.4	12,058.5	16,180.2
		38	9448.3	14,169.2	18,930.7
		42	10,734.5	15,918.6	21,452.5
		46	11,694.9	17,582.8	23,664.1

. The total investment costs were influenced by several factors, including water temperature, shower duration, number of users, and mixed water flow rate. The results demonstrate that implementing a drain water heat recovery system in a single-family home brings significant financial benefits (Variant 2 and Variant 3). Regardless of the variable input parameters of the calculation model, the variants using drain water heat recovery exchangers prove to be financially more advantageous than the traditional installation variant, despite the additional investment cost. Comparing Variant 2 and Variant 3, in only one case is the horizontal DWHR more advantageous than the vertical heat exchanger (34 °C, three people, 5.0 L/min, 5 min). For these data, the LCCs of Variant 3 are only EUR 12.9 lower than those of Variant 1. It should therefore be concluded that the financially optimal solution for domestic hot water production in a single-family building is an installation integrated with a vertical DWHR exchanger.

An analysis of the percentage distribution of components in LCCs (Figure 9) shows that the investment outlays required to implement the heat recovery system represent a significant share of the 15-year total investment costs only in the case of Variant 2 with the shortest bathing duration (l_sh = 5 min). As water consumption increases due to a larger number of users and longer bathing duration, the proportions of investment outlays relative to operating costs decrease. The installation of a vertical DWHR heat exchanger requires approximately twice the investment of a horizontal shower heat exchanger, leading to significant differences in the distribution of LCC components between Variant 2 and Variant 3. For example, in the case of a 5 min shower and three residents (Figure 9a), INV accounts for 32% (46 °C, q_sh = 10.0 L/min) to 56% (34 °C, q_sh = 5.0 L/min), and from 10% to 25%, for Variant 2 and Variant 3, respectively. Under the same conditions, but with a 3-times-longer shower duration (Figure 9c), the share of INV in the total LCCs decreases to 14–29% (Variant 2) and 4–10% (Variant 3).

Based on the calculated annual cash flows resulting from the operating costs of purchasing energy for heating water in the analyzed single-family building, a discounted payback period (DPP) was determined for variants incorporating a wastewater heat recovery system (

Table 10. Discounted payback period (selected calculation cases).

Parameters	Variant	Mixed Water Temperature Tsh, °C	DPP, Years	Parameters	Variant	Mixed Water Temperature Tsh, °C	DPP, Years
qsh = 5.0 L/min, M = 3 people, lsh = 5 min	Variant 3	34	14.50	qsh = 5.0 L/min, M = 3 people, lsh = 15 min	Variant 3	34	3.99
		38	10.75			38	3.11
		42	9.03			42	2.68
		46	7.02			46	2.14
	Variant 2	34	14.93		Variant 2	34	4.09
		38	10.66			38	3.09
		42	8.56			42	2.55
		46	7.27			46	2.20
qsh = 5.0 L/min, M = 5 people, lsh = 5 min	Variant 3	34	7.71	qsh = 5.0 L/min, M = 5 people, lsh = 15 min	Variant 3	34	2.32
		38	5.90			38	1.82
		42	5.03			42	1.57
		46	3.98			46	1.26
	Variant 2	34	7.90		Variant 2	34	2.38
		38	5.86			38	1.81
		42	4.79			42	1.50
		46	4.11			46	1.30
qsh = 10.0 L/min, M = 3 people, lsh = 5 min	Variant 3	34	7.16	qsh = 10.0 L/min, M = 3 people, lsh = 15 min	Variant 3	34	2.32
		38	5.99			38	1.92
		42	4.80			42	1.60
		46	4.21			46	1.41
	Variant 2	34	6.63		Variant 2	34	2.02
		38	5.39			38	1.68
		42	4.58			42	1.44
		46	3.91			46	1.24
qsh = 10.0 L/min, M = 5 people, lsh = 5 min	Variant 3	34	4.33	qsh = 10.0 L/min, M = 5 people, lsh = 15 min	Variant 3	34	1.37
		38	3.57			38	1.13
		42	2.96			42	0.95
		46	2.59			46	0.83
	Variant 2	34	3.77		Variant 2	34	1.20
		38	3.10			38	0.99
		42	2.65			42	0.85
		46	2.27			46	0.74

). The longest payback periods were observed in cases with lower water consumption for bathing, as the annual energy savings were small relative to the financial outlay. It was also noted that although the investment cost (INV) for Variant 2 was more than twice as high as for Variant 3, the heat recovery efficiency of the horizontal DWHR exchanger was only half that of the vertical DWHR exchanger, resulting in negligible differences in DPP between the two variants. Selected calculations are presented in Table 9. In no case did the DPP exceed the 15-year period assumed in the LCC analysis. As the energy demand for domestic hot water production increased, the payback period shortened, reaching its minimum under the maximum input parameters, i.e., q_sh = 10.0 L/min, M = five people, l_sh = 15 min. For these conditions, the DPP was approximately one year.

In accordance with the scenarios presented in Section 2.4, the study was extended to include a sensitivity analysis of the investment variants. Owing to the wide range of input variables in the financial model, the results of the sensitivity analysis are presented for selected calculation cases. As shown in Figure 10 (q_sh = 5.0 L/min, l_sh = 5 min, 3 people), a 4% change in the annual energy price increase (Scenario A_4%) altered the investment profitability hierarchy compared to the baseline scenario, in which Variant 3 was the most advantageous solution at a mixed water temperature of 34 °C. In Scenario A_4%, the most cost-effective option for domestic hot water production was an installation integrated with a vertical DWHR exchanger (Variant 2). A similar change occurred in Scenario A_10%, although the differences in the total LCCs among the variants were larger. A 4% annual increase in energy prices raised LCCs by 13%, 11–12%, and 17–19% for Variant 1, Variant 2, and Variant 3, respectively. In contrast, a change in scenario A_10% increased the total operating costs of the analyzed variants by 45% (Variant 1), 27–29% (Variant 2), and 38–40% (Variant 3).

In all other calculation cases of the financial model, the optimal variant was already in the baseline scenario, Variant 2; therefore, changes in the rate of energy price increase only affected the value of the total LCCs incurred over the 15-year period. The results of the scenario analysis for the highest water flow rate from the shower head are presented in Figure 11. The increase in energy prices in accordance with Scenario A_4% resulted in a 13% increase in the LCC indicator in Variant 1, 9–10% in Variant 2, and 12–13% in Variant 3. Comparing the LCCs of the DHW installation variants and taking into account Scenario A_10% with the costs of the basic scenario, it was observed that there was an increase in the LCC indicator by 45%, 37–38%, and 43–44% for Variant 1, Variant 2, and Variant 3, respectively.

The sensitivity analysis also examined the impact of changes in the discount rate r. In the baseline version of the study, the current discount rate of 6% was assumed when calculating the LCC index. Due to the long-term nature of the investment, the total LCCs of the analyzed domestic hot water preparation variants were also determined for r = 8% (Scenario B_8%) and r = 10% (Scenario B_10%). The results for the case of q_sh = 5.0 L/min, l_sh = 15 min, and M = three users are shown in Figure 12. Increasing the discount rate to 8% and 10% reduced the 15-year operating costs and thereby lowered the LCC index for all variants. It also influenced the share of individual cost components (INV and OMC) in the total Life Cycle Costs of the installation variants. A higher discount rate altered the profitability hierarchy of the analyzed variants, but only for a mixed-water temperature of T_sh = 34 °C. At this temperature, in both Scenario B, the optimal solution was an installation without shower heat exchangers (Variant 1). For all other T_sh temperature values, as in the basic scenario, the most financially advantageous solution was the implementation of a vertical DWHR exchanger (Variant 2). An increase in hot water consumption and thus the energy demand for heating (Figure 13) did not change the profitability hierarchy of individual installation configurations, and Variant 2 remained the optimal option—regardless of the mixed-water temperature.

3.3. Impact of DHW System Variants on the Environment

The results of laboratory studies and the calculated energy consumption for various variants of domestic hot water preparation formed the basis for evaluating the environmental impact of the wastewater heat recovery system. Annual CO₂ emissions associated with the analyzed DHW options are shown in Figure 14. The differences in E_CO2 values arise from both the amount of water consumed during bathing and the heat recovery efficiency of the DWHR exchangers. As expected, greater hot water demand results in higher carbon dioxide emissions.

For a short 5 min shower, the application of a horizontal DWHR allows for an annual reduction in the amount of CO₂ emitted to the atmosphere from 119.7 kg to 341.9 kg (three residents), from 159.6 kg to 432.0 kg (four residents), and from 199.6 kg to 540.0 kg (five residents). A substantially greater reduction in CO₂ emissions is observed with Variant 2, where the DHW system is equipped with a vertical wastewater heat recovery exchanger. This DHW system configuration reduces harmful CO₂ emissions by 265.4–830.6 kg (three residents), 353.9–1107.5 kg (four residents), and 442.4–1384.4 kg (five residents).

Extending the shower duration l_sh to 15 min further increases the energy demand for water heating, which in turn amplifies greenhouse gas emissions. Under these conditions, heat recovery via the vertical DWHR exchanger allows for annual reductions of 796.3–2492.0 kg (three residents), 1061.8–3322.5 kg (four residents), and 1327.2–4153.1 kg (five residents). By contrast, systems equipped with a horizontal exchanger (Variant 3) achieve significantly lower reductions. For households of three residents, the horizontal heat exchanger reduces CO₂ emissions by 359.2 kg (34 °C, 5.0 L/min)–971.9 kg (46 °C, 10.0 L/min). For larger households, the reduction in carbon dioxide emissions is 478.9–1296.0 kg (four residents) and 598.6–1620.0 kg (five residents).

4. Discussion

The research presented in this article has demonstrated that numerous parameters influence the performance of wastewater heat recovery system in showers, affecting energy efficiency, financial viability, and environmental outcomes. A key determinant of the overall effectiveness is the selection of the appropriate type of drain water heat exchanger. This was confirmed by experimental studies conducted on two different DWHR unit designs: a vertical and a horizontal shower heat exchanger. The horizontal DWHR exchanger, developed according to the author’s design, reduced the energy demand for domestic hot water by 21.7% to 29.6%, whereas the commercially available vertical DWHR unit achieved a significantly higher reduction of 49.5% to 64.7%.

This reduction in energy demand was influenced by the flow rate of the shower head and the mixed water temperature. It was found that as the flow rate increased, the heat recovery efficiency decreased, while as the bath water temperature increased, the efficiency increased. These findings are consistent with results reported by other researchers [26,74,75]. In Piotrowska and Słyś’s study [26], a new horizontal heat exchanger demonstrated an average reduction of 22–31% in energy consumption for hot water preparation. In another study by Wong et al. [75], the authors showed that only 4–15% of shower water heat can be recovered through a 1.5 m long single-pass counter-flow horizontal heat exchanger.

To further improve the efficiency of horizontal heat exchangers and reduce energy demand in DHW systems, Pochwat et al. [76] used baffles. This improved the efficiency of the tested exchanger by 5.13 to 5.15 percentage points. Likewise, Kordana-Obuch and Starzec [77] reported that properly designed baffles could increase energy recovery efficiency by up to 40% compared with exchangers without such modifications.

Vertical DWHR exchangers, which represent the most commonly applied solution for wastewater heat recovery, are characterized by significantly higher efficiency levels. According to the literature, their efficiency can reach up to 70% [78]. Comparing the energy demand reduction achieved in this study (49.5–64.7%) to the results of other researchers, it can be concluded that the tested exchanger is an effective device for recovering heat from greywater. For instance, Wallin and Claesson [79] demonstrated that a vertical inline copper coil DWHR exchanger enabled approximately 25% heat recovery from wastewater. Zoloum et al. [80] pointed out that the use of the vertical copper DWHR unit can reduce natural gas usage for domestic hot water preparation by 9–27%. Despite their higher efficiency, vertical wastewater heat exchangers have one drawback in terms of required installation space. Specifically, they require a minimum of 1 m of vertical space under the shower tray, which eliminates them as a potential additional energy source in many existing buildings, especially multi-story buildings.

The recovery of energy from wastewater generates tangible financial benefits through the reduction of electricity or gas consumption for water heating. The financial profitability of implementing a heat recovery system can be determined using various methods [35,46]. In the study, the financial analysis was carried out according to the Life Cycle Cost methodology, which considers both investment expenditures and operating costs incurred over a longer period. The results of this analysis showed that despite the additional costs associated with implementing a DWHR exchanger, the traditional water heating method was the least cost-effective option in all calculation scenarios. The LCC value was primarily determined by water consumption, which depends on the number of residents, the flow rate from the shower head, and the duration of bathing. Similar findings have been reported in other studies [20,26]. Based on the results, it can be concluded that a heat recovery system is financially viable, especially in cases of high water demand for bathing purposes. In such situations, the investment is also more stable and less susceptible to changes in financial parameters over the long term, as demonstrated by sensitivity analysis. This conclusion is also emphasized by Pochwat et al. [27]. Moreover, increasing water consumption for bathing purposes significantly reduces the investment payback period.

Heat recovery from wastewater not only provides financial benefits, but also contributes to a reduction in greenhouse gas emissions. The research results presented in this article show that shower drain water heat recovery exchangers will not achieve carbon neutrality in buildings, but they can be an important technology supporting this process. Implementing a single exchanger in a small building may seem insignificant for the environment, but if we consider the possibility of using DWHR units in buildings characterized by higher water consumption per bath (multi-family buildings, hotels, hospitals, swimming pools, etc.), the potential for CO₂ emissions reduction is enormous.

Vertical exchangers, owing to their higher efficiency in recovering heat from wastewater discharged into the sewer system, offer relevant reductions in emissions of harmful substances, including greenhouse gases. Kordana-Obuch et al. [41] emphasized that one of the most important parameters influencing CO₂ emissions reduction during shower heat exchanger use is the intensity of carbon dioxide emissions. Consequently, the greatest environmental benefits from implementing a wastewater heat recovery system can be achieved in regions where electricity is primarily generated from coal, such as Poland.

This study further shows that total daily shower length (determined by the number of people and the duration of individual showers) is a key factor in determining CO₂ emissions. Over a 15-year period, the tested DWHR units reduced emissions by up to 24,307.5 kg (horizontal DWHR unit) and 62,296.5 kg (vertical DWHR unit). These values highlight the potential of wastewater heat recovery systems to support the decarbonization of the building sector. Additionally, by reducing energy consumption for domestic hot water preparation, DWHR implementation lowers overall pollutant emissions. This aligns with the United Nations Sustainable Development Goals (SDGs), particularly SDG7—Affordable and Clean Energy [6].

Limitations of the Study and Future Research

The results demonstrate that both horizontal and vertical DWHR systems are financially viable and environmentally beneficial. Laboratory tests were conducted for a wide range of variable physical parameters characterizing the shower wastewater heat recovery system. The ambient temperature, which can affect the heat transfer process, was not taken into account. Therefore, this will be implemented as the next stage of the research. Attention should also be paid to weather conditions, which can also influence test results. This is particularly true for outdoor temperatures, which can determine the temperature of cold water supplied from the water supply network. Laboratory tests were conducted only during the winter, so we plan to expand them and examine the efficiency of DWHR exchangers during other seasons. This will allow us to determine the impact of seasonal variations on the operation of wastewater heat recovery systems.

The laboratory test results obtained will form the basis for creating a numerical CFD model of the horizontal DWHR unit and conducting simulations for a wide range of variable technical and physical parameters. These studies will allow for further optimization of the solution from a design perspective. Expanding the research to include these tasks will enable a full evaluation of the new heat exchanger solution and improve its energy efficiency, thus increasing its implementation potential.

The methodology applied for the financial and environmental analysis of several domestic hot water system variants enabled the assessment of multiple factors affecting both the Life Cycle Cost (LCC) and the carbon dioxide emissions associated with energy consumption. However, this study considered only the operational phase of the tested DHW system configurations, excluding the production and decommissioning phases of the DWHR exchangers. It is important to note that the extraction of raw materials, their processing, and the subsequent manufacturing of system components from them are highly water- and energy-intensive, as well as emissions-intensive. Including all stages of the life cycle of the installation options under consideration could influence the final assessment of these options. With this in mind, future research will incorporate a comprehensive environmental analysis—a Life Cycle Assessment (LCA).

The final stage of the planned research will involve the analysis and optimization of a wastewater heat recovery system integrated with other renewable energy sources. Enhancing the overall energy efficiency of buildings remains a critical challenge, and as studies have shown, wastewater heat exchangers can be a valuable solution, though primarily as a complementary technology on the path to achieving low- and zero-emissions buildings.

5. Conclusions

Based on the research results, several main conclusions were drawn.

• The Life Cycle Cost methodology is an effective tool for selecting the financially optimal DHW installation option and reduces the risk of making incorrect decisions based solely on initial investment costs.
• Both vertical and horizontal DWHR exchangers offer lower 15-year Life Cycle Costs than conventional water heating systems. The vertical exchanger reduces energy consumption by up to 64.7%, while the horizontal exchanger reduces energy consumption by up to 29.6%.
• By significantly reducing CO₂ emissions, the tested shower heat exchangers can contribute to reducing the carbon footprint of buildings.
• The significant impact of parameters such as shower duration and shower head flow rate on the final financial and environmental results demonstrates how important individual user preferences and behaviors can be in reducing the negative environmental impact of buildings.

In conclusion, it is also important to highlight the practical aspects of the research. By demonstrating the financial and environmental benefits associated with wastewater heat recovery technology, the results of these studies may provide an impetus for potential investors, designers, owners, and building managers to implement such systems in various types of buildings. Furthermore, the presented research results may contribute to the wider adoption of wastewater heat exchangers, which, despite their numerous advantages, remain underutilized in many countries, given their potential to significantly reduce greenhouse gas emissions from the residential sector.