Next Article in Journal
Observations of the Fine Structural Changes Associated with Merogony and Gametogony in Eimeria necatrix and Localization of Two Gametocyte Proteins
Previous Article in Journal
Current Challenges in Yersinia Diagnosis and Treatment
Previous Article in Special Issue
Two Sporadic Cases of Legionellosis Associated with the Use of Domestic Ultrasonic Humidifiers
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Microorganisms
Volume 13
Issue 5
10.3390/microorganisms13051134
microorganisms-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Review

Legionella in Hot Water Heat Pump (HWHP) Systems

by
Jodi Brookes
1,*,†,
Helena Senior
1,†,
Rebecca J. Gosling
1,
Duncan Smith
2 and
Margaret Wade
3
1
Health and Safety Executive, Science and Research Centre, Harpur Hill, Buxton SK17 9JN, UK
2
Health and Safety Executive, Alnwick House, Benton Park View, Newcastle upon Tyne NE98 1YX, UK
3
Health and Safety Executive, Redgrave Court, Merton Road, Bootle L20 7HS, UK
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Microorganisms 2025, 13(5), 1134; https://doi.org/10.3390/microorganisms13051134
Submission received: 24 February 2025 / Revised: 25 March 2025 / Accepted: 30 March 2025 / Published: 15 May 2025
(This article belongs to the Special Issue Legionella pneumophila: A Microorganism with Thousand Faces, 2nd Edition)
Review Reports Versions Notes

Abstract

:
It is anticipated that by 2028 there will be a significant increase in the use of HWHP systems in Great Britain (GB). Such systems are considered a better, energy-efficient alternative to fossil fuel-based burners and furnaces, as they use electricity. There are concerns that these systems are susceptible to microbial contamination because they hold water at lower temperatures. In particular, the concern is regarding Legionella contamination, as it can potentially cause disease in the general public and those who are maintaining and replacing these systems. Therefore, this review was focused on understanding the potential risk posed by their increased use and maintenance requirements. This review was approached systematically but was not a full systematic review. There were 61 papers that were considered potentially relevant to the research questions. Of these, 40 papers were considered relevant to the topic of Legionella in HWHP and underwent full article assessment and data extraction. The remaining papers were considered useful for background information. The scope of this review established that Legionella are a known risk in hot water systems that can be carried over to HWHP systems, yet there is minimal evidence to suggest that the current control measures are being appropriately applied to reduce the risk of exposure. When considering countrywide legislation and guidance, it appears that the risk is considered lower in single- or multi-family homes that do not require a centralised system. This review included the assessment of information regarding the safety of working with HWHP systems with regards to maintenance and replacement. The authors found a lack of information regarding these safety concerns. This review is among the first to systematically evaluate the risks of Legionella contamination in HWHP systems.

1. Introduction

In Great Britain (GB), the roll out of some 600,000 hot water heat pump (HWHP) systems per year by 2028 has been deemed critical in order to decarbonise residential heating and hot water supplies [1]. The 2021 heating consensus reported that 74% of households in GB use gas-based heating and only a small percentage use other sources, such as electricity [2]. HWHP systems are considered a better alternative, using electricity instead of fossil fuel-based burners and furnaces. Therefore, they will reduce overall energy consumption in line with European Union (EU)-wide targets of full decarbonisation of the building sector by 2050 [3].
The development of HWHP systems is evolving and there are now multiple generations of systems commercially available for single/multiple-family households, district water supplies and commercial buildings. However, there are concerns that these systems are susceptible to microbial contamination because they can hold water at lower temperatures than a standard calorifier. When delivering hot water, these microorganisms may be introduced into facilities, e.g., in households via showers or taps [4]. There is particular concern that microorganisms such as Legionella may establish and potentially pose a public health risk to end-users. Notably, Legionella pneumophila exposure can cause infection if the bacteria are inhaled, e.g., in water droplets formed during shower use [5]. Legionnaires’ disease can be life-threatening without timely treatment and is a notifiable disease in many countries. Diagnostic/exposure testing has developed in recent years such that antigens can now be tested in urine samples [6]. There are many ways an individual can be exposed to the bacteria and acquire the disease [4]. Examples include exposure from cooling towers, hot tubs, swimming pools, water fountains, etc.
Legionella bacteria are generally found in natural water sources, especially in favourable conditions, such as warmer temperatures between 20 °C and 45 °C, stagnant water flow or the presence of free-living protozoa. Protozoa such as amoebae can provide intracellular protection and influence the structure and microbial composition of biofilms. Within a biofilm, Legionella can co-exist with other bacterial species that provide essential nutrients and improve conditions. Cyanobacteria and Flavobacterium breve have been found to enhance Legionella pneumophila growth [7]. An established biofilm can persist and protect Legionella, enhancing survival against physical and chemical stressors. In the presence of an established biofilm, Legionella can be released continually into the water [8]. Legionella can also survive water disinfection procedures [9]. Therefore, they can potentially inhabit potable water in very low numbers. If introduced into a HWHP system, when conditions are appropriate, there is the opportunity for Legionella to proliferate.
Despite evidence that Legionella are a contaminant found in some hot water systems, it is unclear whether they are likely to pose significant risks to the health of workers and the wider general population if there is wider utilisation of HWHP systems. Furthermore, there is little information with regards to Legionella risk to those who work with them. If a HWHP system requires maintenance, replacement of parts or replacement of the whole system, there could be a risk of aerosols being generated in the process and the potential for exposure. It is important to understand the risks posed by the use and maintenance of HWHP systems and, therefore, a rapid evidence review was conducted in order to determine the level of risk with increased instalment of these systems. This review is among the first to systematically evaluate the risks of Legionella contamination in HWHP systems.

Types of HWHP and Risk of Legionella Contamination

HWHP systems can provide buildings with space heating and cooling in addition to the delivery of domestic hot water [10]. They may generate the heat from different sources, such as from air, water or ground. This heat is then amplified by the heat pump system and supplied into the heat distribution system [10]. Air-to-water or ground-to-water HWHP systems need to be able to store water ready for when it is needed. A hot water tank allows the HWHP systems to gradually heat water, with the tank storing it. While most HWHP systems provide water at 55 °C, hot water will periodically need to reach 60 °C to prevent Legionella proliferation. Research has shown that almost 90% of Legionella bacteria will be inactivated when held at 60 °C [11]. Where water contains 100,000 cfu/mL of Legionella or more, the water must be held at 60 °C for 10 min to reduce the bacterial levels below 100 cfu/mL. However, if even small numbers of viable bacteria remain in the system, there is always the chance of recolonisation if the temperature decreases to a more optimal value for the bacteria [12,13].
Some HWHP systems can deliver this temperature; however, most systems also have an electrical immersion heater inside the water storage tank to raise the water temperature periodically when required. Regardless of the heat source, the key concern is that without the correct maintenance and operation of the system, this may provide optimal conditions for the introduction and persistence of microorganisms such as Legionella.
The scope of this review encompasses all types of HWHP systems. If Legionella are a risk in a HWHP system due to running at lower temperatures that would allow them to flourish, appropriate controls are required to ensure mitigation of Legionella growth. For any water heating system, the potential risk of contamination will depend on the following:
The physical design of the system, and whether there is any opportunity for water to remain stagnant in the pipework or a tank for long periods of time, i.e., in the tank or from “dead legs”.
Whether mechanisms are in place to challenge growth such as the use of biocidal treatment, for example, which will prevent or minimise the opportunity for bacteria to grow. The most commonly used mechanism to control Legionella growth in such systems is temperature. For example, boosting temperatures regularly, by raising the temperature usually >60 °C for a time-period, which is a technique referred to as thermal disinfection;
Whether the system is used to produce hot water to which individuals are likely to be directly exposed. This includes those who may maintain the system as well as use it for hot water. For example, is the heated water being used from taps and showers, or only being used to feed a closed central heating system?

2. Methods

2.1. Search Terms and Search Strategy

A rapid evidence review was conducted in relation to Legionella and HWHP systems. The review was approached systematically but was not a full systematic review. Search terms specific to the research questions and the scope of the review were determined. Relevant peer-reviewed papers and research reports were identified using search engines and databases, e.g., Web of Science, PubMed and Google Scholar. The search terms were organised in a hierarchy of topics, sub-topics, synonyms and acronyms. Searches were based on the proximity of key terms irrespective of their order based on Boolean operators. The key terms focused on Legionella and HWHP systems. The searches were expanded for a variety of HWHP types and included all domestic and commercial set-ups. Terms included control measures for Legionella risk as well as any scope of maintenance of the systems. Government guidance and legislation were also added into the search and data extraction to consider what is being circulated in regards to the safety and maintenance of HWHP systems and control of Legionella contamination.

2.2. Data Extraction and Evidence Summary

To ensure consistency, the evidence from each qualifying study was collated using a data extraction table. An assessment of the design and quality of each study was based on criteria developed for scoping evidence reviews [14]. Evidence was considered in relation to each research question and summarised in terms of its strength, quality and reproducibility. Scores from 1–5 were given for study relevance and applied, 5 being the most relevant, and 1 being the least. Emphasis was made on establishing a link between HWHP systems and potential Legionella contamination or exposure.

2.3. Relevant Questions

What potentially harmful microorganisms (particularly Legionella) have been detected in HWHP systems?
Is there any evidence of respiratory disease because of exposure to microorganisms from HWHP systems?
What control measures are currently in place to prevent exposure to harmful microorganisms from HWHP systems to workers and the public?

3. Results

There were 203 papers of potential relevance that were identified through the initial searches. After examining the abstracts and applying inclusion and exclusion criteria, 61 were considered to be potentially relevant to the research questions. Of these, 18 were scored as 5; considered specifically relevant to the topic of Legionella in HWHP systems (Summarised in Table 1). A further 22 papers were scored as 4, considered to be relevant to the topic as they made reference to HWHP safety or Legionella in household water systems. The remaining papers were considered useful for background information. Those used have been referenced; citations [15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35] formed part of the review but not directly referenced [15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35].

3.1. Presence of Legionella and Risk of Exposure

Many papers acknowledged and considered the requirement of controlling Legionella in water heating systems, including those with HWHP systems, but none looked at actual colonisation. Under closer scrutiny, few of these studies experimentally tested for Legionella. Most (n = 12) of the highest-relevance papers were single experimental studies examining the storage of water. One study by Mathys et al. (2008) tested hot water samples from family residences for the occurrence of Legionella. They compared instantaneous hot water heaters to houses that used storage tanks and recirculating hot water [41]. That study showed that there was an increased prevalence (12%) of Legionella in homes that were supplied with storage tanks or recirculated hot water. Legionella pneumophila accounted for 93.9% of Legionella positive samples. A more recent study by Moodley et al. (2023) reported the presence of Acanthamoeba, Legionella pneumophila, Pseudomonas aeruginosa and non-tuberculous mycobacteria in energy-efficient hot water systems [43]. That study compared samples from winter temperatures (20 °C to 30 °C) to summer temperatures (40 °C to 55 °C). Legionella pneumophila were detected in both winter and summer months.
If Legionella have been detected in water heating systems, it is reasonable to assume that that there is a potential risk for exposure. However, no research in the scope of this review has demonstrated a direct link between water from HWHP systems and any disease related to Legionella directly, and this would be difficult to achieve.

3.2. Mitigation of Legionella Growth

The main measure to control the growth of Legionella in HWHP systems is to ensure maintenance of temperatures above habitable conditions for the bacteria. The required temperature is dependent on the size and design of the system. There are certain criteria that are generally referred to in order to determine the minimum temperature required to prevent Legionella growth. This is reflected in guidance and legislation for protecting health.
According to SIA (2011), the risk of Legionella is categorised based on building size and purpose [44]. The temperature of the hot water is considered to be directly related to the presence of Legionella. Single-family dwellings and multi-family dwellings without a centralised hot water supply are considered low-risk. Multi-family buildings with a centralised hot water supply are considered medium-risk. Finally larger buildings such as hospitals are considered high-risk. These risk categories are based on whether there is opportunity for heat loss in piping, how much domestic hot water is required, how long the water is stagnant in the system and whether the temperature can be reliably maintained throughout the system via heat tracing. The design of a system must maintain at least 60 °C if there is a storage tank, and circulating water in pipes must maintain ≥55 °C. At the point of use, i.e., at the tap, the water must be ≥50 °C. There are also specifications with regards to maintaining these systems, such as periodical cleaning of tanks for limescale or visible biofilms/build-up. Swiss standards also refer to a thermal shock treatment by increasing the temperature to >70 °C.
France, Austria and Germany refer to the “3 Liter rule”, whereby the minimum required temperature increases if the tubing/piping length delivering hot water to the point of use exceeds 3 L of water. If so, the minimum required temperature should be ≥50 °C for France and Germany, and ≥55 °C for Austrian standards. In larger systems, these standards also refer to the capacity of a storage tank. For example, in France, if the storage tank exceeds 400 L capacity, the temperature must be kept at >55 °C [11]. By the German standard, if the storage tank exceeds 400 L, it must be kept above 60 °C. Annual testing for Legionella is also considered a must in both Austria and Germany. According to Austrian Standards Institute (ASI) (2023), if the tested Legionella concentration exceeds 100 cfu/mL, restorative action is required [45]. This may include reassessing the system structure, such as replacement of parts, removing “dead legs” or converting to a decentralised domestic hot water system. There are also recommendations for disinfection that include a thermal disinfection at >70 °C. Other options are noted, such as chemical or ultraviolet (UV) disinfection, but thermal disinfection is preferred.
The Japanese technical guidelines from the Ministry of Health, Labour and Welfare identifies measures to reduce the presence of Legionella [46]. Typically, water heating is from systems that require tanks that are heated by electricity [11]. Inside the tank, the water must be maintained at 60 °C, and by the time the water reaches the tap it must be 55 °C. There are also stipulations that there must be a “draining valve” present to prevent stagnation and allow for cleaning. In addition, a “flow valve” is a requirement to ensure constant hot water. The tank must be cleaned annually with regular drainage and ensured testing and adjustment of the valves.
In comparison to the reviewed journal documents that refer to thermal inactivation (n = 9), the most common temperature range reported to control Legionella contamination was between 50 °C and 60 °C. Two documents refer to the use of a “boost”, where the temperature is increased to 60 °C on a daily or weekly basis for the thermal inactivation of Legionella bacteria. The minimum and maximum required temperatures of tank storage and circulating hot water vary. Kleefkens (2020) published a thorough review of Legionella contamination in HWHP systems, collating this information and reporting a “lack of harmonisation” between countrywide legislation and guidance [11].

3.3. Legionella Mitigation and Economic Concerns

Of the reviewed documents, in areas where HWHP systems are used more widely, some papers (n = 8) considered the energy efficiency and energy savings of these systems once in place. The higher temperature requirements to reduce the risk of Legionella in the system have posed concerns with regards to future energy costs. It was commonly stated that having to increase the temperature to the 60 °C benchmark would increase the overall energy demand and therefore overall costs. Therefore, alternative methods were explored to determine whether this temperature could be reduced while retaining the safety provided by reducing the risk of Legionella growth.
Stagnation removal, ensuring that any water storage was continually mixed to maintain the consistent elevated temperature, was highlighted (n = 2) as key in reducing the likelihood of Legionella contamination. When designing a new building, stagnation and other areas such as piping would be considered in the design. However, in areas such as Great Britain (GB), some of these systems are likely to be retrofitted onto an older, pre-existing system. This could allow for a higher chance of stagnation in pipework or “dead legs”.
Two papers reported that an effective method to reduce the likelihood of Legionella being introduced into the system would be to ultrafilter the inlet water. With the addition of ultrafiltration, this also allowed for the system to be kept 5 °C lower, improving energy efficiency [38]. Knapp and Nordel (2017) refer to pasteurising the water prior to introduction with a Duck Foot Heat Exchange model, whereby the model imitates the counter-current heat exchange in the feet of ducks, which preheats the fluid prior to pasteurisation and cools it after. The authors reported minimal Legionella contamination two years after installation [39]. Legionella can take a considerable amount of time to establish in a system. It requires optimal conditions where a biofilm has formed inside. In order to demonstrate that the probability of contamination has been reduced, it is important to ensure periodic testing for Legionella over a longer period of time.
Kaschewski et al. (2020) studied the risk of Legionella and the possible methods of energy saving in domestic hot water systems, based on simulation models of a typical single-family home [37]. The results demonstrated that Legionella risk was mainly dependent on storage volume and tap water volume. Households that utilised 50 L per day and a 150 L storage tank size remained below 100 cfu/mL of Legionella. Simulations based on energy demand for the same system showed that this could be decreased by 62% by operating with “disinfection-on-demand” by boosting the temperature and replacing electric heating of the system with an immersion heater. Furthermore, the authors concluded that integrating a UV Light-Emitting Diode (UV-LED) reduced additional energy demand by 6% and could be an alternative option to thermal disinfection.
None of the reports or publications reviewed expressed or highlighted any concerns relating to the potential for exposure of people who work with or maintain these HWHP systems to Legionella. The risks posed to workers from potential exposure when removing a system that has been in place for several years, where it is more likely to have established a biofilm containing Legionella, are unknown. This would depend on the potential for aerosolization of water in the system during its removal or replacement of parts. This risk could be mitigated by draining the system and depressurising. Without this evidence, it is difficult to determine whether such work activities may pose a risk in the future.

4. Discussion

The review of forty sources demonstrated that Legionella are an expected risk of contamination in HWHP systems, particularly where there are issues maintaining temperature or levels of stagnation. Whilst there is an expected risk, little has been done to demonstrate experimentally that Legionella contamination is appropriately managed, and this may be difficult to achieve. When testing for Legionella, microbiological techniques to test for viable bacteria can be used. However, Legionella can be difficult to grow outside of their established biofilm and therefore they can be underrepresented in a given sample. Further molecular techniques such as quantitative Polymerase Chain Reaction (qPCR) can be used to identify and quantify the presence of Legionella DNA. However, if the bacterial cells are present but inactivated, this can over-report the levels present in a given sample. Therefore, specific techniques would be required in order to gain a representative quantification of Legionella present, which can require specialist testing.
It was difficult to establish evidence of disease directly related to Legionella exposure from HWHP systems. There were no sources that made a direct association between exposure from a water source related to a HWHP and Legionnaires’ disease or other Legionella-related conditions. However, some sources (n = 4) did refer to statistical data that demonstrated an increase in the incidence of Legionnaires’ disease over recent years. In an annual epidemiological report for Legionnaires’ disease, the European Centre for Disease Prevention and Control (2023) reported their highest annual notification rate of Legionnaires’ disease in the European Union, at 2.4 cases per 100,000 population in 2021 [4]. It cannot be ascertained whether the HWHP use has had any impact on this number, as there are other factors that may explain this increase, e.g., an increase in travel after the COVID-19 pandemic. Legionnaires’ disease can also be community-acquired and therefore this increases the difficulty in finding the source of exposure.
In order to establish the link between HWHP systems and Legionnaires’ disease, a water source would need to be positive for Legionella and a patient sample would also need to reflect this. Two of the sources from the review (Mathys et al., 2008; Moodley et al., 2023) experimentally analysed water samples for Legionella, although some of the relevant review sources expect Legionella to be a known contaminant of HWHP systems based on its presence in other hot-water-sourced systems [41,43]. Therefore, whilst there is a lack of evidence, HWHP systems cannot be discounted as a potential source of exposure.

4.1. Control Measures

Whilst there is a lack of consistency as to what the most appropriate minimum effective temperature is in controlling Legionella growth in HWHP systems, the general consensus is that thermal inactivation is key to controlling system contamination. The most commonly stated method is to maintain the temperature consistently at ≥50 °C, with the option to boost the temperature periodically to 60 °C. However, the recommended boosting temperature also varied between 60 °C and >70 °C. The required time to control this temperature varied from 15 min to 1 h. Emerging evidence suggests that temperature regulation may become problematic in the future due to heat shock resistance, whereby bacteria are able to survive sudden changes in temperature.
A study by Liang et al. (2023) tracked the evolution of heat shock resistance in Legionella pneumophila [25]. That study simulated the effects of periodically boosting the temperature against the expression of the heat shock protein present. Temperature treatments were carried out at 50 °C to 59 °C at repeated intervals for approximately 15 min. It was also shown that Legionella were found to survive in temperatures of 63 °C. Therefore, to ensure safety, instead of reducing the temperature for economic efficiency, this boosting temperature could require increasing for a longer time frame in the future. This could be as high as 75 °C to 80 °C to allow for bacterial cells to die off faster [11,47]. If small numbers remain in the system, there is the possibility of recolonisation [12,13].
Furthermore, if further evidence demonstrates that adaptation to temperature may occur after repeated heat cycles, it may be important to explore other control measures outside of thermal treatment. While these non-thermal control measures are considered less practical or more costly, they are discussed below.

4.2. Removal of Bacteria from Water Prior to Introduction into the System

Removing bacteria present in water before it is introduced into the system may be an effective alternative to temperature control measures. The main methods proposed for this were ultrafiltration and pasteurisation of the water. Ultrafiltration physically removes the bacterial cells from the water by allowing only water to pass through, capturing bacteria and other microorganisms on the membrane or barrier that can be removed [48]. The study by Genuardi et al. (2023) demonstrated that with a fitted ultrafiltration device, the hot water supply was continuously found to be free of Legionella [38]. In comparison, the cold water was not fitted with a device and Legionella were detected. However, ultrafiltration systems require their own maintenance and can be subject to fouling, with pores becoming clogged, reducing their efficiency [49,50]. There are newer technologies for anti-fouling and anti-aging of such membranes [50]; however, this could create further obstacles in terms of costs incurred by introducing additional steps whilst trying to solve the initial issue. Knapp and Nordel (2017) proposed pasteurising the water prior to introduction into the system with a Duck Foot Heat Exchange Model [39]. Whilst this system still uses thermal inactivation, by performing this step prior to introduction into a larger water system, this could effectively prevent Legionella contamination taking hold on a larger scale, where exposure could occur. The authors also reported that, with follow-up testing, after two years in circulation there was minimal Legionella contamination. It can take a considerable amount of time for Legionella to establish in a new system, so it is possible that follow-up tests may demonstrate a slow increase in Legionella presence over time. As Legionella proliferation requires optimal conditions where a biofilm has formed within the HWHP system, pre-treatment with ultrafiltration or pasteurisation may aid in reducing other bacteria that may generate a biofilm, therefore reducing the potential for Legionella to survive.

4.3. Ultraviolet (UV) Radiation Treatment

UV treatment has been alluded to in some of the reviewed documents as a possible alternative treatment, but no further analysis was carried out. This treatment method would work by delivering a dose of UV radiation, known to have bactericidal properties, to bacterial cells in the water. UV treatment has been widely researched in water treatment and food preparation [51,52]. However, there are a number of factors that could affect the efficiency of such treatment in HWHP systems, such as ensuring the delivery of the correct dose to efficiently inactivate bacterial cells. Newer technologies are emerging, and UV-LED technology is something that may be explored in more detail in the future. The previously mentioned article by Kaschewski et al. (2020) highlighted that there would be a potential benefit to integrating UV-LED technology in the HWHP system [37]. This was not demonstrated with a clear reduction in Legionella cells, but in an additional reduction in energy demand from the system. The authors suggested this technology could potentially replace thermal disinfection in some circumstances.
Conventional UV treatment is delivered from a mercury lamp that emits UV light at 254 nm and has bactericidal effects. Some challenges when using these are that they can be fragile and break, releasing mercury, which poses health and environmental risks. UV-LED is considered a “friendlier” method, as they are a mercury-free option and have also been considered in the application of the treatment of pathogens on food products [48,53]. In addition, they also possess an adjustable wavelength from 210 nm up to visible light, allowing for a tailored delivery of dosage [48]. As with standard UV treatment, there are factors to consider when integrating the technology with a HWHP system; so, whilst promising, there is not enough evidence in the scope of this review to date to suggest this technology is as effective as current methods. However, with increasing interest, more evidence may come to light.
There remains a lack of clarity as to whether these alternative control measures are likely to be effective at reducing the risk of Legionella contamination in HWHP systems in the future. This review was particularly interested in the potential risk of exposure to Legionella for those who maintain and replace HWHP systems. In the sources reviewed, there was no reference to worker exposure risk during maintenance activities.

5. Conclusions

The scope of this review established that Legionella presence is an expected risk in HWHP systems, yet there is minimal evidence to suggest that the current control measures are being appropriately applied in order to reduce the risk of exposure. Admittedly, this is difficult to achieve without thorough and targeted testing over prolonged periods of time. There has been an increase in cases of disease related to Legionella exposure, but it cannot be ascertained whether this is due to increased instalment of these systems without evidence associating the case(s) of disease with exposure directly from the HWHP system. When considering countrywide legislation and guidance, it appears that the risk is considered lower in single- or multi-family homes that do not require a centralised system. HWHP systems are evolving, and newer generations are being reported to not only reduce Legionella contamination but increase efficiency. Whilst thermal inactivation is currently the easiest and most sustainable control method, there are newer technologies that are incorporating potential alternatives. However, little is currently known about their efficacy. This review included the assessment of information regarding the safety of working with HWHP systems in relation to potential exposures to Legionella during maintenance and replacement. The authors found a lack of information in this area. Further research is required to address this knowledge gap.

Author Contributions

Conceptualization, D.S. and M.W.; Methodology, J.B. and H.S.; formal analysis, J.B. and H.S.; Writing—Original draft, J.B.; Writing—Review and Editing, J.B., H.S., R.J.G., D.S. and M.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

The contents of this paper, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.

Informed Consent Statement

Not applicable.

Data Availability Statement

No new data were created or analyzed in this study.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Barnes, J.; Taylor, M.; Silvonen, T. Domestic Heat Pumps: A Rapid Assessment of An Emerging Market; University of Oxford: Oxford, UK, 2023. [Google Scholar] [CrossRef]
  2. Office of National Statistics Census 2021. Available online: https://www.ons.gov.uk/peoplepopulationandcommunity/housing/articles/census2021howhomesareheatedinyourarea/2023-01-05 (accessed on 15 January 2024).
  3. Monteleone, W.; Ochs, F.; Dermentzis, G.; Breuss, S. Simulation-assisted design of a silent façade-integrated R290 mini-split heat pump. Appl. Therm. Eng. 2024, 243, 122520. [Google Scholar] [CrossRef]
  4. European Centre for Disease Prevention and Control. Legionnaires’ Disease—Annual Epidemiological Report for 2021; European Centre for Disease Prevention and Control: Stockholm, Sweden, 2023. [Google Scholar]
  5. Dufresne, S.; Locas, M.; Duchesne, A.; Restieri, C.; Ismaïl, J.; Lefebvre, B.; Labbé, A.; Dion, R.; Plante, M.; Laverdière, M. Sporadic Legionnaires’ disease: The role of domestic electric hot-water tanks. Epidemiol. Infect. 2011, 140, 172–181. [Google Scholar] [CrossRef]
  6. Viasus, D.; Gaia, V.; Manzur-Barbur, C.; Carratalà, J. Legionnaires’ Disease: Update on diagnosis and treatment. Infect. Dis. Ther. 2022, 11, 973–986. [Google Scholar] [CrossRef]
  7. Farhat, M.; Moletta-Denat, M.; Fr�re, J.; Onillon, S.; Trouilhï, M.; Robine, E. Effects of Disinfection on Legionella spp., Eukarya, and Biofilms in a Hot Water System. Appl. Environ. Microbiol. 2012, 78, 6850–6858. [Google Scholar] [CrossRef]
  8. Abdel-Nour, M.; Duncan, C.; Low, D.; Guyard, C. Biofilms: The Stronghold of Legionella pneumophila. Int. J. Mol. Sci. 2013, 14, 21660–21675. [Google Scholar] [CrossRef]
  9. Murga, R.; Forster, T.S.; Brown, E.; Pruckler, J.M.; Fields, B.S.; Donlan, R.M. Role of biofilms in the survival of Legionella pneumophila in a model potable-water system. Microbiology 2001, 147, 3121–3126. [Google Scholar] [CrossRef]
  10. Toleikyte, A.; Reina, J.; Volt, J.; Carlsson, J.; Lyons, L.; Gasparella, A.; Koolen, D.; De Felice, M.; Tarvydas, D.; Czako, V.; et al. The Heat Pump Wave: Opportunities and Challenges. In JRC Science for Policy Report; Publications Office of the European Union: Luxembourg, 2023. [Google Scholar] [CrossRef]
  11. Kleefkens, O. Legionella and Heat Pump Water Heaters. Sweden: Technology Collaboration Program on Heat Pumping Technologies; Heat Pump Centre: Borås, Sweden, 2020. [Google Scholar]
  12. Bédard, É.; Prévost, M.; Déziel, É. Pseudomonas aeruginosa in premise plumbing of large buildings. Microbiol. Open 2016, 5, 937–956. [Google Scholar] [CrossRef]
  13. Papagianeli, S.D.; Aspridou, Z.; Didos, S.; Chochlakis, D.; Psaroulaki, A.; Koutsoumanis, K. Dynamic modelling of Legionella pneumophila thermal inactivation in water. Water Res. 2021, 190, 116743. [Google Scholar] [CrossRef] [PubMed]
  14. Collins, A.M.; Coughlin, D.; Miller, J.; Kirk, S. The Production of Quick Scoping Reviews and Rapid Evidence Assessments: A How to Guide. 2015. Available online: https://assets.publishing.service.gov.uk/media/5a7f3a76ed915d74e33f5206/Production_of_quick_scoping_reviews_and_rapid_evidence_assessments.pdf (accessed on 15 January 2024).
  15. Boait, P.J.; Dixon, D.; Fan, D.; Stafford, A. Production efficiency of hot water for domestic use. Energy Build. 2012, 54, 160–168. [Google Scholar] [CrossRef]
  16. Baumann, C.; Huber, G.; Alavanja, J.; Preißinger, M.; Kepplinger, P. Experimental validation of a state-of-the-art model predictive control approach for demand side management with a hot water heat pump. Energy Build. 2023, 285, 112923. [Google Scholar] [CrossRef]
  17. Best, I. District Heating Systems for New Residential Areas: On the Impact of System Temperatures; University of Kassel: Kassel, Germany, 2022. [Google Scholar] [CrossRef]
  18. Elmegaard, B.; Ommen, T.S.; Markussen, M.; Iversen, J. Integration of space heating and hot water supply in low temperature district heating. Energy Build. 2015, 124, 255–264. [Google Scholar] [CrossRef]
  19. Hadengue, B.; Morgenroth, E.; Larsen, T.A. How to get your feet wet: Integrating urban water and building engineering for low-energy domestic hot water systems. Energy Build. 2022, 271, 112318. [Google Scholar] [CrossRef]
  20. Heinz, A.; Gritzer, F.; Thür, A. The effect of using a desuperheater in an air-to-water heat pump system supplying a multi-family building. J. Build. Eng. 2022, 49, 104002. [Google Scholar] [CrossRef]
  21. Han, X.Y. Effects of climate changes and road exposure on the rapidly rising legionellosis incidence rates in the United States. PLoS ONE 2021, 16, e0250364. [Google Scholar] [CrossRef]
  22. Kelly, N.J.; Tuohy, P.G.; Hawkes, A.D. Performance assessment of tariff-based air source heat pump load shifting in a UK detached dwelling featuring phase change-enhanced buffering. Appl. Therm. Eng. 2014, 71, 809–820. [Google Scholar] [CrossRef]
  23. Kim, H.; Lee, J.Y. Effects of a Groundwater Heat Pump on Thermophilic Bacteria Activity. Water 2019, 11, 12. [Google Scholar] [CrossRef]
  24. Kim, H.; Mok, J.; Park, Y.; Kaown, D.; Lee, K. Composition of Groundwater Bacterial Communities before and after Air Surging in a Groundwater Heat Pump System According to a Pyrosequencing Assay. Water 2017, 9, 891. [Google Scholar] [CrossRef]
  25. Liang, J.; Cameron, G.; Faucher, S.P. Development of heat-shock resistance in Legionella pneumophila modelled by experimental evolution. Appl. Environ. Microbiol. 2023, 89, e0066623. [Google Scholar] [CrossRef]
  26. Líu, H.; Zhang, H.; Javed, S. Long-Term performance measurement and analysis of a Small-Scale ground source heat pump system. Energies 2020, 13, 4527. [Google Scholar] [CrossRef]
  27. National Institute of Infectious Diseases (NIID); Tuberculosis and Infectious Diseases Control Division, Ministry of Health, Labour and Welfare (MHLW). Legionella (Legionellosis) and control measures in Japan. Infect. Agents Surveill. Rep. 2005, 26, 221–222. (In Japanese) [Google Scholar]
  28. Ottosson, K.; Karlsson, L. Overcoming Issues with Legionella in DHW in LTDH Systems; Lunds University: Lund, Sweden, 2018; Available online: https://lup.lub.lu.se/student-papers/record/8945437/file/8952668.pdf (accessed on 15 January 2024).
  29. Schang, C.; Schmidt, J.; Gao, L.; Bergmann, D.; McCormack, T.; Henry, R.; McCarthy, D. Rainwater for residential hot water supply: Managing microbial risks. Sci. Total Environ. 2021, 782, 146889. [Google Scholar] [CrossRef]
  30. Toffanin, R.; Curti, V.; Barbato, M. Impact of Legionella regulation on a 4th generation district heating substation energy use and cost: The case of a Swiss single-family household. Energy 2021, 228, 120473. [Google Scholar] [CrossRef]
  31. Williams, A.; Thomson, M. Significance of Insulation and Heat Pumps in Decarbonising the UK. J. Energy Power Technol. 2023, 5, 1–12. [Google Scholar] [CrossRef]
  32. Yang, X.; Li, H.; Svendsen, S. Evaluations of different domestic hot water preparing methods with ultra-low-temperature district heating. Energy 2016, 109, 248–259. [Google Scholar] [CrossRef]
  33. Yang, X.; Li, H.; Svendsen, S. Alternative solutions for inhibiting Legionella in domestic hot water systems based on low-temperature district heating. Build. Serv. Eng. Res. Technol. 2015, 37, 468–478. [Google Scholar] [CrossRef]
  34. Yang, X.; Li, H.; Svendsen, S. Energy, economy and exergy evaluations of the solutions for supplying domestic hot water from low-temperature district heating in Denmark. Energy Convers. Manag. 2016, 122, 142–152. [Google Scholar] [CrossRef]
  35. Yang, X.; Li, H.; Svendsen, S. Decentralized substations for low-temperature district heating with no Legionella risk, and low return temperatures. Energy 2016, 110, 65–74. [Google Scholar] [CrossRef]
  36. Arabkooshar, A. Non-uniform temperature district heating system with decentralized heat pumps and standalone storage tanks. Energy 2019, 170, 931–941. [Google Scholar] [CrossRef]
  37. Kaschewski, S.; Breuer, K.; Dünnebier, S.; Horstmeyer, S.; Breuer, N.; Horn, K.; Nürenberg, H.; Müller, M.; Dünnebier, D. Energy savings in hot water supply by legionella modelling. REHVA 2020, 6, 54–59. [Google Scholar]
  38. Genuardi, M.W. Installation of an ultrafiltration plant in a multi-family house to reduce the hot-water temperature and energy demand: A case study in Germany. J. Build. Eng. 2023, 66, 105898. [Google Scholar] [CrossRef]
  39. Knapp, S.; Nordell, B. Energy-efficient Legionella control that mimics nature and an open-source computational model to aid system design. Appl. Therm. Eng. 2017, 127, 370–377. [Google Scholar] [CrossRef]
  40. Lidberg, T.; Olofsson, T.; Ödlund, L. Impact of domestic hot water systems on district heating temperatures. Energies 2019, 12, 4694. [Google Scholar] [CrossRef]
  41. Mathys, W.; Stanke, J.; Harmuth, M.; Junge-Mathys, E. Occurrence of Legionella in hot water systems of single-family residences in suburbs of two German cities with special reference to solar and district heating. Int. J. Hyg. Environ. Health 2008, 211, 179–185. [Google Scholar] [CrossRef]
  42. Schmidt, D.; Kallert, A.; Blesl, M.; Svendsen, S.; Li, H.; Nord, N.; Sipilä, K. Low temperature district heating for future energy systems. Energy Procedia 2017, 116, 26–38. [Google Scholar] [CrossRef]
  43. Moodley, S.J.; Muchesa, P.; Bartie, C.; Barnard, T.G.; Clarke, R.; Masenge, A.; Venter, S.N. Prevalence of free-living acanthamoeba and its associated bacteria in energy efficient hot water systems in South Africa. Water SA 2023, 49, 1–7. [Google Scholar] [CrossRef]
  44. SIA 385/1:2011; Heating Systems in Buildings—Water Heating Systems Requirements. SIA (Swiss Society of Engineers and Architects): Zürich, Switzerland, 2011; pp. 1–28.
  45. ÖNORM B 1921:2023; Microbiological Requirements for Water Quality and Its Monitoring in Drinking Water Heating Systems. Austrian Standards Institute: Vienna, Austria, 2023.
  46. Tanimoto, T.; Takahashi, K.; Crump, A. Legionellosis in Japan: A self-inflicted wound? Intern. Med. 2021, 60, 173–180. [Google Scholar] [CrossRef] [PubMed]
  47. Lee, S. An Overview of the European Technical Guidelines for the Prevention, Control and Investigation of Infections Caused by Legionella species. Perspect. Public Health 2018, 138, 241–247. [Google Scholar] [CrossRef]
  48. Li, X.; Cai, M.; Lei, W.; Niu, F.; Yang, D.; Zhang, G. Evaluation survey of microbial disinfection methods in UV-LED water treatment systems. Sci. Total Environ. 2019, 659, 1415–1427. [Google Scholar] [CrossRef]
  49. Gao, W.; Liang, H.; Ma, J.; Han, M.; Chen, Z.; Han, Z. Membrane fouling control in ultrafiltration technology for drinking water production: A review. Desalination 2011, 272, 1–8. [Google Scholar] [CrossRef]
  50. Chang, H.; Zhu, Y.; Yu, H.; Qu, F.; Zhou, Z.; Li, X.; Yang, Y.; Tang, X.; Liang, H. Long-term operation of ultrafiltration membrane in full-scale drinking water treatment plants in China: Characteristics of membrane performance. Desalination 2022, 543, 116122. [Google Scholar] [CrossRef]
  51. Singh, H.; Bhardwaj, S.K.; Khatri, M.; Kim, K.; Bhardwaj, N. UVC radiation for food safety: An emerging technology for the microbial disinfection of food products. Chem. Eng. J. 2021, 417, 128084. [Google Scholar] [CrossRef]
  52. Li, H.; Osman, H.; Kang, C.W.; Ba, T. Numerical and experimental investigation of UV disinfection for water treatment. Appl. Therm. Eng. 2017, 111, 280–291. [Google Scholar] [CrossRef]
  53. Shin, M.; Na, G.; Kang, J.; Kang, D. Application of combined treatment of peracetic acid and ultraviolet-C for inactivating pathogens in water and on surface of apples. Int. J. Food Microbiol. 2024, 411, 110519. [Google Scholar] [CrossRef] [PubMed]
Table 1. Summary of relevant papers used in this review. The summarised table only includes peer-reviewed articles or reports; other sources from grey literature were discounted.
Table 1. Summary of relevant papers used in this review. The summarised table only includes peer-reviewed articles or reports; other sources from grey literature were discounted.
ReferenceGoal of StudyControl MeasuresGeographical LocationFuture TechnologyType of System Delivery
[36]Novel concept of non-uniform temperature district heating (NUTDH) system with decentralised heat pumps and standalone heat storage units (HPHS)Proposed system will increase temperatures in distribution to 70 °C during short periods of timeCentral EuropeYesDistrict Heating
[37]Energy savings in hot water supplies and Legionella modellingRemoval of stagnation, temperature control and integration of UV-LED technologyWestern EuropeYesMulti-Family Home/District Heating
[38]Ultrafiltration plant installation to reduce water temperature and energy demandAddition of ultrafiltration plant in hot water systemWestern EuropeYesMulti-Family Home
[39]Efficiency of Duck Foot Heat Exchange ModelPasteurisation systemUS/CanadaYesDomestic Hot Water
[40]Examination of Energy Refurbishment Packages (ERPs) to lower temperature needTemperatureSwedenNoMulti-Family Space Heating/District Hot Water/District Heating
[41]Testing of hot water systems in randomly selected residences between two citiesTemperatureGermanyNoSingle-Family Residences
[42]Low temperature district heatingNoWestern EuropeYesDistrict Heating
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Brookes, J.; Senior, H.; Gosling, R.J.; Smith, D.; Wade, M. Legionella in Hot Water Heat Pump (HWHP) Systems. Microorganisms 2025, 13, 1134. https://doi.org/10.3390/microorganisms13051134

AMA Style

Brookes J, Senior H, Gosling RJ, Smith D, Wade M. Legionella in Hot Water Heat Pump (HWHP) Systems. Microorganisms. 2025; 13(5):1134. https://doi.org/10.3390/microorganisms13051134

Chicago/Turabian Style

Brookes, Jodi, Helena Senior, Rebecca J. Gosling, Duncan Smith, and Margaret Wade. 2025. "Legionella in Hot Water Heat Pump (HWHP) Systems" Microorganisms 13, no. 5: 1134. https://doi.org/10.3390/microorganisms13051134

APA Style

Brookes, J., Senior, H., Gosling, R. J., Smith, D., & Wade, M. (2025). Legionella in Hot Water Heat Pump (HWHP) Systems. Microorganisms, 13(5), 1134. https://doi.org/10.3390/microorganisms13051134

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop