The conflicts between energy-saving and water hygiene

Ongoing drivers in environmental sustainability have led to pressure and increasing responsibility on building owners and managers to consider energy saving measures during day-to-day operation of their premises that can contribute toward both carbon zero targets, waste reduction, and demonstrable cost savings.  One area under close scrutiny is the efficient and effective management of hot water installations.

Hot water should be stored at a continuous temperature of 60 °C in the calorifier and with an accepted loss of 5 °C as it moves throughout the system, returning at a minimum of 55 °C to the calorifier according to the Code of Practice in Germany, whereas in the UK hot water can return to the calorifier at 50 °C.1

Achieving and maintaining a consistent circulating hot water temperature of ≥ 55°C requires a significant amount of energy and a correctly operating, hydraulically balanced water system.  Due to the current energy crisis situation, several expert groups have debated the potential implications of decreasing hot water temperatures and it’s likely impact upon managing the risks related to potential microbial contaminations.

German Expert Panel Recommendations

Following their recent positioning paper (February 1st, 2023),the German Society of Hospital Hygiene (Deutsche Gesellschaft für Krankenhaushygiene, DGKH) has issued a warning against reducing hot water temperatures within the build environment.2  According to the DGKH, alternative energy saving measures for water installations should be considered that do not risk compromising this fundamental and widely adopted control measure, which is routinely relied upon, and which has proven to play a pivotal role in the overall strategy for mitigating the risk of microbial amplification, especially with Legionella spp.2

As with the UK, Legionnaires’ Disease (LD) in Germany is nationally notifiable. In 2020, 1281 cases of LD have been reported, which equals to 1.5 cases per 100,000 citizens.4 However, a 15x – 30x underreporting is assumed. Factoring in this likely level of underreporting, the true number of LD cases which occur in Germany will lie between 19,000 and 38,000 per year.4,5

Across the UK, 295 cases of LD have been reported between the months of  January to October 202010, and 513 cases in 2019, with an estimate of 4,000 – 6,000 cases occurring every year according to the British Safety Council.11

Legionella amplifies optimally at temperatures between 25 and 45°C. Temperatures > 48 °C suppress Legionella growth, and at temperatures > 60 °C Legionella become inactive following a relatively short exposure time.3

DGKH confirm that based on scientific and operational findings over the past few decades, it is clear that changing consumer behaviour is essential for both energy and water saving.  They suggest a number of available options for consumers to assist with achieving energy savings without compromising the microbial quality of water.  Amongst these options, it is suggested that the following primary measures be given due consideration:

  • Calorifiers temperatures should be set at 60°C and not higher.
  • Efficiency in the hot water system should ensure it does not lose more than 5°C before returning to the calorifier.
  • The hydraulic balance of the system should be safeguarded2

In March 2023, the German Organisation of Gas and Water (Deutscher Verein Gas und Wasser, DVGW) published a factsheet detailing how to achieve energy savings whilst maintaining a hygienically safe water installation, in which they highlight and reinforce the clear message that protection of health is more important than energy saving4,5

Other key points included:

  • Avoidance of critical temperature ranges is the only measure that has been shown to suppress Legionella growth, where stagnation of the water system is avoided and regular water exchange occurs through normal usage.
  • Patient rooms within hospitals, elderly/long-term care facilities, and retirement homes should be excluded from any energy saving measures due to microbial and hygienic risks associated with these cohorts.
  • In large water systems, the following key actions can deliver energy saving measures:
    • Regular maintenance, especially of the calorifier including cleaning to remove scale, along with the use of energy efficient/saving heat generators,
    • A correct calorifier capacity,
    • The correct isolation of pipes and calorifiers,
    • The setting of the calorifier’s hot water temperature at 60 °C with maximal loss of 5°C within the system, according to the Code of Practice6
    • The use of energy-efficient circulation pumps4,5
    • A reduction of hot water temperature has led to systemic Legionella contamination in several of the buildings observed, whereas no previous detected contamination was found prior to this intervention.

Voelker et al undertook regular sampling across nine buildings with existing Legionella contamination for a period of six months.  They documented the water consumption along with the hot and cold water temperatures at the outlets, collecting 807 drinking water samples in total.  Analysis of data revealed that there is a highly significant correlation between constant and consistent temperature and Legionella concentrations.  It is important to note that only 3 water samples where temperatures were consistently > 60 °C were contaminated with Legionella in the investigated buildings at a level of > 100 cfu/100 ml.7 

Similarly, a long-term hospital study from Spain confirmed that continuous hot water temperature at ≥ 55 °C leads to a more effective outcome for controlling Legionella than that at ≥ 50 °C providing the hydraulic balance and recirculation were effective.8   Further arguments have been made in a similar vein by Bedard et al, who monitored and analysed hot water systems across five healthcare facilities.  The authors confirmed that buildings in which the circulating hot water temperature was maintained consistently at ≥ 55 °C were negative for Legionella by culture or qPCR, whereas in systems where temperature control fell outside of these criteria, a positive and higher rate of Legionella detection by culture and qPCR were evidenced.9

The debate around the conflicting goals of energy reduction and the ongoing obligations to create a safe and comfortable environment for users of healthcare and non-healthcare premises will continue to evolve over time.

In November 2022 the Chartered Institution of Building Services Engineers (CIBSE) hosted a national debate at London Excel in the UK.  This event looked at the challenges of lowering temperature with an initial focus on domestic hot water systems.  This debate took the experiences and input from a panel of experts including David Harper from Harper Water Management Group, and posed some very real questions that require careful consideration.  The discussions covered a wide array of relevant and diverse subjects, and explored the extent to which existing and historical practice can be challenged safely by a range of innovative and energy conscious approaches.

The ongoing follow up to these initial opening discussions is eagerly anticipated and will provide further useful insights which no doubt play an important role in helping to shape and guide our direction in the years to come.

In summary

Based on the evidence available, a variety of energy-saving and energy efficiency options are indeed possible for both existing and new water installations, without incurring a corresponding increase to the risk of associated microbial amplification.

The strong argument to maintain energy efficient and hygienically safe water systems within the built environment is to build, install and commission, operate and maintain water installations according to the existing Approved Code of Practice or better. The important insights and outcome from the German debate sends a clear message. Although well intentioned, any efforts to reduce temperature within hot water systems as a mechanism to achieve energy savings for building owners should be discouraged in practice if we are to continue avoiding potentially dangerous and expensive repercussions.

If you like to further discuss the topics touched upon, please contact us.  The team at Harper Water Management Group would be keen to hear from you regarding your own challenges and experiences with energy savings.


1 Health and Safety Executive HSG274 Part 2, “Legionnaires’ Disease, Part 2: The control of Legionella bacteria in hot and cold water systems”, 2014

2 German Society of Hospital Hygiene, Deutsche Gesellschaft für Krankenhaushygiene, DGKH, “Legionellosen und Energiesparmaßnahmen“,

3 Robert Koch-Institut, “Legionärskrankheit in Deutschland während der COVID-19-Pandemie, Januar – Juli 2020“, Epidemiologisches Bulletin, 44, 2020 accessed 22.03.2023

4 German Organisation of Gas and Water, „Energieeinsparungen in der Trinkwasser-Installation: Gesundheitsschutz geht vor Energieeinsparung”, Deutscher Verein Gas und Wasser (DVGW), 2023, accessed March 5th, 2023

5 German Organisation of Gas and Water, „Information. Energieeinsparungen in der Trinkwasser-Installation – geht das? ”, Deutscher Verein Gas und Wasser (DVGW), 2023, accessed March 5th, 2023

6German Organisation of Gas and Water, „Technical Rule DVGW W551_2004-04: Drinking water heating and drinking water piping systems – Technical measures to reduce Legionella growth – Design, construction, operation and rehabilitation of drinking water installations”, Deutscher Verein Gas und Wasser (DVGW), 2004

7Voelker S. et al., „Modelling characteristics to predict contamination risk – Surveillance of drinking water plumbing systems and identification of risk areas”, Int J Hyg Environ Health, 219(1), 101-109, 2016

8 Gavaldà et al., “Role of hot water temperature and water system use on Legionella control in a tertiary hospital: An 8-year longitudinal study”, Water Res, 149, 460-466, 2019.

9Bédard et al., “Temperature diagnostic to identify high risk areas and optimize Legionella pneumophila surveillance in hot water distribution systems”, Water Res, 71, 244-256, 2015.

10Public Health England, „Legionella  monthly reports – January – October 2020 aggregate report”, National Surveillance scheme for Legionnaires’ Disease in residents of England and Wales”, accessed March 5th, 2023

11British Safety Council, “Legionella risks emerging form lockdown”, 02.08.2020,,every%20year%20in%20the%20UK. accessed 22.03.2023

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