How to Balance Energy Savings with Legionella Prevention

4/22/2025

Over the long history of the sustainability movement, one of the most difficult aspects of moving toward energy efficiency is balancing energy-reducing strategies with other priorities within a facility. One area where that balancing act is exceedingly crucial is generating hot water. Good housekeeping, rational designs/installations and working practices can be effective in saving energy safely. The more complex the solution to energy efficiency, the greater the cost and potential for mechanical and human error. And human error in this space can lead to greater risk for Legionella in your facility. How can facility managers manage these two priorities simultaneously? Here are several solutions and considerations.   

Heat Pumps 

Two main types of heat pump are used to heat domestic hot water. Ground source heat pumps circulate water through pipework installed in the ground, extracting warmth from the ground. Air source heat pumps use fans to pass air across coils to extract the energy from the air. 

Net-zero incentives direct us to heat pumps, and manufacturers share that the energy output from air source heat pumps is about 3kW for each 1kW input.  This, however, is under optimum conditions, which vary with climate.  Note that for this free energy, the heat pump requires 1kWe (electrical energy) to generate 3kWt (thermal energy), meaning the electricity demand will significantly increase if your primary energy is currently gas. Additionally, associated plant and equipment combined with the required plant space is costly and sizeable. 

Heat pump technology continues to improve to overcome the challenge of generating hot water at the required operational temperature.  Manufacturers advise that low-grade heat is a thing of the past. However, it still needs to be checked that temperatures are achievable through the seasons and that achieving 70oC for the pasteurization of the hot water system is also possible. It may be that you will need supplementary heating. 

Solar thermal energy 

Solar water heating starts with solar collectors, panels or tubes that circulate a mixture of water and anti-freeze to and from the hot water cylinder. Climate considerations mean that hot water cannot be guaranteed all year round; therefore, a supplementary heat source, e.g., an immersion heater, is needed to achieve 60oC. 

Solar / Photovoltaic (PV) Panels 

PV panels are the most common renewable energy source. They capture the sun’s energy and convert it into electricity and are a supplementary source to the mains electricity installed; therefore, there is no need for a supplementary heat source for generating hot water. 

Embodied Carbon 

All items of a water system should factor in embodied carbon, the total CO2 from the energy used to create any item from its origin as raw material through to the end of the disposal / recycle process. 

Water systems' design, installation and operation needs to be looked at from a holistic perspective. Identifying the user risk and their specific needs through design risk assessment will help keep decisions rational, minimising the environmental impact. 

Thermal Insulation – keeping the hot water hot and cold water cold 

Piped services and plant and equipment should be adequately insulated. Often, hot and cold water pipes are routed side by side in service risers and ceiling/floor voids. Ensuring pipework is insulated keeps the energy within the hot water pipes, reducing heat gain in the space and reducing the warming impact on insulated cold pipes. Maintaining hot water above 50oC (or 55oC in healthcare premises) and cold water below 20oC is key to lowering legionella risk. 

Pipe Distribution Losses 

Insulated pipes still bring distribution losses, so removing hot water centralised pipework removes the associated distribution losses. Approaches can include using localized mini-plate heat exchangers (heat interface units) served from a centralized heating network or point-of-use electric water heaters can be used. Each approach does, however, come with its varied characteristics and components. Reducing hot water centralized pipework and plant reduces the system volume, energy losses and risk from components. 

‘Value’ Engineering  

True value engineering is where the justification for keeping or adding quality to minimize risk, and Op Ex occurs, i.e. remote monitoring that can provide data to inform if outlets are infrequently used.  Or if we are talking omissions, it’s removing thermostatic mixing valves (TMVs) / taps where the scalding risk assessment has identified this opportunity. 

‘Gloves off’ approach 

Disposable gloves bring increased bacteria risk. They create moisture and warmth on the hands, an ideal environment for bacteria to grow, as well as cross-contamination from contact with various surfaces.  Avoiding unnecessary use of gloves removes the environmental impact from both the disposal aspect and the manufacturing and transportation costs (embodied carbon).  Gloves should not be a substitute for hand washing; therefore, the ‘Gloves Off’ approach could, but should not, bring greater water/energy consumption. 

Summary 

Ensuring the design risk assessment is a true reflection of actual use minimizes waste.  Over-designing for future-proofing adds margins, bringing exaggerated demand, which leads to oversized plant and equipment and inefficiencies. Rational designs allow the hot water generating infrastructure to be sized accurately.

Andy Clews is Senior Consultant for the Water Hygiene Centre.