What Temperature Should a Heat Pump Water Heater Be Set To?
What Temperature Should a Heat Pump Water Heater Be Set To? A heat pump water heater will commonly be set to store domestic hot water at around 50°C to 55°C during normal heat pump operation, but the appropriate temperature depends on the appliance, system design and the required Legionella control strategy. Higher temperatures, often around 60°C, may be used periodically for thermal disinfection or maintained where a specific risk assessment requires it. The key is to balance domestic hot water safety, Legionella control and heat pump efficiency while following the manufacturer’s instructions for the particular heat pump water heater.
Why does the water temperature matter?
The storage temperature has a direct effect on both hygiene and energy consumption.
Higher water temperatures provide greater protection against the growth of Legionella bacteria and increase the amount of usable hot water available from a given cylinder once it is mixed with cold water.
However, producing hotter water requires the heat pump’s compressor and refrigerant circuit to operate at a greater temperature lift. This generally reduces the Coefficient of Performance (COP).
A heat pump water heater operating at 50°C will therefore generally be more efficient than the same unit attempting to maintain substantially higher water temperatures under otherwise identical conditions.
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Is 50°C hot enough for a heat pump water heater?
Many domestic heat pump water heaters can operate efficiently with a normal setpoint around 50°C to 55°C, provided this forms part of an appropriate overall hot water control strategy.
However, there is an important distinction between the everyday heat pump operating temperature and the temperature required for Legionella control.
For higher-risk hot water systems covered by workplace health and safety requirements, HSE guidance uses temperature control as a primary method of controlling Legionella. HSG274 Part 2 advises that calorifier thermostat settings should modulate as close to 60°C as practicable without going below 60°C, with hot water reaching at least 50°C at relevant outlets within one minute.
Those HSE requirements should not simply be applied indiscriminately to every domestic heat pump water heater. The appropriate strategy depends on the type of premises, system configuration and applicable risk assessment.
What is an anti-Legionella cycle?
Many integrated heat pump water heaters incorporate a programmed high-temperature or anti-Legionella cycle.
The heat pump may normally maintain the cylinder at a lower temperature for improved efficiency, then periodically increase the stored water temperature to approximately 60°C or another temperature specified by the manufacturer.
Some heat pumps can achieve this temperature using the refrigeration cycle alone. Others use an electric immersion heater to supplement the compressor at higher temperatures.
Using an immersion heater reduces the energy-efficiency advantage while it is operating, but it can allow the system to reach temperatures that would otherwise be difficult or slow for the heat pump to achieve.
The frequency and temperature of any disinfection cycle should follow the appliance manufacturer’s requirements and the hot water system’s design and risk assessment.
Why not leave the cylinder at 60°C or higher all the time?
You can operate some heat pump water heaters at higher storage temperatures, but there can be an efficiency penalty.
As the required domestic hot water temperature rises, the heat pump has to work against a greater temperature difference between the evaporator air and the water being heated. COP generally falls as this temperature lift increases.
Higher storage temperatures can also increase standing heat losses from the hot water cylinder.
The most energy-efficient setting is therefore not necessarily the highest temperature the appliance can achieve. Equally, simply turning the cylinder temperature down as far as possible is not good practice because water hygiene and usable hot water capacity also need to be considered.
What about the risk of scalding?
Hot water temperatures also need to be considered from a safety perspective.
Water stored at temperatures used for Legionella control can cause scalding, particularly for children, elderly people and other vulnerable users.
Where necessary, thermostatic mixing valves (TMVs) can reduce the temperature delivered at an outlet while allowing water to be stored at a higher temperature.
In England, current Approved Document G guidance requires the hot water supply to a bath in a new dwelling to be limited to a maximum of 48°C using an appropriate temperature control device. This is a specific Building Regulations guidance provision and should not be interpreted as meaning that the entire hot water cylinder should be stored at 48°C.
What is the best temperature for efficiency?
For many domestic applications, around 50°C to 55°C can provide a useful starting point for normal heat pump operation, subject to the manufacturer’s instructions and the required water hygiene strategy.
The optimum setting will depend on the appliance, refrigerant, ambient air temperature, cylinder capacity and household hot water demand.
For example, an R290 heat pump water heater may be capable of producing relatively high domestic hot water temperatures using the compressor, but that does not automatically mean the maximum available temperature is the most economical setting.
Ultimately, the correct heat pump water heater temperature should balance COP, hot water capacity, recovery time, standing heat loss, scald protection and Legionella control. Always use the manufacturer’s recommended settings and ensure any required high-temperature or disinfection cycle remains enabled.