Do Heat Pump Water Heaters Need a Legionella Cycle?

Do Heat Pump Water Heaters Need a Legionella Cycle? A heat pump water heater will often have a Legionella or thermal disinfection cycle if the domestic hot water is normally stored below about 60°C, but what’s actually required depends on a whole lot of factors – the system itself, the premises, the water temperatures, the risk assessment and what the manufacturer is telling you to do. Many heat pump water heaters normally operate at around 50°C to 55°C to keep things energy-efficient, then periodically crank the cylinder temperature up to approximately 60°C or higher as part of a way to keep Legionella under control. It’s important to remember that Legionella control should be part of the whole hot water system picture, rather than assuming every heat pump needs to be set to the same weekly programme.

Why is Legionella Relevant to Heat Pump Water Heaters?

Legionella bacteria can start multiplying away in the water system when conditions are right – particularly when water stays within temperatures that let them grow.

This is relevant to heat pump water heaters because they’re designed to produce hot water efficiently, and efficiency tends to go down as the water temperature goes up.

A conventional electric immersion heater can pretty easily keep the cylinder at 60°C or above all the time. A heat pump has to work a lot harder whenever the difference between the air temperature and the temperature of the water you want gets bigger.

Running continuously at a high cylinder temperature can therefore reduce the heat pump’s efficiency score and increase electricity usage.

Discover our air source heat pump water heating products by visiting the relevant range below:

Compact wall mounted heat pump water heaters
Heat pump water heaters for residential properties
Heat pump water heaters for commercial properties

What Does a Legionella Cycle Do?

A Legionella cycle – also known as an anti-Legionella cycle, pasteurisation cycle or thermal disinfection cycle – periodically cranks the temperature of the stored water up.

For example, a heat pump water heater might normally operate with a setpoint of approximately 50°C to 55°C and then occasionally crank the cylinder temperature up to around 60°C or a bit higher.

The exact temperature, how long to keep it that high, and how often should be determined by the appliance manufacturer’s control strategy and what the particular hot water system needs. You shouldn’t just assume one universal setting will be right for every install.

Does the Heat Pump Themselves Heat the Cylinder to 60°C?

That depends on the heat pump.

Some air source heat pump water heaters can get water up to 60°C or above just using the compressor and refrigerant circuit.

This is something that’s becoming more achievable with high-temperature systems, including some heat pump water heaters using R290 refrigerant – which is basically refrigerant-grade propane.

Other heat pumps operate most efficiently at lower domestic hot water temperatures and use a built-in electric immersion heater to do the high-temperature bit.

This uses direct electric resistance heating, so your efficiency goes down a bit while it’s on. But if it’s only needed occasionally, the heat pump can still provide most of the everyday hot water at a more efficient operating temperature.

Does UK Guidance Require Hot Water to be Stored at 60°C?

This is a bit of a grey area because the rules are different for different types of premises.

The Health and Safety Executive (HSE) provides a comprehensive guide on Legionella control in HSG274 Part 2 for water systems covered by workplace health and safety legislation. For calorifiers using temperature as the control method, HSE guidance includes getting the water to at least 60°C throughout the vessel for a continuous period and keeping the whole hot water system at the right temperatures.

HSG274 is mainly aimed at people who are responsible for premises where health and safety rules apply. It’s not just a blanket rule saying every private domestic heat pump water heater has to store water at 60°C permanently. The HSE itself describes its Legionella guidance as helping people meet their legal responsibilities.

Similarly, Approved Document G is statutory guidance in England on sanitation, hot water safety and water efficiency, including requirements for hot water systems. It’s guidance on meeting the Building Regulations, not a manufacturer-specific instruction for setting a heat pump’s Legionella cycle.

Should You Disable the Legionella Cycle to Save Electricity?

Generally, no. You shouldn’t just turn off the thermal disinfection function to reduce electricity consumption without thinking about how water hygiene is being controlled.

If the immersion heater is running during the cycle, there will be some extra electrical consumption. But turning off a manufacturer’s hygiene function to save electricity could compromise the intended water hygiene strategy.

You should try to optimise your normal heat pump operation while keeping the right Legionella control measures in place.

What’s the Best Approach for a Heat Pump Water Heater?

For a lot of domestic heat pump water heaters, a good strategy is to let the heat pump operate at an everyday domestic hot water temperature while using the manufacturer’s specified thermal disinfection programme when required.

The right arrangement will depend on a whole range of factors, including the cylinder design, storage temperature, temperature stratification, hot water usage, system volume, occupants, controls and manufacturer requirements. In the long run, a heat pump water heater won’t necessarily require an identical Legionella cycle in every situation; you see, what it really needs is a sensible water hygiene and legionella control strategy. Where a manufacturer provides a built-in anti-Legionella programme, it’s usually best to follow their instructions on how to set it up and run it, rather than turning it off just to cut down energy costs.