How does a heat pump cylinder deal with legionella?

What the guidance says

Health and Safety Executive (HSE) guidance on managing legionella sets out the temperatures that matter. The bacteria multiply between 20°C and 45°C where nutrients are available, are dormant below 20°C and do not survive above 60°C. HSE advises that hot water storage cylinders should store water at 60°C or higher, with hot water distributed at 50°C or higher.

A heat pump normally heats a cylinder to around 50°C, so the stored water needs lifting to 60°C from time to time.

What the installation guide says

The Advance Appliances heat pump cylinder installation guide states that storage at lower temperatures, for example with heat pump cylinders, will require thermal disinfection on a weekly or daily basis, and refers installers to guidance from WRAS, the Water Regulations Advisory Scheme. The ECO heat pump cylinder guide says the same, and requires the immersion heater thermostat to be set to 60°C.

Every Advance heat pump cylinder comes with at least one 3kW immersion heater, and the data sheets list the immersion as being there for thermal disinfection.

A weekly 60°C cycle, usually by immersion

A 2013 laboratory study for the Department of Energy and Climate Change found that in many heat pump systems the controls raise the cylinder to 60°C for one to three hours once a week, usually using the cylinder's immersion heater.

The same study found the bottom of the cylinder did not always reach the recommended temperature during an immersion cycle, and suggested running cycles at night to give the water more time to heat through. Ask the heat pump installer how and when the cycle is set on your system.

Looking after the immersion heater

• The element is Incoloy 825, fitted with a thermostat and a safety cut-out, on a non-standard 1 3/4 inch boss.
• Replacement elements come from Advance, such as the 3kW immersion heater for unvented cylinders.
• In hard water areas the element needs descaling periodically unless a softener or descaler is fitted.
• The annual service includes checking the immersion wiring and earth continuity.

An immersion that has failed or scaled up will not lift the cylinder to temperature, so it is worth checking at every service. For the temperature you get day to day, see what temperature your hot water will be.