The Heat Pump Legionella Cycle Explained: Settings, Schedule and Cost

Quick answer: Heat pumps store hot water at around 45–50°C — inside the range where Legionella bacteria can multiply — so once a week the cylinder is heated to 60°C, usually by the immersion heater, to sterilise it. On a typical 200-litre cylinder that costs roughly 60p a week, about £32 a year, at current price cap rates.

Why heat pumps need a sterilisation cycle and boilers don’t

A gas boiler typically stores hot water at 60°C or above, which keeps the tank hostile to bacteria all the time. A heat pump deliberately runs cooler — every degree of extra cylinder temperature costs efficiency, so most systems store water at 45–50°C for daily use (see how COP drives running costs). The catch: HSE guidance notes that Legionella bacteria multiply between 20°C and 45°C and do not survive above 60°C. Storing water in or near that growth band is safe only if the tank is periodically sterilised — hence the weekly cycle, sometimes labelled “anti-legionella”, “sterilisation”, “purge” or “boost” in your controller.

What the cycle actually does

On its scheduled day the system raises the whole cylinder to 60°C (some units target 65°C) and holds it there briefly, killing any bacteria that have taken hold. Because most heat pumps cannot reach 60°C efficiently — or at all — the last part of the temperature lift usually falls to the electric immersion heater built into the cylinder. Newer high-temperature units, including many using R290 refrigerant, can do the full lift themselves at roughly twice immersion efficiency, which cuts the cost of the cycle by about half.

What it costs at July–September 2026 cap rates

The physics is simple: heating water takes about 1.16 kWh per 100 litres per 10°C rise. Assuming the immersion does the final 10°C lift (50°C to 60°C), electricity at the July–September 2026 price cap of 26.11p/kWh, one cycle a week and ignoring standing losses:

If the cylinder starts cooler than 50°C the lift — and the cost — is proportionally bigger, but even from 40°C a 200-litre cycle costs only around £1.20. In the context of total annual heat pump electricity use, the Legionella cycle is a rounding error: real money is won and lost on flow temperatures, not sterilisation.

The settings that matter

• Frequency: weekly is the standard factory setting and suits almost all homes. Some installers set fortnightly on well-used cylinders; do not stretch it further without professional advice.
• Timing: schedule it for a time when hot water demand is low so the 60°C water is not immediately drawn off — and if you are on a time-of-use tariff, put it in your cheap-rate window to cut the cost further.
• Completion: check occasionally that the cycle actually reaches target. A failed immersion heater is the classic silent failure — the controller tries weekly, never gets to 60°C, and nobody notices. Some controllers log an error; see our troubleshooting guide if yours does.
• Do not disable it. The saving is pennies; the risk — Legionnaires’ disease is a serious pneumonia — is not worth it. If anyone in the house is elderly or immunocompromised, treat the cycle as non-negotiable.

Holidays, smart tariffs and other wrinkles

Two practical wrinkles are worth knowing. First, holiday mode: on most systems it suspends normal hot water heating but should still run — or immediately run on your return — a sterilisation cycle, precisely because a lukewarm tank left standing for a fortnight is the textbook growth scenario. Check how your unit handles it rather than assuming. Second, if your home has a smart thermostat or third-party controls layered over the heat pump’s own controller, make sure they are not fighting: the sterilisation schedule lives in the heat pump controller, and external controls that lock out hot water at the scheduled time can quietly stop it completing.

A quick health check for your system

Find the cycle in your controller (ask your installer if it is buried in a service menu), note the scheduled day, and glance at the cylinder temperature after it runs. An annual service should verify the immersion heater and the sterilisation schedule as a matter of course — worth confirming it is on the checklist. And if you are still weighing up a heat pump and wondering about quirks like this, our heat pump guide and cost calculator cover the full ownership picture.

FAQs

Can I run the Legionella cycle less often to save money?

The saving is tiny — moving a 200-litre cylinder from weekly to fortnightly saves roughly £16 a year at cap rates — and it weakens a safety control. Follow your manufacturer’s guidance, and if in doubt keep it weekly.

Why does my hot water smell or scald on one day each week?

That is the day the cycle runs: the cylinder is at 60°C rather than the usual 45–50°C, so taps run hotter than normal. If scalding is a concern — young children, for instance — ask your installer about a thermostatic blending valve, which caps the temperature at the tap.

Does a combi boiler or an air-to-air heat pump need a Legionella cycle?

No. Combi boilers heat water on demand without storing it, and air-to-air heat pumps do not handle domestic hot water at all. The cycle only applies where hot water is stored below 60°C — which is exactly the standard air-to-water heat pump setup.