Legionella is killed by temperature and time together: the bacteria die off in about 32 minutes at 60 °C, in roughly 2 minutes at 65 °C and almost instantly at 70 °C. A heat pump cylinder should therefore be stored at 50–55 °C day to day and raised to 60 °C for at least an hour once a week, then blended down to 43–45 °C at the tap.
What temperature kills legionella?
Not a single number — a temperature and a duration. Below 20 °C the bacteria are dormant, between 20 and 45 °C they multiply, and above 50 °C they begin to die at a rate that accelerates sharply with every degree.
| Water temperature | Effect on Legionella |
|---|---|
| Below 20 °C | Dormant, does not multiply |
| 20 – 45 °C | Multiplies; fastest around 35–46 °C |
| 50 °C | Survives but stops multiplying |
| 55 °C | Dies slowly — hours |
| 60 °C | Dies off in about 32 minutes |
| 65 °C | Dies in about 2 minutes |
| 70 °C | Killed almost instantly — thermal disinfection temperature |
UK ACOP L8 practice codifies this as: store at 60 °C, distribute at 50 °C or above, and get 50 °C to every outlet within one minute of running it. The background biology is covered in legionella and the anti-legionella cycle.
Can a heat pump reach legionella temperature on its own?
It depends on the refrigerant. Older R410A units typically top out near 55 °C water and need an immersion heater to complete the pasteurisation. Units using R290 propane or an EVI compressor reach 70–75 °C directly and can pasteurise unassisted, at a low COP for that hour. Either arrangement is legitimate; what is not legitimate is a system that never reaches 60 °C anywhere in the store because the immersion was left disconnected at commissioning.
The other half of the problem is the cylinder itself. Reaching 60 °C at the thermostat pocket is not the same as reaching 60 °C at the bottom of the tank, where cool water and sediment sit. A cylinder with strong stratification and a coil that extends low into the vessel pasteurises the whole volume; a shallow boiler-era coil heats the top third and leaves a reservoir below it.
How often does the legionella cycle need to run — and what does it cost?
Weekly is the standard default, and weekly is enough for a sealed cylinder with normal daily turnover. Daily pasteurisation, which some controllers ship with as a factory setting, is the most common avoidable cost on a heat pump bill.
The arithmetic is small but not trivial. Raising 200 litres from 50 °C to 60 °C takes 2.3 kWh of heat. Delivered by an immersion heater at COP 1.0 and repeated weekly, that is about 121 kWh a year; done daily it is about 850 kWh. Schedule it overnight, ideally on a cheap tariff, and let the heat pump do as much of the lift as it can before the immersion finishes the job.
Which cylinder design lowers the risk in the first place?
Store less potable water, or none. A hygienic (fresh-water) store keeps the bulk volume as non-potable heating water and heats drinking water instantaneously as it passes through a large corrugated stainless coil, so there is no standing warm potable reservoir to colonise — which is why Solimpeks builds its hygienic combination tanks this way for heat pump systems. Where a conventional store is used, three details matter: a coil that reaches the base of the vessel, a sound magnesium anode so corrosion products do not create biofilm habitat, and a thermostatic mixing valve delivering 43–45 °C to baths and basins. Storing at 60 °C without a mixing valve trades a bacteriological risk for a scalding one.
Frequently asked questions
Does 55 °C kill legionella?
Slowly. At 55 °C the bacteria die over a period of hours rather than minutes, which is why 60 °C for at least an hour, or 65–70 °C briefly, is used for pasteurisation. Storing at 55 °C prevents multiplication but does not disinfect.
Do heat pumps cause legionella?
No. Heat pumps are associated with the risk only because they are most efficient at lower storage temperatures, which sits closer to the growth range. A weekly 60 °C pasteurisation cycle, or a fresh-water cylinder that stores no potable volume, removes the issue entirely.
Can I run the anti-legionella cycle less often than weekly?
Weekly is the appropriate default for a domestic cylinder and should not be extended without a risk assessment. What you can safely change is the timing — run it overnight on a cheap tariff — and the daily-to-weekly frequency if the controller shipped set to daily.
Is 50 °C hot water safe for a family home?
50 °C storage stops Legionella multiplying but does not kill it, so it is only safe as a day-to-day setting when combined with a scheduled pasteurisation to 60 °C. Water delivered to taps should then be blended to 43–45 °C to prevent scalding.