Legionella are waterborne bacteria that multiply in stored water between 20 and 45 °C and are killed within minutes above 60 °C. Hot water systems control them by storing at 60 °C, distributing at 55 °C or higher, or by eliminating stored potable volume entirely. An anti-legionella cycle periodically heats the whole tank to 60–70 °C to disinfect it.
What is legionella and why do hot water tanks matter?
Legionella pneumophila is a bacterium that occurs naturally in fresh water at harmless concentrations. It becomes dangerous when it multiplies in warm, stagnant, man-made water systems and is inhaled as fine aerosol — from showers, spray taps or whirlpools — causing Legionnaires' disease, a severe pneumonia. ECDC surveillance has recorded more than 10,000 cases per year across the EU/EEA in recent annual reports, with numbers trending upward, and building hot water systems are among the recognised sources.
Storage water heaters matter because a cylinder is a near-perfect incubator when run badly: nutrients from biofilm, stagnation in rarely used volumes, and long periods in the bacteria's favourite temperature band.
At what temperature does legionella die?
Temperature is the primary control, and the thresholds are well established in WHO guidance:
| Water temperature | Effect on legionella |
|---|---|
| Below 20 °C | Dormant — survives but barely multiplies |
| 20–45 °C | Growth range — fastest around 32–42 °C |
| 50 °C | Slow die-off over hours |
| 60 °C | Killed within minutes |
| 70 °C | Killed almost instantly |
The classic engineering answer follows directly: store hot water at 60 °C, keep circulating distribution at 55 °C or above, and the population cannot establish itself.
What is an anti-legionella cycle?
An anti-legionella cycle — also called thermal disinfection or a legionella purge — is a controller function that periodically heats the entire storage volume, including the cool lower zones, to 60–70 °C and holds it there long enough to disinfect. Weekly operation is the common default in national guidance. On heat pump cylinders the cycle typically needs the electric backup element, because many heat pumps reach 55–60 °C only inefficiently or not at all. Two caveats: the cycle disinfects the tank, not the pipework beyond it, and dead legs — capped-off or rarely used branches — remain the classic hiding place no tank function can reach.
What do regulations actually require?
Rules are national, but converge on the same numbers. Germany's DVGW Worksheet W551 classifies systems with more than 400 litres of storage or more than 3 litres of pipe content between heater and tap as large systems, requiring 60 °C storage and at least 55 °C on the circulation return. UK HSE guidance (HSG274) works to the same 60/55 °C logic with routine temperature monitoring. At EU level, the recast Drinking Water Directive (EU) 2020/2184 added legionella to the parameters national risk assessments must cover, with a reference value of 1,000 CFU per litre. For homeowners the practical translation is simple: do not run a conventional domestic hot water store permanently below 50 °C.
How do heat pumps and solar systems handle legionella?
Heat pumps lose efficiency at 60 °C, so good system design avoids brute force. The main strategies:
- Scheduled thermal disinfection — run at 50–55 °C for efficiency, purge to 60–65 °C weekly with the backup element.
- Fresh-water principle — a hygienic tank stores only heating water and heats drinking water instantaneously in a coil, leaving no stored potable volume to protect. Storage temperature can then drop to 50 °C without hygiene risk.
- Store hot, deliver safe — solar systems routinely push stores past 60 °C in summer, which is free disinfection; a thermostatic mixing valve at the outlet caps delivery temperature so the hot store never reaches the tap as a scald.
Solimpeks builds both conventional enamelled cylinders and hygienic combination stores at its Konya factory precisely because the right legionella strategy differs between a gas-boiler retrofit and a low-temperature heat pump system.
Frequently asked questions
How often should an anti-legionella cycle run?
Weekly is the common default in national guidance for stores that run below 60 °C. A well-used cylinder held permanently at 60 °C does not need a separate cycle — the standing temperature already does the job.
Is 50 °C hot water storage safe?
Not on its own. At 50 °C legionella dies only slowly, and rarely used volumes remain at risk. Either purge the store to 60 °C or higher periodically, or switch to a fresh-water arrangement where no drinking water is stored at all.
Can legionella grow in a buffer tank?
Buffer tanks hold closed-circuit heating water that is never drawn at taps or aerosolised in showers, so they are not a meaningful exposure route. The hygiene battleground is stored drinking water and the pipework that delivers it.
Do instantaneous water heaters have legionella risk?
Very little — legionella needs stored, warm, stagnant volume and time. Instantaneous heaters and fresh-water stations hold only a litre or two in the exchanger, which is exactly why hygiene rules focus on storage systems.