What temperature kills legionella?

Legionella is killed at 60°C and above: at 60°C most of the bacteria die within about two minutes, and at 70°C the kill is effectively immediate. At 50°C they stop multiplying and die off only slowly, over hours, and below 20°C they are not killed at all but lie dormant. These relationships, set out in the HSE's HSG274 guidance, are the entire basis of temperature control: store hot, distribute hot, keep cold genuinely cold.

The temperature ladder

The effect of temperature on legionella is a ladder, and each rung explains one of the control figures used across this site:

The legionella temperature chart presents the same figures alongside the monitoring targets, and temperature control explains how they are applied day to day.

Why the system stores at 60 but delivers at 50

The two hot-water figures exist because kill and suppression are different things. Storage at 60°C or more means the reservoir itself is hostile: bacteria entering the calorifier or cylinder are killed within minutes. Distribution at 50°C or more, arriving at the outlet within one minute, means the water in the pipes never sits in the growth range, even though 50°C kills too slowly to be relied on alone. If a real system loses a few degrees around a long loop, water that left the vessel at 60°C still arrives above the growth ceiling. Dropping the storage temperature to save energy removes that margin and converts the calorifier from a kill zone into a warm incubator.

Dormant is not dead

The cold-water figure carries a common misunderstanding. Below 20°C legionella stops multiplying, but it survives indefinitely and resumes growth the moment the water warms. A cold supply is therefore only a control while it is genuinely cold. Cold water standing in a warm roof space, running alongside hot pipes, or sitting in a storage tank in summer can hold above 20°C for months, quietly converting dormant water into growth-range water, which is why cold outlets should reach below 20°C within two minutes of running and tanks are checked for heat gain. The growth conditions behind all of this are covered in what causes legionella to grow.

Thermal disinfection, and the scalding balance

Raising the hot water and flushing the whole system at 60°C or more, known as thermal disinfection or pasteurisation, is a recognised way to kill legionella through a colonised system, but it is a remedial exercise done under controlled conditions with outlets out of use, because water at 60°C scalds skin within seconds. The same physics that makes heat the primary legionella control makes it a burn hazard at the tap, and the two duties are reconciled by storing hot and blending down at the point of use with thermostatic mixing valves, a balance worked through in scalding versus legionella and TMV servicing.

Knowing the kill temperatures tells you what to aim for; the evidence that you are achieving it is the recorded readings. Log flow, return and outlet temperatures in a log book or with the temperature checker, because temperature control that is not recorded cannot be evidenced.