Temperature is the single most important factor in controlling Legionella — so what temperature kills Legionella, and what are the safe ranges for your water system? This guide sets out the temperatures at which Legionella grows, survives and dies, and the control regime that keeps water safe.

What temperature kills Legionella?

Legionella bacteria are controlled by temperature as follows:

Below 20°C — Legionella is dormant and does not multiply.
20–45°C — the danger zone, where Legionella grows most readily (fastest around body temperature, ~37°C).
At 50°C — Legionella survives but does not multiply, and is killed slowly.
At 60°C — around 90% of Legionella is killed within about two minutes.
At 70°C and above — Legionella dies almost instantly.

The Legionella danger zone

The critical range to design out is 20–45°C. Any part of a water system that sits in this range — a warm cold-water tank, tepid stored hot water, or lukewarm water in a rarely used pipe — gives Legionella the conditions it needs to multiply. Keeping water either hot enough or cold enough is the foundation of control.

Safe water temperatures: the control regime

The recognised temperature control regime under ACOP L8 and HSG274 is:

Hot water — stored at 60°C or above, and distributed so it reaches at least 50°C (55°C in healthcare settings) within one minute at the outlet.
Cold water — kept below 20°C, again within one minute of running the outlet.

Because water hot enough to control Legionella can also scald, thermostatic mixing valves (TMVs) are used at the point of use to deliver a safe temperature without lowering the temperature stored in the system.

Why temperature is the primary control

Temperature control is recognised as the main way to manage Legionella risk in most water systems. It is reliable, measurable and continuous — which is why routine temperature checks are central to any monitoring programme.

How to keep temperatures in the safe range

Insulate and lag pipework to hold temperatures.
Monitor temperatures regularly as part of water hygiene monitoring.
Flush little-used outlets so water does not sit and cool or warm.
Remove dead legs and redundant pipework.
Service TMVs and check stored hot-water temperatures.

Temperature isn't the only factor

Temperature is the primary control, but not the only one. Stagnation, biofilm, scale and nutrients all play a part, which is why a full programme combines temperature control with a risk assessment, monitoring and, where needed, Legionella testing.

Keeping your water safe in London

London Water Treatment provides Legionella risk assessments and control services across London, including temperature monitoring, sampling and remedial work. Get in touch for advice or a quote.

Frequently asked questions

Does 60°C kill Legionella instantly?

Not instantly — at 60°C around 90% of Legionella is killed within about two minutes. It is only at around 70°C and above that Legionella is killed almost immediately.

Is cold water safe from Legionella?

Cold water is low-risk only if it is kept below 20°C. If cold water warms into the 20–45°C range — for example in a poorly insulated tank — it can allow Legionella to grow.

What Temperature Kills Legionella? Safe Hot & Cold Water Ranges