What is an anti-legionella valve and when do I need one?
Updated 12 September 2026
Short answer
An anti-legionella valve is a thermostatic mixing valve designed to remove stagnant water from the tempered (blended) pipe that runs between the valve and the outlet. It reduces legionella risk in installations where the blended pipe would otherwise sit at body temperature and support growth. Most often used in healthcare premises with TMV3 mixers on baths and showers.
The phrase is used loosely in the trade and can mean slightly different things depending on context. Three separate products get called "anti-legionella" valves, and they solve overlapping but different problems.
Thermostatic mixing valves (TMVs) with anti-stagnation features
A standard TMV blends hot and cold water to deliver a safe temperature at the outlet, typically 41 degrees Celsius for a shower. The blended pipe between the TMV and the outlet is at exactly the temperature that supports fastest legionella growth. Where the outlet is used frequently this is not a problem because the pipe is flushed through by normal use. Where it is not, the pipe can become a colonisation site.
An "anti-legionella" TMV incorporates features to address this: a thermal disinfection mode that allows the blended pipe to be purged with high-temperature water, an integrated check valve to prevent backflow, and often a self-cleaning element that exercises the valve automatically. TMV3-approved products designed for healthcare use normally include these features.
End-of-line anti-legionella valves
These are valves fitted at the end of a pipe run rather than at an outlet, and are used in healthcare premises to allow thermal disinfection of a zone without needing to open every outlet. They open automatically when the flowing water temperature exceeds a set threshold, allowing high-temperature flushing, and close again when it drops.
Anti-legionella expansion vessels
Expansion vessels on hot water systems can support legionella growth in the stagnant water inside the vessel. "Anti-legionella" expansion vessels are through-flow designs where the system water continuously passes through the vessel rather than sitting static. They are becoming standard on new commercial hot water installations.
When you need one
The specification of any of these products should come from the water hygiene risk assessment or from the design of a new installation, not from a generic rule. A standard office TMV does not need to be an "anti-legionella" model if the outlet is in daily use. A hospital en-suite bath TMV should be.
Legal basis
HSG274 Part 2 paragraph 2.54 on thermostatic mixing valves. BS 7942 on TMV2. NHS Model Engineering Specification D 08 on TMV3 for healthcare premises.
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