Thermostatic Mixing Valves and Legionella: What You Need to Know

A thermostatic mixing valve, or TMV, blends hot and cold water so that it comes out of the tap at a safe temperature. It protects people from scalding. It also creates a short length of pipe full of warm water, which is where legionella grows. Managing a TMV means controlling both risks.

Why TMVs are fitted

To control legionella, hot water is stored at 60°C or above. Water at that temperature can scald in seconds, and children, older people and anyone with reduced sensation are most at risk. A TMV lets the system run hot while the outlet stays safe. They are common in nursing homes, hospitals, schools and childcare settings.

Why they create a legionella risk

• Water downstream of the valve sits in the range where legionella grows, between 20°C and 45°C.
• The valve contains strainers and small passages that collect scale and debris.
• If the outlet is rarely used, that warm water stands still.

How to reduce the risk

1. Fit them only where they are needed. A scalding risk assessment should decide which outlets need a TMV. Staff-only sinks may not.
2. Keep the pipe after the valve short. The closer the TMV is to the outlet, the less warm water there is to stagnate.
3. Avoid one valve serving many outlets where possible.
4. Flush little-used outlets on a set schedule, typically weekly.
5. Service the valves. Clean and descale the strainers, disinfect the internals and replace worn parts as the manufacturer directs.
6. Test them. Check the outlet temperature is as set, and that the valve shuts off the hot supply if the cold supply fails.
7. Record each service and test.

Signs a TMV needs attention

• Outlet temperature too hot, too cold or drifting.
• Poor flow at the tap.
• Temperature that changes when other taps are used.
• No record of when it was last serviced.

Where they fit in your risk assessment

Your legionella risk assessment should list every TMV, where it is, what it serves and how often it is serviced. Where residents are vulnerable, outlets with TMVs are often chosen as sampling points.

The wider picture

TMVs are one part of temperature control. See our guide to hot and cold water temperatures for legionella control, and for care settings, legionella risk assessments for nursing homes.

Safer Solutions includes thermostatic mixing valves in every legionella risk assessment. Call 086 275 9728, email info@safersolutions.ie or request a quote.