How often should a thermostatic mixing valve be serviced?

A thermostatic mixing valve (TMV) should be serviced at least once a year. The precise interval is set by your legionella risk assessment, and in healthcare, care homes and other higher-risk settings six-monthly servicing is common. TMV3 valves also carry short in-service checks in the first few months after they are commissioned.

An annual service is the floor, not a target. A TMV is the one component in the system that is deliberately kept warm, so it needs more attention than a plain tap, not less. Skipping the service does not just risk scalding if the valve drifts out of setting; it leaves a warm, scale-filled component sitting right at the outlet, where any bacteria it shelters are delivered straight to the user.

Why a TMV is both a scald control and a legionella risk

A TMV blends stored hot water with cold to deliver water at a safe, stable temperature, usually somewhere around 38 to 46°C depending on the outlet. That is what protects a child, an elderly resident or a patient from scalding at the tap or shower.

The problem is that this blended range sits squarely inside the legionella growth range of roughly 20°C to 45°C. The valve produces, by design, water at the temperature the bacteria most like. Add the internal parts, the strainers, the flexible tails and the scale that builds up on warm wetted surfaces, and you have a component that both creates ideal growth conditions and gives bacteria somewhere to shelter. Our legionella temperature chart shows the full range.

This is the honest point most guidance buries: a TMV does not control legionella. It controls scalding, and in doing so it introduces a legionella hazard that then has to be managed. That management is partly design (keep the valve as close to the outlet as possible so there is very little warm blended water sitting downstream) and partly maintenance (service it on schedule).

What a TMV service actually involves

A service is more than a temperature check. A competent service covers:

• Inspection of the valve body, strainers, check valves and connections for wear, damage or debris.
• Cleaning and descaling of the internal components, because scale both harbours bacteria and stops the thermostatic element moving freely.
• Disinfection of the valve and any in-line strainers.
• The fail-safe test: isolating the cold supply and confirming the valve shuts the flow off quickly, so it cannot deliver scalding hot water if the cold ever fails.
• Recording the inlet temperatures, the blended outlet temperature and the fail-safe result.

The fail-safe (cold-supply shut-off) test is the check that matters most for safety, and a valve that fails it needs cleaning, re-setting or replacing. If the blended temperature has crept up by more than about 2°C from the commissioned setting, that too is a sign the valve needs attention.

How often a TMV should be serviced

Frequency is risk-based, which is why HSG274 and the wider guidance point you back to your risk assessment rather than a single fixed number. In practice:

• Annual servicing is the usual minimum for lower-risk, general premises where valves are performing reliably.
• Six-monthly is common in healthcare, care settings and anywhere serving people who are more vulnerable to infection or scalding.
• After commissioning, TMV3 valves are typically checked at 6 to 8 weeks and again at 12 to 15 weeks. If they hold their setting, the interval can be extended to the annual or six-monthly cycle.

Between full services, a simple monthly or quarterly temperature check at the outlet helps you spot drift early, and the reading goes in your log book alongside the rest of your monitoring.

TMV2 or TMV3: which scheme applies

Approved TMVs are certified under one of two schemes, and knowing which you have tells you what standard the valve and its servicing should meet.

• TMV2 covers general and domestic applications and is certified to BS EN 1111 and BS EN 1287. You will find these in homes, offices, schools and most commercial premises.
• TMV3 covers healthcare applications under the NHS D08 scheme and sits alongside HTM 04-01. These valves meet tighter performance tolerances and carry the stricter in-service checking regime described above.

What temperature a TMV should run at

A common mistake is to try to run the TMV hotter to control legionella. You cannot, because raising the blended temperature removes the scald protection that is the valve's whole purpose. The blended outlet is set for safety, not for disinfection: roughly 41°C at a basin or shower and up to around 44°C at a bath, though the commissioned figure for your specific outlet is what counts.

Legionella control is achieved around the valve, not at it. The hot water feeding the TMV should still be genuinely hot: stored at 60°C and reaching the valve inlet at 50°C or above (55°C in healthcare) within one minute. Keep the run of blended pipework after the valve as short as possible, and keep cold water below 20°C right up to the cold inlet. Showers fed through TMVs also need their heads and hoses cleaned and descaled, as covered in our guide to legionella in shower heads.

Where TMV servicing fits in your legionella controls

TMV servicing is one line in a written scheme of control, not a standalone task. ACOP L8 requires you to assess the risk and put controls in place, and HSG274 Part 2 sets out the temperature monitoring, cleaning and inspection that surround the valve. Record each service, each fail-safe test and each temperature reading, so the evidence sits together with the rest of your legionella compliance records. The service proves the valve was maintained on the day; it does not by itself make a system safe, which is why it works alongside temperature control, flushing of little-used outlets and periodic review.