Thermostatic mixing valves: do you need one, and which type?

What a thermostatic mixing valve actually does

A TMV has a hot inlet, a cold inlet and a single mixed outlet. Inside is a temperature-sensitive element, usually wax, that expands and contracts as the mixed water warms or cools. It moves a shuttle that opens one inlet and closes the other, holding the outlet temperature steady even when someone runs another tap or flushes a toilet and the pressure shifts.

The safety feature that matters most is what happens when the cold supply stops. A proper thermostatic valve closes down the hot flow quickly instead of letting full-temperature water through. A basic blending or tempering valve, which simply mixes in a fixed ratio, does not react like that, and that is why the two should not be treated as the same thing.

Many thermostatic shower mixers already have a TMV built in, so a separate valve is not normally added to the shower. Separate TMVs are fitted in the pipework feeding a bath, basin or group of outlets, often under the bath panel or in a cupboard close by. If a shower mixer is the one drifting, see our guide to thermostatic shower temperature problems.

When a TMV is required by the rules

In England, Approved Document G of the Building Regulations requires that the hot water supplied to a fixed bath is limited to a maximum of 48°C. That applies when a dwelling is newly built or formed by a material change of use, such as a house split into flats or a commercial building converted to homes. In practice a TMV or a bath filler with a built-in thermostatic limit is how that is met.

Replacing a bath in an existing home does not trigger the same requirement. Even so, if you are refitting a bathroom it is the cheapest point to add one, because the bath panel is off and the pipework is exposed.

Rented homes, HMOs and care settings bring extra duties. Landlords have to manage legionella risk, which means hot water stored hot, and duty holders in health and social care are expected by HSE to control scalding risk where vulnerable people bathe. In those settings TMVs are normally expected rather than optional.

TMV2 or TMV3: which do you need?

TMV2 and TMV3 are approval schemes, not different mechanisms. A TMV2-approved valve has been tested to the British and European standards for domestic use. A TMV3-approved valve has been tested to the more demanding NHS specification for healthcare, including tighter performance when supply pressures change.

For a family home, a TMV2 valve is normally what you want. For a care home, hospital, nursery or supported housing, TMV3 is the usual expectation, and the valve has to be approved for the actual supply pressures on site, whether that is a low-pressure tank-fed system or mains pressure from a combi or unvented cylinder.

That last point catches people out. A valve approved only for high pressure may not perform properly on a gravity-fed system, and the reverse is also true. Check what hot water system you have before choosing a valve. Our unvented vs vented cylinder guide explains the difference if you are not sure.

What temperature should it be set to?

The Building Regulations limit is a maximum of 48°C at the bath. Healthcare guidance goes lower, commonly around 44°C for a bath and 41°C for showers and basins, because people in those settings may not be able to react to hot water or get out of the bath quickly.

In a family home, most people set the bath valve somewhere in the low to mid 40s. Set it with a thermometer at the outlet, not by feel, and do not turn the storage temperature of the cylinder down instead. HSE guidance is to store hot water at 60°C or above to control legionella, and the TMV is what makes that safe at the tap.

If a bath or basin suddenly runs scalding hot, stop using it and turn the hot supply off at the isolation valve if you can reach it safely. A TMV that no longer limits the temperature has failed at the one job it is there to do.

Where it should be fitted

Fit the valve as close to the outlet as practical. The pipe between the TMV and the tap carries water at body-warm temperatures, which is ideal for bacteria if it sits still. HSE's legionella guidance for hot and cold water systems asks for that length of blended pipework to be kept short.

The valve needs to stay reachable. It has isolation valves, strainers and non-return valves on the inlets that need cleaning, and the temperature has to be checked and adjusted. A TMV sealed behind tiling with no access panel becomes a problem the first time it needs a service. Our guide on where to plan plumbing access panels before finishing a room covers where to leave them.

Signs a TMV needs servicing or replacing

London's hard water is the main reason TMVs drift. Scale builds up on the element and shuttle, and the strainers clog with debris after work on the mains. Common signs are:

• Water that is only lukewarm at the bath, even with the cylinder or combi working normally
• Temperature that swings when another tap is run or a toilet is flushed
• Very slow flow from one outlet while others are fine
• Water that stays hot when the cold supply is turned off during testing, which means the fail-safe is not working
• A drip or weep from the valve body or its unions

Getting a TMV fitted, serviced or checked

Loyal Plumbers fits and services thermostatic mixing valves on baths and basins across London and everywhere inside the M25, usually as part of bathroom fitting or general plumbing work, or when a bath runs too hot or too cold. We check what hot water system feeds the valve, fit one that suits those pressures, set it at the outlet and leave access for future servicing.

Call us or send an enquiry with where the valve is, what hot water system you have and what has changed. If it is the shower that has lost its heat rather than the bath, our guide to no hot water in the shower but the taps are fine covers the likely causes first.