Thermostatic mixing valves (TMVs): what they do and who sets them up

Stored hot water is kept hot for good reasons, but water that hot can scald, particularly children, older people and anyone who cannot move away quickly. A thermostatic mixing valve, or TMV, sits between the hot supply and the outlet and blends in cold water so the water that comes out is held at a safer temperature. If the supply temperature or pressure changes, the valve adjusts.

How a TMV works

A TMV has a hot inlet, a cold inlet and a mixed outlet. Inside is a temperature-sensitive element that moves a shuttle to change how much hot and cold gets through. You set a target outlet temperature when the valve is commissioned, and the valve tries to hold it.

The important safety feature is what happens if one supply fails. A good TMV is designed to shut down, or nearly shut down, the flow if the cold supply is lost, rather than sending pure hot water to the outlet. How well it does that, and how quickly, depends on the valve and on it being installed and set up correctly.

A TMV is not the same as:

• a thermostatic shower valve or thermostatic bath shower mixer, which has a thermostatic cartridge built into the valve you operate (see electric vs mixer showers);
• an ordinary mixer tap, which mixes hot and cold but does not control the temperature for you (see mixer and bath taps);
• a thermostatic radiator valve, which controls a radiator, not drinking or washing water.

Some TMVs are separate valves hidden under a bath or behind a panel. Others are built into a tap.

Where they are used

• Baths, especially in new homes and conversions (see the rules below).
• Bidets and some basins, where a very hot supply could scald.
• Care homes, hospitals, schools and other care settings, where the people using the water may be at higher risk.
• Homes adapted for an older or disabled person, or where young children use the bath.

What the rules say about bath temperatures

In England, requirement G3(4) of Schedule 1 to the Building Regulations 2010 says the hot water supply to any fixed bath must be designed and installed with measures to make sure the temperature of the water that can be delivered to that bath does not exceed 48°C. That requirement applies when a dwelling is erected, or formed by certain material changes of use, such as a conversion into flats.

How to meet it is covered in Approved Document G on GOV.UK. A TMV on the bath supply is a common way, but read the Approved Document, or ask your installer or building control body, rather than assuming one particular product or setting.

Wales, Scotland and Northern Ireland have their own building regulations. Check the rules where you live. Care settings often follow additional health and safety guidance, set by the organisation running them.

Keeping stored hot water hot enough matters too, for reasons of its own. A TMV lets the stored water stay hot while the water at the tap is cooler. That trade-off is one reason they are used.

Fitting, commissioning and servicing

This is where TMVs differ from most tap jobs. A TMV only does its job if:

• it suits the water system it is on (for example, gravity fed, mains pressure or a combi supply);
• it is fitted the right way round, with any strainers and check valves the maker specifies;
• it is commissioned: set to the right outlet temperature with the actual hot and cold supplies, and tested to show it responds correctly if one supply fails;
• it is checked periodically, because scale and debris can make it sluggish or stop it working.

Commissioning involves measuring temperatures at the outlet and checking the fail-safe response. A plumber who fits TMVs regularly, or a specialist, should do this and leave you a record of the setting. In care settings there are usually formal test schedules. We do not give a fit-and-forget method here, because a wrongly set TMV can be worse than none: it looks like protection but is not.

If you have a TMV already, the useful things a householder can do are:

• notice changes, such as the bath running hotter, cooler or more slowly than it used to;
• keep the maker's instructions and any commissioning record;
• ask a plumber to check it if anything changes, or as the maker recommends.

In hard water areas, scale can affect TMVs as it does taps and showers; see limescale.

TMVs and your hot water system

If you have a hot water cylinder, how hot the stored water is and how it is heated affect how a TMV behaves. Our hot water cylinders and immersions guide explains the parts. With an unvented cylinder, any change to the hot water system also involves a competent installer; see unvented cylinder basics.

We have not linked a TMV product here. We could not find one on the retailer pages we checked today, and a TMV should be chosen by the person fitting and commissioning it.

Do I have to have a TMV on my bath?

For a new dwelling or certain conversions in England, the Building Regulations require measures to stop the water delivered to a fixed bath exceeding 48°C, and a TMV is a common way to meet that. For other homes, ask your installer or building control body what applies to your work.

Can I adjust my TMV myself?

Some have an adjustable cap, but changing the setting without measuring the outlet temperature and checking the fail-safe is not a good idea. Ask a plumber to recommission it.

Why has my bath gone lukewarm?

A TMV that is partly blocked with scale or debris, or a strainer that needs cleaning, can reduce flow or change the temperature. So can a change in the hot water supply. Ask a plumber to check and recommission the valve rather than taking it apart.