TMV Installation & Commissioning Guide
A thermostatic mixing valve blends hot and cold to a safe outlet temperature and shuts down if the cold fails — but only if it is installed on equal pressures and commissioned properly.
A thermostatic mixing valve, or TMV, sits between the hot and cold supplies and delivers a blended output at a safe, stable temperature. Its purpose is scald prevention: stored hot water is held hot enough to control legionella, which is too hot to touch, so the TMV brings it down at the point of use and fails safe by shutting off the flow if the cold supply is lost.
This guide covers what a TMV does, where one is required, the difference between TMV2 and TMV3 approvals, how to install it correctly, and how to commission it — including the cold-water failure test that proves it fails safe. It is written for the installer who has to leave the valve set, tested and recorded, not just plumbed in.
Key takeaways
- A TMV blends hot and cold to a safe outlet temperature and fails safe by shutting off if the cold supply fails.
- In domestic bathrooms, Part G of the Building Regulations limits the hot water temperature at a bath to a maximum of around 44°C, which a TMV delivers.
- TMV2 is the approval scheme for domestic use; TMV3 (BuildCert) covers healthcare and care settings per the relevant HTM / D08 guidance.
- Install the valve accessibly, with isolation and strainers, and on balanced (equal) hot and cold pressures for stable blending.
- Commission by setting the blend temperature and carrying out a cold-water failure test, then record the results.
What a TMV does and where it is required
A TMV modulates the mix of hot and cold water to hold the outlet at a set temperature, and its thermostatic element reacts to a change in either supply. The critical safety behaviour is the cold-water failure response: if the cold supply is lost, the element shuts the valve down rather than letting scalding hot water pass through to the outlet. That fail-safe is what makes it a scald-prevention device rather than just a mixer.
In domestic work, Part G of the Building Regulations requires the hot water temperature at a bath to be limited to a maximum of around 44°C to prevent scalding, and a TMV is the standard way to achieve that. In healthcare and care settings the requirement is more stringent and is governed by the relevant Health Technical Memorandum and D08 guidance, which set out where TMVs must be fitted and how they are managed for vulnerable users.
Blend at the point of use
The TMV does not replace storing water hot enough to control legionella — it lets you store hot and still deliver safe. Keep the stored temperature where it needs to be and let the valve bring it down at the tap or bath.
TMV2 versus TMV3
TMVs are certified against two schemes. TMV2 is the scheme for domestic and general commercial applications, testing the valve against the performance requirements for ordinary use. TMV3 is the higher-duty scheme, administered under BuildCert, for healthcare and care environments where users may be unable to react to a scald — it tests to the tighter tolerances and duties set out in the healthcare guidance.
Specify the right class for the setting. A domestic bathroom is a TMV2 application; a hospital, care home or other healthcare setting calls for a TMV3 valve installed and maintained in line with the HTM and D08 regime. Fitting a TMV2 valve where TMV3 is required does not meet the standard, so confirm the intended use before you order the valve.
Installation
Site the valve where it can be reached for commissioning and maintenance — behind an access panel rather than buried in a wall. Fit isolating valves on the hot and cold inlets so the valve can be shut down and serviced, and fit the line strainers supplied with the valve to keep debris off the thermostatic element and seats, since grit is a common cause of drift and failure.
Blending stability depends on balanced supplies, so aim for equal, or nominally equal, hot and cold pressures at the valve — a high-pressure hot against a low-pressure cold, or vice versa, will make the blend wander. Keep the pipe runs from the valve to the outlet as short as is practical to reduce dead-leg and give a quick response, and observe the manufacturer’s marked flow direction and orientation.
Commissioning and recording
Once installed, set the blend temperature to the value required for the application — for a domestic bath that means the maximum in the region of 44°C called for by Part G — measuring the mixed water at the outlet with a calibrated thermometer and adjusting the valve to suit. Let the temperature stabilise before you take the reading rather than reacting to the first flush of water.
Then carry out the cold-water failure test: with the outlet running, isolate the cold supply and confirm the valve shuts down so that the outlet temperature does not rise beyond the small permitted increase. A valve that lets the temperature climb has failed the test and must be rectified or replaced before use. Record the set temperature, the failure-test result and the date so the commissioning is evidenced and the valve can be re-checked on schedule.
Issue the certificate on site
TradePlanr lets you capture the set blend temperature and the cold-water failure result and produce the TMV commissioning certificate before you leave, so the record travels with the valve rather than getting lost.
Frequently asked questions
What temperature should a domestic bath TMV be set to?
Part G of the Building Regulations limits the hot water temperature at a bath to a maximum of around 44°C to prevent scalding. Set and measure the blended outlet at the required value and record it during commissioning.
What is the difference between TMV2 and TMV3?
TMV2 is the certification scheme for domestic and general use, while TMV3, administered under BuildCert, is the higher-duty scheme for healthcare and care settings where users may not be able to react to a scald. Specify the class that matches the setting.
How do you commission a TMV?
Set the blend temperature at the outlet with a calibrated thermometer, then carry out a cold-water failure test by isolating the cold supply and confirming the valve shuts down before the outlet temperature rises significantly. Record the set temperature, the test result and the date.
From guidance to action
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