How to Fit a Thermostatic Mixing Valve — UK Guide
A TMV blends hot and cold to a safe pre-set temperature at the outlet. Mandatory for new UK bath and shower installations under Part G.
Step-by-step method
1. TMV2 vs TMV3
TMV2 (BuildCert) for domestic. TMV3 for healthcare/care homes. Both use same principles.
2. Fit close to outlet
Under sink or behind bath panel. Blended pipe run to outlet <2m for accurate temperature.
3. Wire strainers on inlets
TMVs are damaged by debris. Every install must have inline strainers on both hot and cold inputs.
4. Connect and set
Hot to hot inlet, cold to cold inlet, blended to outlet. Set dial to 43°C (bath) or 41°C (shower). Test with a digital thermometer at outlet.
5. Annual thermostat test
Turn cold off — hot should reduce to a trickle within 5 seconds. If not, replace the cartridge.
Frequently asked questions
Is a TMV legally required?+
Yes for new baths (Part G4), and for hot water outlets in care homes/schools. Retrofit not required but strongly advised.
TMV vs thermostatic shower valve?+
TMV lives in pipework and blends for any outlet. Thermostatic shower valve is a TMV built into the shower mixer body. Functionally the same.