What a TMV does, why hot water is stored hot but delivered cooler, and where one is expected.
A thermostatic mixing valve, or TMV, solves a real conflict in hot water systems: water needs to be stored hot enough to discourage bacteria, but that same water is hot enough to scald. The TMV lets you have both.
It works by blending. The valve mixes stored hot water with cold to deliver a chosen, steady temperature at the outlet, and a temperature-sensitive element keeps adjusting the blend so the delivered temperature stays put even if pressures change a little.
That is why systems store water hot for hygiene but deliver it cooler at the point of use. The storage temperature controls bacteria; the TMV controls what reaches the skin. HSE's HSG274 Part 2 describes TMVs blending to a temperature that guards against scalding, typically between 38 °C and 46 °C depending on what the outlet is used for. It also warns that blended water downstream of a TMV can let legionella multiply, so their use follows a risk assessment. For the same reason, fit each TMV as close to its outlet as possible, ideally built into the tap, so that little blended water is left standing; one valve serving several outlets raises the risk. TMVs need routine maintenance by a competent person to the maker's instructions, annually unless the risk assessment sets another frequency, with their strainers or filters cleaned and descaled.
A suitable anti-scald TMV shuts off its hot flow if the cold supply fails, as long as it is working within its specified conditions: the hot-to-mixed temperature difference, the flow and the inlet pressure limits. Prove that safety response at commissioning and at later checks.
Mixing valves are certified under two UK schemes. TMV2 is designed, built and tested for domestic housing; its valves conform to BS EN 1111 for high-pressure supplies and/or BS EN 1287 for low-pressure supplies, such as a gravity-fed shower from a loft cistern, so choose one approved for the supply it will serve. TMV3 meets the higher NHS Estates D08 specification and is widely specified in healthcare and in places with a duty of care, such as care homes.
The diagram illustrates a new-dwelling bath in England: hot storage at 60 °C or more for the conventional hygiene regime, and a bath supply capped at 48 °C, the maximum for a fixed bath in a new dwelling. That figure is a ceiling: where vulnerable people are at risk during whole-body immersion, water must not exceed 44 °C and may need to be cooler. Select the outlet limit for the users and application. The guidance to the Water Fittings Regulations sets one such limit: terminal fittings and communal showers in schools, public buildings and other facilities used by the public are supplied through thermostatic mixing valves so that the water at the outlets does not exceed 43 °C.
Prove the protection during commissioning: check the supply conditions against the valve limits, measure the actual outlet temperature and test its cold-supply-failure response. Keep the valve maintained and checked. The hot inlet and mixed outlet have different temperature targets: for hygiene checks, measure the unblended hot supply before the valve; the cooler mixed outlet is checked for a different reason.
Where the cylinder thermostat sits, and what happens if it drifts
A traditional strap-on cylinder thermostat is commonly positioned about a third of the way up from the base, with firm thermal contact. This lets it sense the stored water away from the hottest upper layer and cold-feed entry. Use the position and fixing method specified for the actual cylinder and control; built-in sensor pockets and other designs follow their own instructions.
A setting around 60 °C is common under the conventional regime for Legionella control. Check the stored water actually reaches the required temperature: do not rely on the dial. Higher storage temperatures increase the outlet scald hazard; lower settings must still satisfy the design's hygiene controls.
If a newly fitted TMV runs weak and lukewarm
Check the hot and cold ports are connected the right way round, and check the blend setting, inlet strainers and check valves. Measure the inlet temperatures, flow and working pressures against the valve’s requirements. A low blend setting explains cool water but not weak flow, so find the cause of both. After the repair, recommission the temperature control and the cold-failure response.