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.
A suitable anti-scald TMV shuts off its hot flow on cold-supply failure when it operates within the specified conditions. The required hot-to-mixed temperature difference, flow and inlet pressure limits matter; prove the safety response during commissioning and subsequent checks.
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.
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 monitoring, check the unblended hot supply before the valve, because checking the cooler mixed outlet serves a different purpose.
Where the cylinder thermostat sits, and what happens if it drifts
A strap-on cylinder thermostat is normally fitted about a third of the way up the cylinder from its base, with the strap pulled tight for good thermal contact, so it responds to the bulk of the stored water rather than only the hottest layer at the top or the cold feed entering at the bottom. It is typically set to around 60°C: set it too high and you add to the scald risk downstream, set it too low and you increase the risk of Legionella growth.
If a newly fitted TMV runs weak and lukewarm
Verify the hot and cold ports, blend setting, inlet strainers and check valves. Measure the inlet temperatures, flow and working pressures against the valve’s requirements. A low blend setting can explain cool delivery without explaining weak flow, so account for both symptoms. After repair, recommission the temperature control and cold-failure response.