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Scald risk and the thermostatic mixing valve

Why hot water is stored hot but delivered cooler through a TMV.

Schematic showing scald-risk control. A hot store kept at around 60 °Celsius and a cold supply both feed a thermostatic mixing valve, which blends them to a safe delivery temperature; a fixed bath is limited to a maximum of 48 °Celsius under Approved Document G.
Scald risk and thermostatic mixing valve schematic

Stored hot water has to satisfy two opposing needs. It must be hot enough to limit bacteria such as Legionella, but that same temperature can scald at the tap — and the hotter the water, the faster it scalds.

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