A thermostatic mixing valve is a temperature control valve, that works by mixing hot water from your hot water system, with cold water to provide a constant safe temperature at your water faucet outlet.
In order to prevent bacteria from breeding in the water supply, water must be stored at a very high temperature, often above 60 °C. Because this is too hot for safe general use; the thermostatic mixing valve adds the right amount of cold water to the water flow, bringing the water temperature down to a more user friendly level of around 48 °C.
Most thermostatic mixing valves are now made using a wax thermostat. To prevent injury, they are designed with an auto shut-off function in the event of a hot or cold water supply failure.