A thermostatic mixing valve mixes hot and cold water to make sure that what comes out of the shower or bath faucet does not scald the user. This tool not only helps to prevent burns, but it also prevents thermal shock, which causes shower users to fall down as a reaction to scalding hot water.
The water that a home uses needs to be kept at an extremely high temperature, usually above 140 degrees Fahrenheit, to prevent Legionella from breeding. A thermostatic mixing valve makes sure that the temperature comes out at a comfortable temperature, which is usually 120 degrees. Most thermostatic mixing valves use a wax thermostat.