How does a thermostatic mixing valve work?

Updated April 17, 2017

A thermostatic mixing valve mixes hot water with cold water in a hot water heat distribution system to prevent scalding when the water is in use and to maintain safe temperatures that kill bacteria within the system.


Fluctuations in water temperature alternately expand and contract the thermostat element. This causes movement of the slide valve to regulate the amount of hot and cold water in the thermostatic mixing valve to maintain the temperature of the mixed water.


Thermostatic mixing valves feature a rapid shut-off in case of a supply failure of either hot or cold water. This acts as a safeguard against scalding to users and against thermal shock to the thermostat element.


A thermostatic mixing valve helps maintain a constant temperature when the water is in use. Water stored at high temperature kills bacteria that would otherwise thrive in standing water.

Common Uses

Thermostatic mixing valves are used in single point applications such as showers, tub fillers and hand wash basin mixers.

Other Uses

The high flow rates of thermostatic mixing valves make them ideal for use in underfloor heating systems that use water as a medium.

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About the Author

Chris Passas is a freelance writer from Nags Head, N.C. He graduated from East Carolina University in 2002 with a bachelor's degree in journalism. He has written online instructional articles since September 2009.