A thermostat is a device for regulating the temperature of a system so that the system's temperature is maintained near a desired set point temperature.

The thermostat does this by controlling the heat flow (often the water flow) into or out of the system. That is, the thermostat controls the output of heating or cooling devices needed to maintain the correct temperature.

Thermostats can be constructed in many ways and use a variety of sensors to measure temperature. 
Common sensors include:

  • Gas filled sealed bellows
  • Bi-metal mechanical sensors
  • Expanding wax pellets
  • Electronic sensors