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The power is supplied by a transformer, and when the thermostat makes contact between the 24 volt power and one or two of the other wires, a relay back at the heating/cooling unit activates the corresponding heat/fan/cool function of the unit(s).
Along with the radiator cap, the thermostat is the second of the 5 major components that provide system monitoring and control making it a very important component of the system. The operation of a thermostat is controlled by a sealed copper cup containing wax and a metal pellet.