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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).
A thermostat is a component of a HVAC control system which senses the temperature of a system so that the system's temperature is maintained near a desired setpoint.he thermostat does this by switching heating or cooling devices on or off, or regulating the flow of a heat transfer fluid as needed, to maintain the correct temperature. A thermostat may be a control unit for a heating or cooling system or a component part of or air conditioner.