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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).
When the thermostat (via outdoor temperature sensor) senses the outdoor temperature is too cold for the heat pump to be used as the primary heat source, it automatically switches to a secondary heating source (gas, propane, oil or electric).