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dual fuel, heat pump w/gas back-up. I installed 15 of those system this past year and have one in my house too. You'll have some sort of outside temperature sensing, either via a sensor through the thermostat or a temp switch at the heat pump. For instance; anything above 35 degF runs in the heat pump mode and outside temps below 35 degF runs the gas furnace. The systems are usually accompanied by "condensing" gas furnaces, using schd 40 PVC as the flue...