A dual Fuel System combines a Heat Pump with a conventional Gas/Electric System. In the Heating Mode, the heat pump will be used until the temperature outside drops below about 35ºF. Below 35º, the system will switch to gas. Comfort wise, this type of system is the best of both worlds; combining the energy efficiency of a Heat Pump during mild weather and the warm air comfort of Gas when it’s cold outside. I usually recommend dual Fuel for Propane users. The high cost of Propane is minimized by only using it when it is really cold outside. Temperatures in East Tennessee are typically below 35º less than 1000 hours per year. The rest of the heating duty is handled by the heat pump.