Dual fuel systems consist of a heat pump and gas/propane furnace. In the summer time, they will condition the air like an air conditioner. Then, when the system calls for heat, it will run in heat pump mode. At a certain set point, the system will switch to the furnace for heat. The set point for the switch can be set to either outside temperature or a calculation for which utility would be cheaper to operate. A dual fuel system offers you the best of both worlds by giving you the low operational cost of a heat pump and when the temperature drops low, the comfort of a gas furnace.