A dual-fuel heat pump is an electric air-source heat pump that uses an alternative natural fuel source like natural gas, LPG or oil, as an auxiliary or backup heat instead of electric resistance coils. It can be accomplished within a single integrated unit, or pieced together by combining a standard air-source heat pump with a furnace via a controller. The controller will determine which unit to operate based on outside temperature, efficiencies, and relative cost of fuels.