Heat pump systems can be combined with natural gas, propane, or oil-fired heating systems. These are known as dual-fuel, piggy-back, or hybrid heat pumps. During the heating season, when the outdoor temperatures drop below the thermal "balance point" of the heat pump, the heat pump turns off and the gas or oil furnace comes on to provide heating. In other words, the electric resistance auxiliary heat found in conventional air-to-air heat pump systems is replaced by the gas or oil furnace.
The dual-fuel heat pump unit is popular for installation on older homes with central systems that were not previously air conditioned, or for homes whose cooling system needs "replacing" and the furnace is still in good shape. Besides providing air conditioning, dual-fuel heat pumps increase the "overall efficiency" of the heating system because both the heat pump and the fossil furnaces are operating at their "optimal efficiency" levels.
For new construction, this heat pump system may also be more cost effective than an all-electric heat pump, particularly in colder climates.