Heat
pump systems can be combined with natural gas, propane, or oil-fired heating
systems. These are known as add-on, dual-fuel, or hybrid heat pumps. During
the heating season, when the outdoor temperatures drop below the thermal (or
economic) "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 add-on 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, add-on 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.
