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Add-on Heat Pumps
Heat pump systems can be combined with natural gas, LP, or oil fired heating
systems. These are known as add-on, piggy-back, 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 may increase the overall efficiency of the heating system because both the heat pump and the fossil furnaces are operating at their optimal efficiency levels.
