Heat pumps commonly use an outside source of air to evaporate or condense the refrigerant. These are known as air-to-air Heat Pumps. They operate differently in the heating or cooling cycles, reversing the evaporation of the condensing process from indoors to outdoors, transferring the heat. Essentially, they are air conditioners, with the added ability to reverse the refrigerant flow and to the condensing or evaporating cycle. Split air-to-air Heat Pump Systems are comprised of both an outdoor unit, which will collect or discharge heat. A Heat Pump is composed of a compressor, outdoor coil, fan, and reversing valve, all of which are normally located within the outdoor unit. In some instances, compressors are located on a third indoor section of a house.
Most air-to-air Heat Pumps are installed along with an existing secondary, or backup heat source. This secondary heating can be provided by an oil or gas furnace, or a simple air handling unit with electric elements or an additional hydronic coil that can transfer extra heat via a hot water supply.