These systems use air on both sides (on coils) and provide heating or cooling. In the cooling mode, heat is removed from the air in the space and discharged to the outside air. In the heating mode, heat is removed from the outside air and discharged to air in the space. In these units, it is necessary to provide defrost controls and periods to maintain maximum efficiency. These are the most popular systems for residential and commercial applications because of easy economical installation and lower maintenance cost.
Depending on climate, air-source heat pumps (including their supplementary resistance heat) are about 1.5 to 3 times more efficient than resistance heating alone. Operating efficiency has improved since the 1970s, making their operating cost generally competitive with combustion-based systems, depending on local fuel prices. With their outdoor unit subject to weathering, some maintenance should be expected.
The most popular heat pump is the air-source type (air-to-air) which operates in two basic modes:
• As an air-conditioner, a heat pump’s indoor coil (heat exchanger) extracts heat from the interior of a structure and pumps it to the coil in the unit outside where it is discharged to the air outside (hence the term air-to-air heat pump), and
• As a heating device the heat pump’s outdoor coil (heat exchanger) extracts heat from the air outside and pumps it indoors where it is discharged to the air inside.