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Air Source Heat Pumps
Air source heat pumps have traditionally been used in warmer climates than Alaska for both heating and air conditioning. However, recent models of ASHPs are able to provide space heating at lower temperatures, even below 32°F. ASHPs take heat from the ambient air and use electricityto “step up” the heat to a temperature usable for
space heating. They deliver heat through forced air distribution. In a traditional ducted ASHPsystem, heat is distributed to individual rooms by ducts. Mini-split ductless ASHPs have indoor wall-
mounted units that deliver heat directly to a room. Air-to-water heat pumps provide heated water for a hydronic distribution system or for domestic hot water uses.
How it works
An ASHP acts like a refrigerator running in reverse, using a refrigeration cycle to gather heat from the outdoor air and transfer it to a home’s interior. In an air-to-air heat pump, fluid (liquid refrigerant) travels through coils of pipe in the outside unit, heats up and evaporates into a gas. Then the gas is compressed (in a compressor) until it is hot enough for space heating. The heated gas passes through a set of indoor coils, where it transfers heat to the indoor air and condenses back into a liquid. A fan is used to move the heat into a room or through ducts, and the fluid moves through the cycle again. In an air-to-water heat pump, the heat from the refrigerant is transferred to water instead of air.
The efficiency of ASHPs is measured by the coefficient of performance, or COP. A higher COP indicates a more efficient appliance. ASHPs in warmer parts of Alaska might have a COP of 2-3, which corresponds to an efficiency of 200-300%. ASHPs can have efficiencies over 100% because unlike other appliances that convert fuel to heat, heat pumps instead use electricity to intensify naturally occurring heat. This means that occupants receive more heat than is contained in the electricity used to run the heat pump.
The COP of a heat pump depends on the outdoor temperature, and thus will fluctuate throughout a winter. Lower outdoor temperatures result in less efficient heat pumps, because there is less heat available in the atmosphere. Thus, it is important to select a model of ASHP that can work efficiently even at the coldest temperatures of the winter.