By definition, a heat pump is a machine which moves heat. Heat exists in all air at all temperatures down to “absolute zero” (-460 degrees F). In the winter, the heat pump draws heat from the outdoor air and circulates it through the ducts of your home. During the summer, it reverses the process and draws heat from your interior air and releases it outdoors. It will also dehumidify the indoor air as it cools it.
Because a heat pump does not burn fuel, it is safer and cleaner to run than a gas-powered furnace. A heat pump provides a more uniform temperature throughout a building. It does not produce a sudden blast of hot air as traditional furnaces do each time they kick on.
In the heat mode, heat pumps do not dry out the air the way traditional heaters do. The higher humidity maintained by heat pumps during cold weather provides for a healthier environment. Heat pumps are more efficient and cost less to run than electric furnaces. Because heat pumps are used year round (for cooling as well as heating needs), they cost less per hour of use (cost of purchase and installation divided by total number of hours used per year) than do individual heating and cooling systems, which each sit idle for a good part of the year.
The heat pump serves as an air conditioner by absorbing heat from indoor air and pumping it outdoors. The heat pump contains an indoor coil, which, in turn, contains a very cold liquid refrigerant. As indoor air passes over the indoor coil, the refrigerant-cooled coil absorbs heat from the air and so quickly cools that air. The cooled air cannot hold as much moisture as it did at a higher temperature. The excess moisture condenses on the outside of the coil, resulting in the dehumidification of the air. The cooled, dehumidified air is then fan forced into the duct system, which, in turn, circulates it throughout the building.
At the same time, the absorption of heat by the refrigerant turns the refrigerant from a liquid into a vapor. A compressor pumps the heat laden vapor through a vapor line to an outdoor coil which discharges the heat extracted from the indoor air. As the heat is discharged, the vapor is cooled and changes back into a liquid refrigerant. The refrigerant is then pumped back through a liquid line to the indoor coil and the cycle is repeated.
In addition to serving as an air conditioner, the heat pump contains a reversal valve which reverses the flow of refrigerant and thus allow the heat pump to serve as a heater during cold weather.
The heat pump serves as a heater by absorbing heat from outdoor air and pumping it indoors. All air, even cold winter air, contains a certain amount of heat. As the outdoor air passes over the outdoor coil, heat from that air is absorbed by the refrigerant contained inside the coil. This absorption of heat changes the refrigerant from a low temperature liquid to a low-temperature, low-pressure vapor. The vapor then passes through a compressor where it is compressed into a high pressure, high-temperature vapor. The hot vapor then circulates into the indoor coil. As indoor air passes over the indoor coil, it absorbs heat from the coil. The warmed air is then redistributed through the duct system.
As outdoor air passes over the outdoor coil, its heat is absorbed by the refrigerant contained inside that coil. The temperature of the outdoor air passing over the outdoor coil is reduced by about 10 F. This means that even if the outdoor temperature is above freezing (35-40) the air closest to the outdoor coil will be reduced to below freezing. This reduction in temperature will cause the moisture contained in that air to freeze and to form frost on the surface of the outdoor coil. When the coil is iced over, it must be defrosted.