Innovative and efficient, air source heat pumps (ASHPs) use outside air as a heat source and heat sink. They utilize a compressor and a condenser to absorb heat in one place and release it in another. When heating a space during the winter, an ASHP absorbs heat from outside air and releases it inside. Remember that outside air at any temperature above absolute zero contains some heat. Air source heat pumps simply move this heat to provide space heating. In summertime, the system reverses the refrigerant flow to cool the interior of the home or building.
ASHPs offer a full central heating and cooling option, and they can provide fairly low-cost space heating. Standard air source heat pumps found in most homes can extract useful heat down to about 5°F.
Air source heat pumps contain two main components: the outdoor heat exchanger coil, which extracts heat from ambient air, and the indoor heat exchanger coil, which transfers heat into the indoor heating system.
Lifespan and Maintenance
Although the life expectancy of air source heat pumps varies, many last for 20 years or more. In fact, heat pumps installed two to three decades ago are still in service today, even in places with extremely cold winters. To extend the life of your air source heat pump, don’t neglect maintenance duties. For example, the outdoor unit and fan must be kept free from debris, and filters on the indoor section must be changed from time to time.
Auxiliary and Emergency Heat Systems
ASHPs are often paired with auxiliary or emergency heat systems to provide backup heat when outside temperatures are too low for the pump to work efficiently. They also come in handy in the event the pump malfunctions. Propane, natural gas, and electric furnaces can provide this auxiliary heating requirement.
Sometimes the outdoor section on units will “frost up” when outdoor temperatures are 32-41°F or colder and there is sufficient moisture in the air, which restricts air flow across the outdoor coil. Note that as the temperature drops below freezing, the need to defrost the outdoor section typically lowers due to the air’s reduced relative humidity.
When defrosting is needed, the heat pump unit can use a defrost cycle in which the system switches to A/C mode to transfer heat from the home to the condenser to melt the ice. This requires the supplementary heater (resistance electric or gas) in the indoor section to activate and temper the distributed cold air. Although the defrost cycle reduces the efficiency of the heat pump significantly, newer control systems are more intelligent and require less defrosting.
A high-efficiency heat pump can provide up to four times as much heat as an electric heater using the same energy. The warmer the outside temperature, the more efficient the air source heat pump will be.
To measure the efficiency of an air source heat pump, you will need to determine the coefficient of performance (COP). A COP of three conveys that the heat pump produces three units of heat energy for every one unit of electricity it consumes. In mild weather, the COP of an air source heat pump can reach up to four. On a very cold winter day, it takes more work to move the same amount of heat indoors than on a mild day, so you may see a COP closer to one.