Air-source heat pumps draw heat from the outside air during the heating season and reject heat outside during the summer cooling season.
How Does an Air-Source Heat Pump Work? An air-source heat pump has three cycles: the heating cycle, the cooling cycle and the defrost cycle.
The Heating Cycle During the heating cycle, heat is taken from outdoor air and "pumped" indoors.
At the outdoor ambient balance point temperature, the heat pump's heating capacity is equal to the heat loss of the house.
Below this outdoor ambient temperature, the heat pump can supply only part of the heat required to keep the living space comfortable, and supplementary heat is required.
When the heat pump is operating in the heating mode without any supplementary heat, the air leaving it will be cooler than air heated by a normal furnace. Furnaces generally deliver air to the living space at between 55°C and 60°C. Heat pumps provide air in larger quantities at about 25°C to 45°C and tend to operate for longer periods.
The Cooling Cycle The cycle described above is reversed to cool the house during the summer. The unit takes heat out of the indoor air and rejects it outside.
During the cooling cycle, the heat pump also dehumidifies the indoor air. Moisture in the air passing over the indoor coil condenses on the coil's surface and is collected in a pan at the bottom of the coil. A condensate drain connects this pan to the house drain.
The Defrost Cycle If the outdoor temperature falls to near or below freezing when the heat pump is operating in the heating mode, moisture in the air passing over the outside coil will condense and freeze on it. The amount of frost buildup depends on the outdoor temperature and the amount of moisture in the air.