September 10, 2014
How do air source heat pumps work? Air source heat pumps have three cycles: Heating cycle: providing heat energy to the building; Cooling cycle: removing heat energy from the building; Defrost cycle: defrosting
Heating cycle
Components of the heating cycle of an air source heat pump
During the heating cycle, heat is taken from the outdoor air and "pumped" indoors.
1. The liquid refrigerant passes through the expansion device and becomes a low-pressure liquid/vapor mixture. It is then delivered to the outdoor coil, which acts as the evaporator coil. The liquid refrigerant absorbs heat from the outdoor air and boils, becoming a low-temperature vapor.
2. The vapor passes through the reversing valve into the accumulator, which collects any remaining liquid before the vapor enters the compressor. The vapor is then compressed, reducing its volume and warming it up.
3. The reversing valve delivers the now hot gas to the indoor coil, which is the condenser. The heat from the hot gas is transferred to the indoor air, causing the refrigerant to condense into liquid. This liquid returns to the expansion device and the cycle repeats. The indoor coil is located in the ductwork, near the furnace.
Note:
The ability of an air source heat pump to transfer heat from the outdoor air to the indoors depends on the outdoor temperature. As the temperature drops, the heat pump's ability to absorb heat also drops. For many air source heat pump units, this means there is a temperature (called the thermal balance point) when the heat pump's heating capacity equals the heat loss from the house. Below this outdoor ambient temperature, the heat pump can only provide part of the heat needed to keep the living space comfortable, and supplemental heat is required.
It is important to note that the vast majority of air source heat pumps have a minimum operating temperature below which they cannot operate. For newer models, this ranges from -15°C to -25°C. Below this temperature, an auxiliary system must be used to heat the building.
Cooling Cycle
Parts of an Air Source Heat Pump Cooling Cycle
In the summer, the above cycle is reversed to cool the house. The unit removes heat from the indoor air and expels it outdoors.
1. In the heating cycle, the liquid refrigerant passes through an expansion device, becoming a low-pressure liquid/vapor mixture. It then enters the indoor coil, which acts as an evaporator. The liquid refrigerant absorbs heat from the indoor air and boils, becoming a low-temperature vapor.
2. The vapor passes through the reversing valve into the accumulator, which collects any remaining liquid, and then into the compressor. The vapor is then compressed, reducing its volume and warming it up.
3. The now hot gas passes through the reversing valve into the outdoor coil, which acts as a condenser. The heat from the hot gas is transferred to the outdoor air, causing the refrigerant to condense into a liquid. This liquid returns to the expansion device, and the cycle repeats.
Note:
During the cooling cycle, the air source heat pump also dehumidifies the indoor air. The moisture in the air passing through the indoor coil condenses on the surface of the coil and collects in a pan at the bottom of the coil. A condensate drain connects this pan to the house drain.
