An air source heat pump moves heat rather than generating it, using refrigerant and a compressor to capture thermal energy from the outdoor air and deliver it indoors for heating and domestic hot water. This technology extracts warmth even when outside temperatures are cool, making it an efficient alternative to traditional combustion-based systems in many climates.
Core Components and Basic Layout
The primary elements of an air source heat pump include an outdoor coil, an indoor coil, a compressor, and an expansion valve, arranged in a closed loop that circulates refrigerant. Refrigerant absorbs heat as it evaporates in the outdoor evaporator, is compressed to a higher temperature and pressure, releases warmth in the indoor condenser, and then returns to the outdoor unit to repeat the cycle.
How Refrigerant Moves Heat
Evaporation in the Outdoor Coil
In the outdoor evaporator, low-pressure liquid refrigerant expands and evaporates, drawing in thermal energy from the ambient air even at temperatures well above absolute zero. A fan pulls air across the coil fins, improving heat transfer as the refrigerant changes from liquid to vapor without reaching boiling point at the prevailing pressure.
Compression and Temperature Rise
The vapor refrigerant is drawn into the compressor, where mechanical work raises its temperature and pressure significantly, transforming it into a high-temperature, high-pressure superheated vapor. This step is crucial because it moves the refrigerant to a temperature level that allows heat to flow into the indoor space when the condenser coil is colder than the refrigerant.
Condensation in the Indoor Coil
Inside the building, the hot refrigerant vapor flows through the indoor condenser, where it condenses back into a liquid as it transfers heat to the indoor air or to a hydronic heating distribution system. As the refrigerant changes phase, it releases the thermal energy collected outdoors plus the energy added by compression, providing efficient space heating and often domestic hot water preheating.
Expansion and Return to the Outdoors
After giving up its heat, the high-pressure liquid passes through an expansion valve or capillary tube, where its pressure and temperature drop sharply before returning to the outdoor evaporator. This drop prepares the refrigerant to absorb more outdoor heat, completing the closed-loop cycle that continuously moves thermal energy from outside to inside.
Heating and Cooling Modes
By reversing the flow of refrigerant with a four-way valve, the same outdoor and indoor coils can switch roles to provide cooling in summer and heating in winter. In cooling mode, the indoor coil acts as the evaporator that absorbs heat from indoor air, while the outdoor coil releases heat to the exterior, functioning much like a conventional air conditioner but often with greater efficiency.
Efficiency Factors and Performance Considerations
Coefficient of Performance depends on temperature differential, refrigerant properties, airflow, and system design, with well-installed units often delivering two to four units of heat for each unit of electricity consumed. Cold climates may require specialized refrigerants, larger outdoor coils, or auxiliary backup heating to maintain performance as outdoor temperatures fall, while proper sizing and airflow are essential to avoid efficiency losses and ensure consistent comfort.
