The air source heat pump is positioned outside and has a fan that draws in outdoor air over a large heat exchanger which harnesses the heat energy in the air, even at minus degrees, and transfers it to the refrigerant, triggering the refrigeration cycle. See ‘How a Heat Pump Works’. Cold air is expelled from the heat pump once the heat energy has been extracted.
There are a number of different types of air source heat pump:
Air to Water Air Source Heat Pump
The refrigeration cycle takes place inside the outdoor unit with a hot water flow and return into and from the building via insulated pipe work delivering heating and domestic hot water.
Split System Air Source Heat Pump
The refrigerant is heated across the heat exchanger in the outdoor unit and then passes inside the building where it is compressed in an indoor unit transferring heat to water for space heating and domestic hot water.
Air to Air Heat Pump
The refrigeration cycle takes place in the outdoor unit and hot gas (or cold gas when cooling) is passed inside to a fan convector mounted on the wall or in the ceiling. Hot or cold air is distributed into the room via the fan convector. This type does not heat water.
The fan in the outdoor part of the air source heat pump can be heard and therefore where it is located requires consideration. Higher quality air source heat pumps tend to be quieter and function at lower temperatures, typically down to - 20° C but the efficiency reduces at colder temperatures. Having said that the efficiency should be looked at across the year as the converse is true and efficiencies are high when the ambient air temperature is higher.
The advantage of air source heat pumps is that there is no collector pipe or groundworks required and it is ideal where space and access are limited. Installation is simpler than with ground source heat pumps and therefore faster and cheaper. The efficiency of the latest generations of air source heat pumps is high but doesn’t quite match the efficiency of ground source heat pumps.