Air Source Heat Pumps


An air source heat pump (ASHP) is able to extract heat from the external air and by using the principles of vapour compression refrigeration, transmit this heat for use inside a building. It basically functions the same way as your fridge, but in reverse.

Outside air carries heat which can be extracted by the ASHP, right down to absolute zero, although in practice the majority of systems will function down to -15˚C. The air source heat pump can also be used to provide cooling for a building, in the UK this rarely required, due to our temperate climate.

The air source heat pump consists of three main pieces of equipment, the external heat exchanger coil (to extract heat from the air), the internal heat exchanger coil (to emit heat into the building) and some kind of control panel, which is quite often installed in the internal unit.

The heat pumps are able to transmit heat by refrigerant circuit or by using a circulated water system, the latter being more popular due to the ability to connect into an existing ‘wet’ heating system.

The benefit of an air source heat pump is down to its efficiency of 4:1, meaning that 4 units of heat are generated from one unit of energy input. The efficiency drops off as the external temperature falls and some systems will stop working altogether at temperatures around 0˚C, which is quite often when you need heating the most.

It is for this reason you should carefully research the technology in relation to your project, picking the wrong piece of equipment can have disastrous consequences. It is also worth treating this renewable technology as a supplementary heating source, making the most of the efficiency, but also having a backup supply of heat for when the ASHP is out of action.

Another point to note, is that air and ground source heat pumps, maintain the stated efficiencies with a flow temperature of 35˚C to 45˚C. This is fine if you have underfloor heating, as you will have exactly the right temperature required, however, domestic hot water and traditional radiator circuits will require a higher flow temperature (minimum 60˚C for hot water and 80˚C for heating).

You can oversize radiators to account for the lower flow temperatures, although, this takes up more space. The flow temperature of the heat pump can be turned up, however, this then adversely affects the efficiency. When you add in the fact that most homeowners will use an electric immersion heater to bring the domestic hot water temperature up to the minimum 60˚C, to reduce the risk of legionella, the energy efficiency benefits for air source heat pumps are not as good as they appear.