Air source heat pumps absorb heat from outside air into a fluid in the pump.

Requires: External space for installation.

The fluid is then compressed by the pump and heats up. It is this heat that is transferred to the heating circuits of the building. Air source heat pumps are much less efficient when it is cold outside. In these conditions, air source heat pumps will often need to run continuously but they can still extract heat in external temperatures as low as -15°C, although less efficiently (and therefore at higher running costs). (1)

Air source heat pumps can be air-to-water or air-to-air. Air-to-water extracts heat energy from the air outside and transfers it into water, which is then circulated around a building using a wet central heating system. Air-to-air on the other hand extracts heat energy from the air outside and transfers it into air inside, which is then circulated around a building using a warm air central heating system.

Controllable air-to-air systems, with the ability to modulate how much heating (or cooling) they deliver, where, and when, are called variable refrigerant flow systems. (2) As air-to-air systems cannot provide hot water, they are typically used in conjunction with solar water heating. They are less common than air-to-water systems in the UK. (1)

Air source heat pumps have external units. Ideally the external unit should be installed in a sunny spot, against a wall or on the ground outside a building with ample space to have good air circulation. The external units are then connected through the wall to internal units which link with the building’s warm air central heating system. These internal units are usually smaller than standard fossil fuel boilers.

The noise levels created by air source heat pumps are often cited as a concern, however the sound characteristics (and those of all types of heat pumps) are stipulated by law. Nevertheless, the design of the installation must consider whether even this low level of noise could be problematic.

(1) Energy Saving Trust, Generating Renewable Energy, Air Source Heat Pumps. Available from: https://energysavingtrust.org.uk/advice/air-source-heat-pumps/ [Accessed 26^th January 2021].

(2) Washington State University Energy Program [sic], Emerging Technologies, Variable Refrigerant Flow (VRF) Heat Pumps, Summary. Available from: http://e3tnw.org/ItemDetail.aspx?id=200 [Accessed 26^th January 2021].