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Last updated: 19 April 2022

How Do Air Source Heat Pumps Work?

Air Source Heat Pumps Explained

Air source heat pumps (ASHP) is a process that by using the principle of vapour compression, transfers the hot air from a place to another exactly in the same way as the system of a refrigerator does.

AIR SOURCE HEAT PUMPS - How Do They Work? │GreenMatch

Before looking at the details of the technology, it is important to note that air at a temperature above absolute zero always contains some heat and many of these heat pumps manage to extract heat even at low temperatures as -15 C degrees.

Air source heat pumps systems consist of four major elements that allow the refrigerant to pass from the liquid state to the gas:

  1. A compressor
  2. A condenser
  3. An expansion valve
  4. An evaporator

When the refrigerant passes through the heating system, the high temperature (usually 100 degrees or more) transforms it into vapour or gas while the energy produces heat.

The gas then goes through the compressor that increases its temperature, and then through the expansion valve that makes the hot air enter the building.

Next, the hot air passes in a condenser that turns the gas into liquid again. The heat produced by the energy in the evaporation phase passes through the heat exchanger again to restart the cycle and it is used to make the radiators work, for underfloor heating (air-to-air system) or for domestic hot water (air-to-water heat pump system).

The interactive graphic below illustrates how the process works. You can click each point to read about the steps in more detail:

1 1: An outdoor unit draws in heat from the ambient air with fans, and then blows it over a heat exchanger coil. 2 2: The heat is transferred to a cold coolant flowing in the coil. Due to its very low boiling point, the heat only turns the coolant into a slightly warm vapour. 3 3: The slightly warm coolant vapour passes through a compressor that increases its temperature and compresses it to a denser vapour. 4 4: This hot coolant vapour is then transferred to the indoor unit(s) through internal piping systems. 5 5: The indoor unit blows out the heat from the coolant to the indoor air with fans, thereby warming up the room. 6 6: Once the coolant has transferred some of its heat to the air, it cools back into a liquid. 7 7: The liquid coolant passes through an expansion valve, which forces it to let go of more of its heat and mix with air, thereby creating a liquid vapour mixture. 8 8: The cold liquid vapour coolant mixture is then circulated back to the outdoor unit, to be heated again.
ASHP for Heating

Measures of Efficiency and Benefits of Air Source Heat Pumps

Air source heat pumps performances are measured through a Coefficient of Performance (COP) that can have different values that mean how many units of heat are produced using one unit of energy.

There are many advantages of air source heat pumps, both on environmental and economic sides. There are also differences between air to air and air to water heat pumps, and we have compared them extensively in the video below.

Air to Water VS Air to Air Heat Pump: A Comprehensive Comparison

First of all, air source heat pumps don't have an environmental impact as significant as the heat they use for the process is extracted either by air, water or ground and it is continuously regenerated although they still make use of electricity in the process.

On the financial side, air source heat pump cost can be reduced with the help of the State through the Renewable Heat Incentive, and householders can reduce carbon emissions by cutting on harmful fuels.

Furthermore, this technology does not need frequent maintenance but it usually works smoothly after the installation and it is cheaper to install than ground source pumps as it does not need any kind of excavation site.

However, it could be less efficient than the ground pump and its performance can be negatively affected by low temperatures and it usually needs a longer time and bigger surfaces to heat the interiors.

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Aris Vourvoulias
Written by Aris Vourvoulias, Former Writer

Aris Vourvoulias is the Head of Content in GreenMatch. Aris is a passionate author and marketer with an educational background in journalism. He continuously writes, reviews, and educates himself in the areas of business, finance, and renewable energy. He has managerial experience in many European markets, including UK, Denmark, Sweden, and Finland. He and his content team have been featured on reputable sites like GreenPeace, Guardian, iNews, Gizmodo, and more