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Last updated: 01 April 2022

Air and Ground-Source Heat Pumps: Which Is the One for You?

Basic Principles of Air and Ground-Source Heat Pumps

Naturally, heat flows from a higher to a lower temperature. However, heat pumps are able to force the heat flow in the opposite direction, using a small amount of high-quality drive energy, such as electricity, fuel or high-temperature waste heat. This is how heat pumps can transfer heat from natural sources in the surroundings (air, ground and water) to a household or industrial building. Heat pumps have a cooling function as well. In this case, heat is sent in the opposite direction, from the device where it is cooled, into the outdoors at a higher temperature. In some cases, the excess heat made up from cooling is used to meet simultaneous heat demand. Both ground and air source heat pumps are eligible for payments through the Renewable Heat Incentive (RHI).

Ground-Source Heat Pumps

The new modern heat pump systems are very efficient. For each kilowatt of electricity used by the heat pump, 3 to 4 kilowatts of heat are delivered into the building. The efficiency of a ground-source heat pump (GSHP) system relies on the quality of the design and installation, but it can be improved even more when using a solar recharge of the ground.

1 1: A mixture of water and antifreeze is pumped into underground pipes, and the antifreeze mixture is warmed up by the constant temperature of the ground. The pipes are placed horizontally in trenches about 1-2 metres deep 2 2: The slightly warmed antifreeze mix is fed to a heat exchanger unit, where it transfers the heat to a refrigerant. The two liquids don’t mix, but the heat is transferred through thin heat exchanger plates. 3 3: The refrigerant gets heated up by the antifreeze mix and transforms into a gas form. A compressor heats up the gas further. 4 4: The hot refrigerant moves on to the condenser, which transfers the heat from the refrigerant gas to a water tank through another set of heat exchanger plates. 5 5: Hot water from the tank is circulated to your wet central heating system. 6 6: Cooled water is transported back to the water tank. 7 7: The cool water lowers the temperature of the refrigerant in the condenser. The refrigerant also passes through an expansion valve that further cools it down, returning it back into its liquid form. 8 8: The cooled refrigerant then in turn cools down the antifreeze mix in the heat exchanger, and the cold antifreeze mix continues its way back to the underground pipes, to be heated up again.
Horizontal GSHP

The GSHP systems are becoming pretty common especially in the USA, Canada, Sweden, Switzerland and Germany. The principles revolving around ground-source heat pumps were first developed by William Thomson, also known as Lord Kelvin, in the 1850s when he formulated the first and second law of thermodynamics. Since then, there has been continuous development in the field and the principles discovered by Lord Kelvin have been used commercially for more than 50 years now, being improved regularly on efficiency and reliability.

Henceforth, they now provide a proven, safe, cost-effective and environmentally friendly alternative to fossil fuels.

All the new buildings constructed in the UK were designed to meet the Building Regulations and are able to profit from a ground-source heat pump. These regulations have been introduced to ensure the conservation of fuel, reduce heat losses and create greater energy efficiency, in order to make sure all modern buildings need less heating.

For a well-insulated property, the size of the heat pump will be smaller and it will require smaller ground loops, thus will be less expensive.

A GSHP uses a ground loop or borehole in order to extract heat from the ground. The advantage of this system is that a constant temperature is available all through winter if and only if the ground loop is big enough. For larger buildings, the alternatives can also be attractive, including open-loop systems that extract heat from open water or an aquifer system:

  • water-source heat pumps.
  • marine-source heat pumps.

There is the possibility of exchanging heat with the outside air, but an air-source heat pump is less efficient when the outdoor temperature is low. In the UK, 40% of the CO2 emissions derive from heating and cooling of properties, thus the government is looking to implement sustainable and environmentally friendly technology such as heat pumps. Ground source heat pumps make up for 9% of heat pumps market share in the UK, and the scope for growth is much higher especially with the new builds. 

The video below explains how a ground source heat pump works:

Ground Source Heat Pumps - How do they work?

Like every piece of hardware, the ground-source system should be covered by an annual maintenance agreement with your supplier. It should be said that routine maintenance requirements are very low given to the fact that the life expectancy of such a heat pump is over 20 years (longer than a combustion boiler) - whilst the ground heat exchanger should be expected to last for at least 50 years.

Air-Source Heat Pumps

Air-source heat pumps (ASHP) are usually placed outdoors either at the side or back of a property. The heat is taken from the air and boosted to a higher temperature using a heat pump. Although the pump requires electricity in order to run, it should use less electrical energy than the heat it produces. Most of the ASHPs on the market are eligible for payment through the Renewable Heat Incentive (RHI). Air source heat pumps represent 87% of heat pump units in the UK and are the most sought after option among heat pumps. This is also because air source heat pumps are great for both retrofits and new builds.

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

Air to water heat pumps works much better with underfloor heating systems. If this system is not accessible, large radiators are recommended. This is required due to the fact that the heat developed by the heat pump is not as high as that delivered by a conventional boiler, thus a bigger surface area is needed to achieve identical temperatures in your household.

It is better suited to brand new properties because air source heat pump costs can be deducted if the heat pump is part of the building specification, rather than having to modify the underfloor heating later on. It is essential that your house is insulated, otherwise the heat the pump is generating escapes more easily.

The video below explains how an air source heat pump works:


If your budget does not allow you too much room for comfort, ASHPs are known to be cheaper than ground-source heat pumps. The estimated cost of installing a typical air-source heat pump at somewhere between 8,000 and 18,000 pounds.

The payback period varies from user to user. Firstly, it depends on how efficient your system works, how you will be using the heat generated by the pump, the type of system you are replacing and whether you can get money with the RHI.

For example, the EST estimates that an ordinary performing air-source heat pump in an average 4-bedroom home could save:

  • £545-£880 a year if replacing oil.
  • £550-£1,060 a year if replacing electric heating.

The RHI would also pay an extra £805-£1,280 a year if the appropriate air-source heat pump is used.

There are 2 main types of ASHPs:

  • Air to water systems - heat from the outside air is fed to your central heating system. As the heat produced is cooler than that from a conventional boiler, you may need to install larger radiators or underfloor heating in your home.
  • Air to air systems - takes heat from the outside air and feeds into your home through fans. This type of system, unfortunately, cannot produce hot water.

Both of the systems mentioned above require very little maintenance. You may also want to consider solar PV panels for a greener source of electricity if the area in which you live in is suitable.

ASHPs can help lower the carbon footprint that your household produces because it uses a renewable, natural source of heat-air. The amount of CO2 emissions you will end up saving depends on the fuel you are replacing it with. For example, it will be higher when replacing electric heating rather than natural gas.

In conclusion, whichever source of heat pump you choose, we at GreenMatch are here to help you make the best decision for you and your budget. Just fill out the form at the top right corner of this page, and get up to 4 different quotes. It is free and with no obligation.