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Air Source Heat Pumps: A Sizing Guide


Air source heat pumps are becoming more and more popular with homeowners looking to reduce their heating bills and/or reduce their carbon emissions, but one of the most frequently asked questions is about size. How powerful does an air source heat pump need to be to adequately heat your home?

Air source heat pumps use a small amount of electricity to power a fan which draws in latent heat from the air outside the home. This heat is then compressed to increase the temperature even further so it can be passed to a heat exchanger to heat water in a cylinder. This hot water can then supply your taps and radiators or underfloor heating.

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What Size Air Source Heat Pump Do I Need?

Most air source heat pumps are between 4 and 15 kW in output which represents how powerful they are. The bigger your home and the higher your demand for hot water, the higher the output should be.

With the right size of system you should be able to get most or all of your hot water from an air source heat pump. However, if you get a heat pump which is too big it will only need to run for a short period to heat water to the desired temperature and will switch itself off and on again regularly. This is not an efficient way to operate and will cost you more in energy bills. A heat pump which is too small will not be able to produce enough hot water for your home.

To ensure you get the right size of air source heat pump you should find an MCS accredited heat pump installer as they will be able to accurately calculate the best size for your home. To give you a rough idea of the size range you might be looking at, here is some of our industry research. We found some average sizes based on the size of home and how well or poorly insulated it is.

Size of Home / Quality of InsulationApproximate ASHP Output (kW)
2 bed house / flat5
3 bed house – poorly insulated9
4 bed house – well insulated9
4 bed house – poorly insulated16
5 bed house – well insulated16

Sizing an air source heat pump is not a simple process, however, and requires an experienced professional to calculate the best size for your home.

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How Air Source Heat Pump Size is Calculated

  • Size & Type of Home

    A bigger home needs a bigger heat pump but the type of home you have will also factor in the sizing calculation. Each home has different sizes of room and it’s likely that you won’t want your whole home to be the same temperature. For example, you may prefer your bedroom to be cooler than your lounge and bathroom. On average most UK homes can be adequately heated by a heat pump between 4-12 kW in output.

  • Heat Losses

    Insulation is crucial when you have an air source heat pump. This is because heat pumps produce hot water but not to the same high temperature as a boiler. The water is slightly cooler but is delivered consistently rather than in one surge. If your home is poorly insulated the heat will be escaping as quickly as it is being produced.

    This is why newly built homes with higher insulation can install less powerful heat pumps than older homes. In order to retrofit a heat pump in an existing home, the insulation may need to be improved to satisfy building regulations. To qualify for the Renewable Heat Incentive (RHI) payments from the government you will need to provide a current Energy Performance Certificate to ensure your home is adequately insulated.

  • Type of Heat Delivery System

    The heat delivery system is how the heat is emitted in your home, i.e. radiators. This is important because heat pumps deliver hot water at a lower temperature than a boiler. To achieve the same levels of comfort you will need a system with a larger surface area such as bigger radiators, more radiators or underfloor heating.

  • Seasonal Outdoor Temperature

    As air source heat pumps are extracting heat from the air outside the temperature where you live will also need to factor into the installer’s calculations. The colder the air is (on average) where you live, the harder the heat pump will need to work to take in heat and the higher the output needs to be.

Finding the Best Air Source Heat Pump for Your Home

There is no heat pump size calculator as such because there are so many variable factors to consider. On average, it seems that professionals start their calculations based on 1 kW for every 10 metres squared and then take the home’s heat losses into consideration. Once the installer has the right size of heat pump installed in your home you will be able to enjoy heating and hot water all year round with lower costs and significantly less carbon emissions.

To get bespoke heat pump quotes with accurate size recommendations you should get no-obligation installation quotes from an MCS accredited heat pump installer in your area.

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