The bigger the size of heat pump, the better…right? Not really. When it comes to finding the right size heat pump for your house, there’s only a small margin for error: too big and the heat pump will short cycle; too small and the system will use the backup heater too often. Both cases will leave you with an inefficient and costly system.

It takes an experienced and certified heat pump installer with quality design calculation software to size a heat pump correctly. In fact, most underperforming heat pumps are the direct result of a poor understanding – or a complete lack of – suitable design software. Examples of this include the Energy Saving Trust’s heat pump survey and the BBC’s Rip Off Britain.

As this seems to happen all too often, we’ve put together a short guide to heat pump sizing, and how to ensure you get the right size heat pump for your house. If you’d like to find more information about the physical size of an air source heat pump or the area you’ll need for a ground source heat pump, take a look at the article ‘how much space do I need for a heat pump’.


Heat pump sizing

Heat pump sizes range from roughly 6kW – 15kW, and are usually quoted at the industry standard temperatures, which is 7oC/35oC. This means a 10kW air source heat pump will deliver 10kW of heat to your home if the air temperature is 7oC and the indoor flow temperature is 35oC.

Be it a small 6kW or large 15kW system, the size of heat pump you need for your home will ultimately depend on 3 things:

  • Outdoor design temperature
  • Desired room temperature
  • Flow temperature


Outdoor design temperature

Air source heat pumps absorb heat from the air by passing icy-cold refrigerant liquid through heat exchanger coils. The warmer the air temperature outside, the better the heat pump will perform. You can find out more about the heating system’s mechanics in the article ‘how does a heat pump work’.

To ensure that the heat pump can meet your home’s demand for heat all year round, heat pumps are sized based on the coldest air temperatures of the year for your area. For example, heat pumps installed in Manchester use an outdoor air temperature of around -2.6oC.

The outdoor design temperature for your area will be taken from the MCS database – the higher the coldest temperature is, the smaller your heat pump needs to be. Unfortunately, there’s not a lot you can do to change this, since the minimum outdoor temperature will depend on where your home is located in the country.

The only downside to this is whenever the temperature is above this ‘worst case’ air temperature, the heat pump is technically oversized for your heating demand. Other factors that weigh into the outdoor design temperature include the wind chill factor, which measures how sheltered or exposed to the elements your home is.

Surprisingly, the vast majority of inefficient heat pump installations are actually undersized, which is a result of the UK’s inefficient housing stock. Click here to find out more about home efficiency and what you can do to improve this.


Desired room temperature

The lower the room temperature, the better.

The desired room temperature is decided by the homeowner. Typically, most people want their homes to be a warm and cosy 21oC, but lower room temperatures will require less heating, so a smaller size heat pump can be used in your house.

The difference between outdoor and indoor temperatures can then be used to find out the total heat loss from a property.


Flow temperature

The smaller the heat loss, the smaller the size heat pump required. Similarly, the larger the radiators, the lower the flow temperature.

The flow temperature is simply the temperature water needs to be circulating around the radiators in your home to meet the desired room temperature.

This is affected by the size of your radiators (and underfloor heating if you have it) and the level of home insulation. If the radiators are too small, the flow temperature will need to be higher to meet the desired room temperature. Similarly, the better the insulation, the slower heat can dissipate outside, and the less heating is required to meet your indoor design temperature.

Once all these factors are established, the heat pump is sized to match both the heat demand (which is taken from the EPC) and the property’s heat loss. As a rule of thumb, a well-insulated house requires 1 kilowatt of heat for every 25m2 to be heated, whereas a poorly insulated house will heat only 10m2. You can use this to figure out what size heat pump your home should need, although naturally there are several other variables that need to be considered that may also influence the size of heat pump.


Ensure the right size heat pump is installed in your house

Confirm your installer’s credentials.

The installer should be MCS-certified (this is also crucial to receiving money from the Renewable Heat Incentive), and they should be a member of one of the two consumer codes; the Renewable Energy Consumer Code (RECC) or Home Insulation & Energy Systems Contractors Scheme (HIES).

It’s also worth finding out other heat pump installations the installer has carried out, and even getting in touch with the homeowners of these heat pumps. You can also check whether the heat pump comes from a reliable manufacturer, and the model is MCS-approved. However, if the installer’s credentials hold up, they should only recommend MCS-approved models.


Getting the correct size heat pump for your home isn’t as straightforward as you might think, however this step is crucial to achieving a high efficiency heat pump installation. We accurately size the heat pump based on the heat loss calculations and hot water sizing of your property. To find out more, just fill out our contact form. One of our Technical Account Managers will soon be in touch for a quick chat about your project and explain how we can help.


  • Hello
    Could you please tell me how big do i need air heat pump for five bed room house three stories terrace house

    • Hi there Harbhajan, many thanks for your enquiry.

      Heat pumps are accurately sized based on the heat demand of your home, which is why it’s very tricky to accurately estimate how big your heat pump would need to be. If you’d like to know for sure or get a free quote for your home, I can pass your email along to Sivvy, one of our representatives who can arrange a site visit to calculate the heat demand?

      Kind regards,

      Evergreen Energy

  • Please can you advise of the size of heating system required for underground heating for a garden room extension only, with a floor area of approx 26 sq m? Also what would it cost?

    • Hi Karen, thanks for getting in touch.

      Can you please clarify whether you’re enquiring about a ground source heat pump or underfloor heating for your garden room extension?

  • i have a 4 bedroom house 180 sq meters roughly how big air source will i need and how much would it approximate cost to install please

    • Unfortunately it’s difficult to say how big of an ASHP you would need just based on number of bedrooms and square metres. This is due to sizing of ASHP’s being highly dependent on the levels of insulation more than anything else. If such a property is poorly insulated with very old inefficient radiators you would probably need a 16kW ASHP with new radiators (anything from £10-£13k approximately) whereas if such a property is well insulated you might only need a 9kW-12kW ASHP with a couple of new radiators (£8-£11k). Please be advised that the Renewable Heat Incentive will cover a large portion of this cost.

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