What Size Boiler Do I Need?

While it’s difficult to define a boiler sizing rule of thumb, it is possible to work out approximately what size boiler you need based on some basic information. Here’s how to find the right size boiler for your home…

The size of the boiler you require is also known as the output. It’s measured in kW (kilowatts) and generally the higher the number of kW, the more powerful the boiler will be. Domestic boilers usually range from 24kW to 42kW in size.


You need to find a boiler that is just right; a boiler that is too small will not be able to heat your home adequately or produce enough hot water. A boiler that is too big will use more fuel, making your energy bills more expensive and your carbon emissions unnecessarily high.

Boiler Size by Number of Bedrooms

We regularly see questions like ‘what’s the right size boiler for a 4 bed semi-detached house’ or ‘best boiler for a 2 bed terrace’. In reality, rather than recommending a boiler based on whether your home is terraced, semi-detached or detached, installers will give more consideration to the number of:

  • Radiators
  • Bathrooms
  • People in the home.
  • However, there are some general guidelines which you can base your research on; we’ve put together this rough sizing guide to help you.

    Best Boiler for a 2 bed house

    Best Boiler for 2 Bed House

    A Combi boiler with an output between 24-27kW is best suited to a small terraced house, flat or apartment with 1 or 2 bedrooms. A 24kW Combi boiler will comfortably run up to 10 radiators and supply hot water for 1 bathroom. Get quotes today.

    Best Gas Boilers for a 2 Bed House

    Manufacturer Model / Size Type Energy Efficiency Standard Warranty Typical Price
    Viessmann B2KB Vitodens 200-W 26kW Combi Gas Boiler 26kW Combi 97.95% 3-5 years £1415
    Ideal Logic Heat 18kW Regular Gas Boiler 18kW Regular 98.5% 5 years £791
    Worcester Bosch Greenstar 27i System Gas Boiler 27kW System 92% 5 years £981

    Best Oil Boilers for a 2 Bed House

    Manufacturer Model / Size Type Energy Efficiency Standard Warranty Typical Price
    Grant Vortex Pro Internal Combi 36kW Oil Boiler 26kW Combi 94.5% 2 years £2430
    Worcester Bosch Greenstar Camray Utility 25 Regular Oil Boiler 25kW Regular 93.2% 5 years £2030
    Grant Vortex Eco Utility 26kW System Oil Boiler 26kW System 91.7% 2 years £1649


    Best Boiler for a 3 bed house

    Best Boiler for 3 Bed House

    A Combi or System boiler with an output between 28-34kW is best suited to a medium terraced house or semi-detached house. A 34kW boiler will run up to 15 radiators and supply hot water for 2-3 bathrooms. Get quotes today.

    Best Gas Boilers for a 3 Bed House

    Manufacturer Model / Size Type Energy Efficiency Standard Warranty Typical Price
    Ariston Clas One 30kW Combi Gas Boiler 30kW Combi 92% 8 years £1012
    Potterton Assure Heat 30kW Regular Gas Boiler 30kW Regular 93% 2 years £1342
    Ideal Vogue 32kW System Gas Boiler 32kW System 93% 10 years £981

    Best Oil Boilers for a 3 Bed House

    Manufacturer Model / Size Type Energy Efficiency Standard Warranty Typical Price
    Worcester Bosch Greenstar Danesmoor 32 System Oil Boiler 32kW System 91% 2 years £2100
    Warmflow Kabin-Pak K120HE 33kW Regular Oil Boiler 33kW Regular 90.7% 7 Years £1635
    Firebird Enviromax Combi Internal 26kW Oil Boiler 26kW Combi 90.7% 2 years £1945


    Best Boiler for a 3 bed house

    Best Boiler for 4 Bed House

    A System or Regular boiler with an output between 35-42kW is best suited to a large semi-detached house or detached house. A 42kW boiler will run up to 20 radiators and supply hot water for 3 or more bathrooms. Get quotes today.

    Best Gas Boilers for a 4 Bed House

    Manufacturer Model / Size Type Energy Efficiency Standard Warranty Typical Price
    Baxi EcoBlue Advance Combi 40 Gas Boiler 40kW Combi 89.1% 10 years £1260
    Worcester Bosch Greenstar Fs 42CDi Regular Gas Boiler 42kW Regular 92% 5 years £1640
    ATAG IS40 40kW System Gas Boiler 40kW System 89.8% 10 years N/A

    Best Oil Boilers for a 4 Bed House

    Manufacturer Model / Size Type Energy Efficiency Standard Warranty Typical Price
    Mistral Outdoor Combi Standard CODC4 41kW 41kW Combi 92% 2 Years Parts – 5 Years Heat Exchanger £2040
    Firebird Enviromax Kitchen Internal 35kW Regular Oil Boiler 35kW Regular 92% 2 years £1800
    Grant Vortex Pro Kitchen/Utility 36kW System Oil Boiler 36kW System 91.7% 2 years £1860


    Best Boiler for a 3 bed house

    Best Boiler for a 2-3 Bathroom House

    If you have 2 or more bathrooms needing hot water at the same time then a Combi boiler may struggle to cope.

    A professional engineer may recommend that you install a System or Conventional boiler sized between 35-42kW (or even bigger) depending on your hot water demand. These heating systems include a hot water storage cylinder which means multiple taps, showers and baths can be supplied at the same time without losing pressure.

    NOTE: An electric shower doesn’t rely on the boiler for hot water so doesn’t need to be included in any calculations.

    So is there a Boiler Sizing Rule of Thumb?

    In general the following summary table gives you a great starting point when you’re shopping for the right size boiler for your home.

    Size / Type of House Number of Bedrooms Number of Bathrooms Number of Radiators Recommended Boiler Size
    Small terrace, flat or bungalow 1-2 1 Up to 10 24-27kW
    Medium terrace, bungalow or semi-detached house 3-4 1-2 Up to 15 28-34kW
    Large semi-detached or detached house* 4+ 2+ Up to 20 35-42kW

    REMEMBER: It’s important that your home and hot water demand is assessed by a professional to be sure you’re getting the right size as every home (and family) is unique. Get quotes from local, recommended heating engineers today.

    More Boiler Sizing Tips

    Current Boiler Size

    It’s a good idea to know the size of your current boiler as a starting point but don’t assume you need to stick with the same size going forward. Things may have changed since it was installed such as the household’s water demands, the level of insulation or the addition of an extra bathroom. Or you may have plans for the future such as an expanding family, a loft conversion or an extension.

    If you find your home is regularly running out of hot water it could mean that your current size of boiler is not up to the task and you need to think bigger. However, if your current boiler is more than 8 years old then it may just be a case of inefficiency rather than a lack of capacity.

    Combi, System or Regular?
    Make sure you’re getting the right type of boiler for your home. In addition to ensuring the boiler you choose uses the correct fuel (gas, oil, electric, LPG, biomass), you have 3 types to choose from: Regular (Conventional), System or Combi. Each has a different setup and follows a slightly different process to produce your hot water and heat your home.

    For example, Combi boilers may not be suitable for larger homes with 2 or more bathrooms as they only provide a single flow of water that weakens when split between outlets. In homes with a higher demand for hot water it’s often advisable to install a System or Regular boiler which incorporates a hot water cylinder to store a higher volume of hot water.

    Depending on your circumstances, you may qualify for a free boiler grant towards the cost of a new boiler. To find out if you qualify and to apply, visit boilergrants.co.uk. Please note that funding is extremely limited now.


    Mains Flow Rate
    While Regular boilers take their water supply from a large hot water cylinder, Combis and System boilers are fed directly from the mains. For this reason, if you are replacing an older system with a Combi or System boiler, the size you need will be influenced by the flow rate (pressure) of the water coming from the mains. You need to make sure the boiler you choose is big enough to cope with the amount of water passing through it to be heated.

    You can get a rough idea of your flow rate with the help of a bucket with measurements. Run cold water from one of your taps for 1 minute and measure how much you collect in litres. Boiler manufacturers will specify the maximum flow of water in litres per minute so you will be able to see if your new boiler is a good match. The more you collect, the faster the flow from your mains supply and the harder your boiler is going to need to work. The harder it needs to work, the higher the output needs to be.

    Alternatively, a professional installer will be able to measure the flow rate and temperature of your mains water for you.

    Boiler Efficiency Tips

    A poorly insulated house will lose heat through the roof, walls, floors or windows and your boiler will have to work harder (i.e. use more energy) to keep your home warm. To keep energy bills as low as possible you may want to think about investing in some more effective insulation or replacement windows.

    What Radiator Size?
    Not all radiators are created equal as some have higher outputs (measured in British Thermal Units or BTUs) than others and will vary in size. The bigger the surface area of the radiator the quicker it will heat a room, so each radiator should be adequate for the size of room.

    Smart Controls
    Many boilers are now compatible with smart controls which enable you to adjust the temperature of your home from any location via smartphones or tablets. This technology is not only much more convenient but gives you much more visibility and control of your energy use so you can reduce wastage. Some systems can even automatically compensate for the weather outside.

    Annual Boiler Servicing
    To keep a boiler running at its best for as long as possible it’s really important to get it serviced by a professional once a year. This will catch problems early on before they can develop into bigger problems or cause long term damage.

    Need more help?

    Calculating the right boiler size is something that installers will be able to advise you on so it’s important to get multiple quotes so you can compare their advice.

    Boiler Guide has a network of over 5,000 reputable and highly recommended installers ready to provide you with a quote. Send us your enquiry today and we will provide you with up to 3 quotes from Gas Safe registered installers.