What size boiler do I need?

Before you think of changing your boiler or heating system it’s good to know what type of boiler you need.

Here in our quick guide you can see roughly what you would need.
The size of the combination or combi boiler is measured in KW (Kilowatts) and generally the higher the KW the more powerful the boiler will be. Domestic combi boilers usually range from 24KW to 42KW in size. We are here to help you find the one that’s just right for you.

A boiler that is too small will not be able to heat your home effectively or cope with the hot water demands. A boiler that is too big will use more fuel; increasing energy bills and producing unnecessary carbon emissions as well leaving you with a more expensive installation bill.

Boiler size calculator
We make our calculations based on how many radiators, bathrooms and people there are in the
house. We’ve put together this rough sizing guide and our recommendations for a boiler choice.
Note: An electric shower does not rely on the boiler for hot water so does not need to be included in any calculations.

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The best boiler for a two-bed house
This relates to a small terraced house or a one or two-bedroom apartment or flat. A combi boiler with an output of between 24-27 KW will comfortably run up to 10 radiators and supply hot water for 1 bathroom.
Our recommendations:
24KW – Baxi 624 with 7 years warranty
24KW – Ideal Logic+ with 7 years warranty
25KW – Worcester Bosch Greenstar with 5 years warranty

Best boiler for a three-bed house
This relates to a medium sized terraced or semi-detached house or a three-bedroom apartment or flat. A combi boiler with an output if between 28-34 KW will comfortably run up to 15 radiators and supply hot water for 2 bathrooms.
Our recommendations:
30KW – Baxi 630 with 7 years warranty
30KW Ideal Logic+ with 7 years warranty
30KW Worcester Bosch Greenstar with 5 years warranty

Best boiler for a four-bed house
This relates to a large semi-detached or detached three, four or five-bedroom house. A combi boiler of between 35-42KW will comfortably run up to 20 radiators and supply hot water for 2 bathrooms.
Our recommendations:
40KW Baxi Ecoblue Advanced with 10 years warranty
40KW Ideal Vogue Gen 2 with 8 years warranty
42KW Worcester Bosch Greenstar 42CDI Classic with 5 years warranty

Best boiler for a house with two or more bathrooms
If you have 2 or more bathrooms needing hot water at the same time a combi boiler may not be the best solution and may struggle to cope with the demand. We recommend that in larger properties or
where the demand for hot water is higher, a system boiler should be installed with a hot water storage cylinder. This means multiple taps, showers and baths can be used at the same time without losing pressure.

Please be aware this is general boiler guidance and you should always consult a gas safe registered engineer to carry out a site survey.

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