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BoilerCodes

What size boiler do I need?

Tell us about your home and we'll show the boiler size manufacturers recommend, in kW, and whether a combi suits you.

Combi boilerkW

The combi sizes manufacturers recommend

Viessmann's published bands. Ideal's by-radiator guide matches the first two.

SizeTypical home
24–27 kWSmall one to two bedroom house or flat, with up to 10 radiators and one bathroom
28–34 kWMedium three to four bedroom house, with up to 15 radiators and one to two bathrooms
35–42 kWFour or more bedroom large house, with up to 20 radiators and two or more bathrooms

Bigger isn't better

Viessmann and Ideal both warn against choosing a boiler larger than you need, because it wastes energy and money. These figures are guides. The final size should come from an installer's heat loss calculation, and they'll also check your mains water flow, which limits how much hot water a combi can deliver.

Common questions

What size combi boiler do I need for a 3 bedroom house?

Viessmann's guide puts a three to four bedroom house with up to 15 radiators and one or two bathrooms at 28–34 kW. Ideal's radiator guide gives the same range for 10 to 15 radiators.

Is a bigger boiler better?

No. Viessmann and Ideal both say choosing a boiler much bigger than you need wastes energy and money. An installer's heat loss calculation should decide the final size.

When is a combi boiler not suitable?

Manufacturers say combis can struggle when several taps or showers are used at once, so homes with two or more bathrooms often suit a system or regular boiler. Low mains water pressure or flow is another reason, which your installer will check.

Sources