What size boiler do I need?

Most homes with a combi need 24 to 34 kW: 24 to 27 kW for 1 to 2 bedrooms with one bathroom and up to 10 radiators, and 28 to 34 kW for 3 to 4 bedrooms with 2 to 3 bathrooms, on the guide the boiler maker Vaillant publishes.

Vaillant, "What size boiler do I need?", read 5 October 2026; the page is not dated. Vaillant calls these general guidelines and recommends asking a heating engineer. Its figure for the largest homes is as published, 23 to 43 kW; Viessmann gives 35 to 43 kW for a house with about twenty radiators.

Viessmann, another maker, gives a similar picture by radiators: an average three or four bedroom house has about ten radiators and would need a 24 to 30 kW combi, a larger house with more radiators and bathrooms 30 to 35 kW, and a house with as many as twenty radiators 35 to 43 kW, or better, a system boiler. It says that with more than four bedrooms and three bathrooms a combi is unlikely to be the best choice.

A system or regular (heat-only) boiler is smaller for the same home: 9 to 18 kW for 1 to 2 bedrooms and 18 to 26 kW for 3 to 4, on Vaillant's guide, because it heats a cylinder of stored water ahead of time and does not have to heat water on demand.

Vaillant, "What size boiler do I need?", read 5 October 2026: the same guide gives one table for system boilers and regular boilers, which Vaillant also calls open vent boilers.

Viessmann puts the heat load of an average home at around 10 to 20 kW, which is why these boilers can be sized smaller than a combi. With stored hot water the size of the cylinder matters as much as the boiler: ask the installer to size both for the number of people and bathrooms.

A combi is sized for hot water, not for the radiators. Our heat model puts the heating load of a typical 3 bed semi at about 9 kW, yet the makers' guides suggest a 24 to 30 kW combi for it, because a combi must heat cold mains water to a hot tap's temperature the moment the tap opens.

Heat needed a year is the gas a typical gas heated home of that type uses (DESNZ National Energy Efficiency Data-Framework 2026) at a boiler efficiency of 90%. Heating load is that heat spread over 1,100 hours a year at full output, the rule our heat model uses to size a heat pump, so treat it as a guide to scale, not a design figure. Combi sizes are Vaillant's guide for 1 to 2 bedrooms and Viessmann's for a three or four bedroom house.

Viessmann says as much: a combi's capability is expressed as how much water it can heat a minute, for example 12 litres a minute at a 35 degree rise, and the higher the flow you want for baths and showers, the larger the boiler. Combis end up oversized for the heating as a result, it says, which makes them less efficient at heating the home than a system or regular boiler.

A boiler bigger than the home needs costs more to buy and more to run. Viessmann says choosing a bigger boiler than you need is not recommended: it could result in wasted energy and is likely to leave you paying more than you need to for gas. Too big, it says, and the boiler may use too much energy heating water you are not going to use; too small, and you may run out of hot water or be left with a house that will not heat up properly.

It costs more upfront too. Heatable, an online installer, lists output as one of the things that set the price: bigger homes need a boiler with a higher output, which means a higher price. Its installed prices run from £1,800 to £3,500 for a like-for-like combi swap to £3,000 to £4,500 for a premium model.

The old habit was to add size to make up for heat lost through the roof, walls and windows. Viessmann says that with modern boilers this is no longer the advice. If a home loses a lot of heat, the fix that lasts is insulation: see our insulation savings calculator.

The tables above are a starting point. The right size comes from a heat loss calculation by the installer, which Viessmann calls a much better way to choose than the size of the house: it takes in the floor area, the number of radiators, the number and type of windows and the insulation, and works out how much heat the house loses on the coldest day of the year.

Two homes with the same bedrooms can differ a good deal. In our heat model a well insulated 3 bed semi needs a heating load of about 7.5 kW and a poorly insulated one about 10 kW. Ask each installer how they arrived at the size they quote, and be wary of one who matches the old boiler's output without checking.

How much hot water you use at once decides the type, and the type decides the size. One shower at a time suits a combi. Two running together, or a bath filling while someone washes up, favours the stored hot water of a system or regular boiler.

Heatable, "New boiler installation costs 2026", updated 25 September 2026. The Energy Saving Trust gives the same advice in words: combis are less suitable for houses with several bathrooms or high hot water use.

Water pressure matters as well. A combi has to be big enough for the water passing through it, Viessmann says, and some older homes have poor mains pressure, which can change the type and size of boiler that suits. Our guide to combi vs system boiler covers which type suits which home.

For the type and size that usually suit a particular home, with its gas use, the saving and the payback, see our pages on a new boiler for a 2 bed flat, 2 bed terrace, 3 bed semi, 4 bed detached house and bungalow. The new boiler cost calculator prices a swap or a conversion, and our new boiler cost guide covers what a quote should include.

• Vaillant, What size boiler do I need? (not dated): recommended combi, system and regular boiler output by bedrooms, radiators and bathrooms, given as general guidelines. Read 5 October 2026.
• Viessmann, What size of boiler do I need for my home? (22 March 2022): boiler size by radiators and bedrooms, the heat load of an average home, why an oversized boiler is not recommended, and that a regularly serviced boiler should last up to around 15 years. Read 5 October 2026.
• Heatable, New boiler installation costs 2026 (updated 25 September 2026): typical installed prices from 2,000 of its own quotes, January to June 2026, its example estimates by home and its price ranges by brand. Heatable is an online boiler installer, so these are one firm's prices. Checked 5 October 2026.
• Energy Saving Trust, Boiler types explained (last updated 20 May 2026): yearly savings for a gas heated home fitting an A-rated condensing boiler with a programmer, room thermostat and thermostatic radiator valves in place of an older boiler, by the old boiler's rating, at July 2026 fuel prices; £3,500 typical cost of replacing a G-rated gas boiler. Checked 5 October 2026.
• Gas use, heating load and energy bills from our heat model, js/heat-model.js, and bill model, js/bills.js: median gas use by type of home and bedrooms from the DESNZ National Energy Efficiency Data-Framework 2026, priced at the Ofgem cap. See our methodology.