What size boiler do I need?
Sizes in kW by radiators and bathrooms, from the makers.
Work out your boiler sizeFour quick taps
The makers' sizes are in the table below, or ring us and we'll ask about your home.See the makers' tableCall us now
The size for a home like yours
A combi of 24 to 27kW
A system or regular boiler of 9 to 18kW
From the makers' guides, for one or two bedrooms, up to about 10 radiators, one bathroom.
A combi of 28 to 34kW
A system or regular boiler of 18 to 30kW
From the makers' guides, for three or four bedrooms, about 10 to 15 radiators, two or three bathrooms.
A combi of 32 to 42kW
A system or regular boiler of 27 to 40kW
From the makers' guides, for more than four bedrooms, 15 to 20 radiators or more, more than three bathrooms.
Your answers fit more than one size of home, so this is the larger: the boiler has to meet your biggest need.
For a home with more than four bedrooms and three bathrooms, Viessmann suggests a system or regular boiler with a cylinder.
Three boilers we fit in that size
A combi
- Top
Greenstar 4000, 25kW - Mid
ecoTEC plus 826, 26kW - Budget
Logic+ Combi2 C24, 24kW
A system or regular boiler
- TopGreenstar 4000 System, 15kW
- MidecoTEC plus 615, 15kW
- BudgetLogic+ System2 S15, 15kW
A combi
- TopGreenstar 8000+, 32kW
- MidecoTEC plus 832, 32kW
- BudgetLogic+ Combi2 C30, 30kW
A system or regular boiler
- TopGreenstar 4000 System, 21kW
- MidecoTEC plus 620, 20kW
- BudgetLogic+ System2 S24, 24kW
A combi
- TopGreenstar 8000+, 36kW
- MidecoTEC plus 836, 36kW
- BudgetLogic+ Combi2 C35, 35kW
A system or regular boiler
- TopGreenstar 8000+ System, 35kW
- MidecoTEC plus 635, 35kW
- BudgetLogic+ System2 S30, 30kW
Each maker's own figures
One or two bedrooms, up to about 10 radiators, one bathroom
A combi
- 24 to 27kW
- 24 to 27kW, for a one-bedroom flat
24 to 27kW
A system or regular boiler
- 9 to 18kW
- 12 to 15kW
12 to 18kW, for two bedrooms
Three or four bedrooms, about 10 to 15 radiators, two or three bathrooms
A combi
- 28 to 34kW
- 28 to 34kW, for two or three bedrooms
- 28 to 34kW, for up to 15 radiators
A system or regular boiler
- 18 to 26kW
- 18 to 24kW
- 18 to 24kW for three bedrooms, 25 to 30kW for four
More than four bedrooms, 15 to 20 radiators or more, more than three bathrooms
A combi
- 32 to 40kW, for 15 to 20 radiators
- 35 to 42kW
A system or regular boiler
- 27 to 40kW
- 30 to 40kW, for 15 to 20 radiators
- 30kW or more, for five bedrooms or more
Each maker calls its sizes a general guide. Our engineer works out the right size with a heat loss calculation, and the survey visit is free.
Where shall we ring you?
Your answers and your boiler size come with your request.
Calls are answered 24 hours a day.
- What size boiler a home needs, in kW
- Why a bigger boiler isn't a better one
- All our engineers are Gas Safe registered
Makes we repair
Boiler size by radiators and bathrooms
The size of boiler a home needs is its output in kilowatts (kW). The makers' guides put a combi at about:
- 24 to 27kW for a home with one or two bedrooms, up to about 10 radiators and one bathroom
- 28 to 34kW for three or four bedrooms with two or three bathrooms.
A system or regular boiler is usually smaller, as its hot water is stored in a cylinder. Each maker calls its figures a general guide, and says to have a heating engineer look at the home before you decide.
A boiler's size means its output, not its height or width. A kilowatt measures how much heat a boiler can give out. The higher the output, the more radiators, taps and showers it can serve. The makers size a boiler by its radiators, bedrooms and bathrooms, and the baths and showers. They also go by the kind of home and how well it's insulated.
The makers' bands differ from one another, so every figure below carries its maker's name. The homes are described roughly, as each maker draws its bands a little differently.
One or two bedrooms, up to about 10 radiators, one bathroom
A combi
Vaillant: 24 to 27kW. Ideal: 24 to 27kW, for a one-bedroom flat. Viessmann: 24 to 27kW
A system or regular boiler
Vaillant: 9 to 18kW. Ideal: 12 to 15kW. Glow-worm: 12 to 18kW, for two bedrooms
Three or four bedrooms, about 10 to 15 radiators, two or three bathrooms
A combi
Vaillant: 28 to 34kW. Ideal: 28 to 34kW, for two or three bedrooms. Viessmann: 28 to 34kW, for up to 15 radiators
A system or regular boiler
Vaillant: 18 to 26kW. Ideal: 18 to 24kW. Glow-worm: 18 to 24kW for three bedrooms, 25 to 30kW for four
More than four bedrooms, 15 to 20 radiators or more, more than three bathrooms
A combi
Ideal: 32 to 40kW, for 15 to 20 radiators. Viessmann: 35 to 42kW
A system or regular boiler
Vaillant: 27 to 40kW. Ideal: 30 to 40kW, for 15 to 20 radiators. Glow-worm: 30kW or more, for five bedrooms or more
The makers' own size guides: Vaillant's, Ideal's power calculator, Viessmann's and Glow-worm's.
By radiators alone, Glow-worm's guide to its regular boilers gives:
- 12kW for up to 6 radiators
- 15kW for a home with several bathrooms and about 8 radiators
- 25kW for a larger home with more than 15 radiators
- 30kW for three or more bathrooms and 20 or more radiators.
Bathrooms and hot water set a combi's size. A combi heats water as it runs, so its output decides how much hot water flows. Baxi's guide for installers gives a 24kW combi about 9.8 litres a minute and a 40kW combi about 16.4, both at a 35°C temperature rise. The more hot water a home wants at once, the bigger the combi.
A home with several bathrooms in use at once may be better served by a system or regular boiler and a cylinder. Viessmann suggests one for a home with more than four bedrooms and three bathrooms. A badly designed combi installation can struggle with more than one tap running. Combi, system or regular, and which suits which home.
Sources: the makers' size guides above, Ideal's combi and regular boiler guides, Viessmann's combi FAQ, Glow-worm's regular boilers, Baxi's installer guide and Gas Safe Register.
Worth knowing
The radiators, the bedrooms and the bathrooms are the first figures a boiler's size is worked out from, and they're easy to count. We'll ask for them when you ring, and our quote form asks for the bedrooms and the bathrooms.
Why a bigger boiler isn't a better one
A bigger boiler isn't a better one, because a boiler sized well above what the home needs wastes fuel.
- Ideal says a boiler that is too big overheats the home. It sends heat into the system faster than the heat is used, so the boiler keeps switching on and off.
- Viessmann says an oversized boiler makes more heat than the home needs, which raises bills, wastes energy and adds to emissions.
The Building Regulations' guidance in England asks for the same. Approved Document L says a heating system should be specified from a heat loss calculation and a sizing method that takes account of the home. Systems "should not be significantly oversized", in its words. A combi should also be chosen to turn its output down to the home's usual heating load.
- The old habit: oversizing was once usual, and Viessmann says today's boilers make it unnecessary. Glow-worm says a boiler that is too big burns fuel the home won't use.
- Combis, the exception: a combi is often sized larger to give a strong flow of hot water for a shower, Viessmann says. A system or regular boiler can be smaller, as its hot water is stored.
- Turning down: a modern boiler turns its output down to suit the load. Viessmann's Vitodens 050-W heats at anything from 3.2 to 35kW, and Navien's NCB700 from 34kW down to 3.4kW. That's how a combi with a big hot water output can still meet the guidance on the heating load.
- The mains water: more kW can't give more hot water than the cold mains brings in. Where the mains gives 12 litres a minute, Baxi's guide for installers says a combi above 28kW doesn't suit the home. That changes if the supply is improved, or stored hot water is fitted.
A combi's number is usually its hot water output, not its heating output, so look at both. These are the makers' own figures:
Worcester Bosch Greenstar 4000 25kW
Heating output
24kW
Hot water, as its maker gives it
9 litres a minute, at a 40°C rise
Worcester Bosch Greenstar 28CDi and 36CDi Compact
Heating output
24kW each
Hot water, as its maker gives it
10 and 12.9 litres a minute, at a 40°C rise
Worcester Bosch Greenstar 8000+ 40, 45 and 50kW
Heating output
33.7 to 33.8kW
Hot water, as its maker gives it
15 to 16 litres a minute, at a 40°C rise
Navien NCB700, the 42K and the 54K
Heating output
34kW
Hot water, as its maker gives it
An output of 42 and 54kW
Alpha E-Tec Plus NX 28, 33 and 38
Heating output
26.1, 30.4 and 34.8kW
Hot water, as its maker gives it
An output of 28, 32 and 36.9kW
Baxi 224
Heating output
21kW
Hot water, as its maker gives it
An output of 24kW
Older and solid-wall homes
An older home with solid walls may need a more powerful boiler than its size suggests. It loses heat faster than a newer, insulated home. No maker gives a figure to add, as the home's heat loss sets it. A thin single brick wall loses heat more quickly than a modern insulated cavity wall.
Which homes have solid walls
Solid walls are most likely in homes built before about 1920. The Energy Saving Trust gives 1920, and the English Housing Survey 1919. The survey adds that older homes can be harder and costlier to insulate than new ones.
What it means for the size
A home that loses heat faster needs more output to stay warm. Glow-worm says older homes with poor insulation need a more powerful boiler to heat every room. Ideal says even a smaller home may need a larger boiler if it's less well insulated. The insulation and the ceiling height are part of the sizing.
Insulation before the boiler
Insulating the home before the boiler is chosen can let a less powerful boiler do the job, Glow-worm says, as well as saving energy.
Water pressure in older homes
Some older homes have poor water pressure, which can change the type and the size of boiler that suits. A lack of mains pressure or flow can affect a combi's hot water.
Regular boilers in older homes
Regular boilers are common in older UK homes, especially those built before the 1970s. A regular boiler heats a cylinder fed from a tank in the loft. How a regular boiler differs from a combi or a system boiler.
After an extension or a conversion
An extension or a conversion changes the size a home needs. When a boiler is replaced as part of one, the old boiler's size is probably not the right one. The home now has more space, radiators, taps or showers.
Sources: the Energy Saving Trust, the English Housing Survey, Glow-worm, Ideal's regular boiler, combi and power calculator guides, Viessmann and Gas Safe Register.
What the engineer measures before quoting
An engineer sizing a new boiler works out how much heat the home loses and how much hot water it uses. Approved Document L asks for a heat loss calculation and a sizing method that takes account of the home. The makers list what goes into it.
The heat the home loses
A heat loss calculation takes in the floor area, the radiators, the number and type of windows, and the insulation. It works out the heat the home loses on the coldest day of the year. The radiator each room needs comes from the same sum, room by room.
The hot water it uses
The hot water a home uses follows its bedrooms, baths and showers. A small home with a lot of people in it may need more hot water than its size suggests.
The mains water, for a combi
A combi needs good mains water pressure. Its hot water flow is limited by what the cold mains brings in.
The gas supply
The gas pipe from the meter to the boiler has to be big enough, allowing for the home's other gas appliances.
Plans for the home
Plans for an en-suite or an extension go into the sizing, along with the insulation and the ceiling height.
A whole new heating system
A wet heating system that is new or fully replaced should be sized to heat the home with water at 55°C or less. That covers the boiler, the radiators and the pipework. Where that can't be done, it should be the lowest temperature that still meets the home's needs. A boiler swap on its own isn't a full replacement.
A new boiler from us is quoted from photos of your old boiler, or after a free survey visit. We fit every make and model. Our engineers size it with a heat loss calculation, and a quote from photos is confirmed at a survey before the boiler is fitted. When they fit it, they set its heating output to suit the home and balance the radiators fully. Ring us on 020 8064 0067 and we'll ask about the bedrooms, the bathrooms and the radiators. Having a new boiler fitted, and how it is quoted.
Sources: Approved Document L, Viessmann's size guide and FAQ, Ideal's calculator and combi guide, Baxi's installer guide and manual, and Gas Safe Register.
Do you cover my part of London?
Yes: we fit, service and repair boilers all over Greater London, in every borough north and south of the Thames. Pick your side of the river to see the areas.
North of the river
Every borough north of the Thames: Barnet and Enfield at the top, Hillingdon in the west, Havering in the east, and central London in between.
South of the river
Every borough south of the Thames: Richmond and Kingston in the west, Bexley in the east, and down through Croydon to Sutton and Bromley.
Questions about boiler sizes
What does kW mean on a boiler?
The kW on a boiler means kilowatts, the unit its output is given in. A kilowatt is 1,000 watts. A boiler's size in kW is the heat it can give out, not its height or width. Ideal draws a line between the input, the energy going into the boiler, and the output, the heat you can use. A combi has two outputs, one for the radiators and one for hot water, and the number in its name usually follows the hot water.
Is a 30 kW boiler too big for a flat?
A 30kW combi isn't necessarily too big for a flat. The makers' tables put a combi for a one or two bedroom home at about 24 to 27kW. A combi's number, though, is mostly its hot water output. Worcester Bosch's Greenstar 28CDi and 36CDi Compact both heat the radiators at 24kW. The guidance asks for a combi that turns down to the home's usual heating load. A bigger combi gives more hot water when the mains can supply it. A heating engineer works the size out from the flat's heat loss and hot water use. How a new boiler is quoted, with a free survey visit.
Does a combi need to be bigger than a system boiler?
A combi usually needs a bigger output than a system boiler, as it heats water the moment a tap runs. A system or regular boiler stores its hot water in a cylinder and can be smaller, Vaillant's and Ideal's guides show. How a combi, a system and a regular boiler differ.
Can a boiler be too small?
A boiler can be too small. Ideal says one without enough output leaves the radiators lukewarm and too little hot water. Viessmann says it can leave a cool house that never properly warms up, and run out of hot water. Ideal adds that a boiler too small for the home won't keep it warm through the winter months.
Does the make change the size I need?
The size a home needs comes from its heat loss and the hot water it uses, whatever the make. Each maker's range comes in its own steps, though. Vaillant's ecoTEC plus combi comes at 26, 32, 36 and 40kW, for one, and Worcester Bosch's Greenstar 8000+ from 32 to 50kW. The main boiler makes compared.
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