Glossary · Boilers and boiler types
What size boiler do I need?
Boiler size is the heat output in kilowatts (kW), not the physical dimensions of the unit. The right size depends on how much heat your home loses and how much hot water you use. Rules of thumb based on radiators and bedrooms give a starting point, but a heat loss calculation is the proper method.
Key points
- In the heating industry, boiler size means kW output, not how big the boiler is on the wall.
- Approved Document L says heating systems should be sized from a heat loss calculation for the home and should not be significantly oversized.
- The 2026 edition of Approved Document L, which applies to work under the 2026 standards, says sizing a replacement boiler from the old one's output is not appropriate.
- Manufacturers' rules of thumb differ. Ideal Heating gives 24 to 27 kW for a combi with up to 10 radiators.
- Bigger is not better: Ideal Heating says a boiler far larger than you need wastes money to buy and to run.
How boiler size works
In the heating industry, boiler size has nothing to do with physical dimensions. It is the kilowatt (kW) output, the amount of energy the boiler can provide to your heating and hot water system.[1]Ideal Heating, What size boiler do I need? Try our power calculator A boiler’s specification shows a power input and a power output. Input is the energy going in. Output is how much of it becomes usable heat. Efficiency is the ratio of output to input.[1]Ideal Heating, What size boiler do I need? Try our power calculator
Ideal Heating says that as a general rule, the larger the house and the more radiators, taps and showers, the more powerful the boiler. But bigger is not better. A capacity far higher than you need wastes money on purchase and running costs, while an underpowered boiler leaves radiators tepid and hot water short.[1]Ideal Heating, What size boiler do I need? Try our power calculator Vaillant agrees that the most expensive models with big outputs may be oversized and as a result will not work properly.[4]Vaillant, What is a condensing boiler?
The proper method
The current edition of Approved Document L, the 2021 edition incorporating 2023 amendments, says the specification of a space heating system should be based on an appropriate heat loss calculation for the dwelling and a sizing method that takes account of the properties of the home. Systems should not be significantly oversized.[3]Ministry of Housing, Communities and Local Government, Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments The 2026 edition, which takes effect on 24 March 2027 for most building work in England, is more specific. It names the BS EN 12831-1 heat loss calculation and says systems should not be oversized. It adds that when only a heating appliance is being replaced, a whole dwelling heat loss calculation is appropriate, and that sizing a replacement appliance from the heat output of the one already installed is not appropriate.[6]Ministry of Housing, Communities and Local Government, Approved Document L: Energy and greenhouse gas emissions, Volume 1: Dwellings (2026 edition)
For hot water, the same document says domestic hot water systems should be sized for the anticipated demand and should not be significantly oversized.[3]Ministry of Housing, Communities and Local Government, Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments
Rules of thumb from manufacturers
These are starting points only. Ideal Heating’s guide by radiator count gives these ranges.[1]Ideal Heating, What size boiler do I need? Try our power calculator
| Radiators | Combi boiler | System or heat only boiler |
|---|---|---|
| Up to 10 | 24 to 27 kW | 12 to 15 kW |
| 10 to 15 | 28 to 34 kW | 18 to 24 kW |
| 15 to 20 | 32 to 40 kW | 30 to 40 kW |
Combi boilers need more power because they heat water instantly. System and heat only boilers heat a cylinder more slowly.[1]Ideal Heating, What size boiler do I need? Try our power calculator Vaillant’s guide uses bedrooms, radiators and bathrooms, and gives 24 to 27 kW for a combi in a one or two bedroom home with up to 10 radiators and one bathroom.[2]Vaillant, What size boiler do I need?
Other tables differ. Vaillant’s guide for heat only boilers ties the size to the number of bedrooms and the level of insulation. It gives 7 to 8 kW for two to three bedrooms with excellent insulation, and 16 kW for more than four bedrooms with poor insulation.[5]Vaillant, What is an open vent boiler? The gap between manufacturers shows why a rule of thumb is not a quote. Both stress that a professional should decide.[1]Ideal Heating, What size boiler do I need? Try our power calculator[2]Vaillant, What size boiler do I need?
Example
Ahmed has a three-bedroom semi in Leeds with 9 radiators and one bathroom. By Ideal Heating’s table a combi would sit in the 24 to 27 kW band.[1]Ideal Heating, What size boiler do I need? Try our power calculator A system boiler with a cylinder would sit in a much lower band, 12 to 15 kW.[1]Ideal Heating, What size boiler do I need? Try our power calculator The engineer then checks the actual heat loss of the house, the insulation and the hot water demand. Ahmed’s old boiler might have been a larger or smaller unit than the house needs, so its output is not the starting point.[3]Ministry of Housing, Communities and Local Government, Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments
What people get wrong
The most common mistake is matching the new boiler to the old one. The 2026 edition of Approved Document L says that sizing a replacement appliance from the output of the one already installed is not appropriate.
The 2026 edition gives that rule on replacement sizing, and both editions base sizing on a heat loss calculation.[6]Ministry of Housing, Communities and Local Government, Approved Document L: Energy and greenhouse gas emissions, Volume 1: Dwellings (2026 edition) A second mistake is thinking a higher kW is a safety margin. A boiler that is too big for the heat the house needs may not work properly.[4]Vaillant, What is a condensing boiler? See modulating boiler for how modern boilers turn their output down. Radiators matter too. See radiator BTU output.
The bottom line
Boiler size is kW output. The right figure comes from a heat loss calculation and your hot water demand, not from the old boiler or a rule of thumb. Manufacturers’ tables give a rough guide, and they differ. Do not pay for more output than the home needs, because an oversized boiler wastes money and can work badly.
Common questions
What does boiler size mean?
Ideal Heating says that in the heating industry size has nothing to do with physical dimensions. It is the kilowatt output, which is the amount of energy the boiler can provide to your heating and hot water system.
Is a bigger boiler better?
No. Ideal Heating says a capacity much higher than you need wastes money on the purchase and the running costs, while an underpowered boiler leaves radiators tepid and hot water short.
Can I size a new boiler from my old one?
No. Approved Document L expects a heat loss calculation for the dwelling. The 2026 edition, in force from 24 March 2027 for most building work, adds that sizing a replacement appliance from the old one's heat output is not appropriate.
Why are combi boilers a higher kW than system boilers?
Ideal Heating and Vaillant both say combis heat water on demand, so they need a larger output. System and heat only boilers heat a stored cylinder over a longer time.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Ideal Heating. What size boiler do I need? Try our power calculator. Checked 11 November 2026. ↩
- Vaillant. What size boiler do I need?. Checked 11 November 2026. ↩
- Ministry of Housing, Communities and Local Government. Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments. Checked 11 November 2026. ↩
- Vaillant. What is a condensing boiler?. Checked 11 November 2026. ↩
- Vaillant. What is an open vent boiler?. Checked 11 November 2026. ↩
- Ministry of Housing, Communities and Local Government. Approved Document L: Energy and greenhouse gas emissions, Volume 1: Dwellings (2026 edition). Checked 11 November 2026. ↩