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Boiler size is measured in kilowatts (kW). Get it right and you'll have strong showers and a warm home without paying for power you don't need. The right size depends mainly on the type of boiler, how many bathrooms you have and how many radiators it needs to heat.
Quick answer
- Combi, flat or small house with 1 bathroom: 24 to 26kW, giving around 10 litres of hot water a minute
- Combi, typical 3-bed home with a bathroom and a separate shower: 28 to 30kW, around 11.5 to 12.5 litres a minute
- Combi, larger home with 2 bathrooms: 35kW or more, around 14 to 15 litres a minute
- System or regular boiler: usually smaller, as it's sized for your radiators rather than your shower, often 12 to 24kW for most homes
These are the sizes our online quote recommends for typical homes. We confirm the right size for your home when we quote.
Why combis and system boilers are sized differently
A combi boiler heats water instantly as you turn on a tap, and that takes far more power than heating radiators. So a combi's kW rating is mostly about hot water: the higher the kW, the more hot water it can deliver each minute.
A system or regular boiler heats a cylinder of stored hot water in the background, so it doesn't need that burst of power. It's sized for the heat your home loses: the number and size of your radiators, plus a bit extra for the cylinder.
What size combi boiler do I need?
Manufacturers quote a combi's hot water flow rate in litres per minute, usually based on heating the water by 35°C. That's the clearest way to compare them. Among the boilers we fit, a 24kW combi typically delivers around 10 litres a minute, a 30kW about 12, and a 35 to 36kW about 14 to 15 (correct in October 2026).
A combi can only deliver what your mains water supply provides, so if the water pressure coming into your home is low, a bigger boiler won't make your shower stronger. If you have three or more bathrooms, or several people often shower at the same time, a system boiler with a hot water cylinder is usually the better choice. Our guide to combi vs system boilers explains the difference, and if you're thinking of swapping a cylinder for a combi, see our guide to converting to a combi boiler.
What size system or regular boiler do I need?
| Your home | Radiators | Typical system or regular boiler |
|---|---|---|
| Flat or 1 to 2-bed house | Up to about 10 | 12 to 15kW |
| 3 to 4-bed house | About 10 to 15 | 18 to 24kW |
| Larger home | 15 to 20 or more | 24 to 30kW or more |
A well-insulated home needs less than these figures suggest, and a draughty older home may need more. The most accurate way to size a system or regular boiler is a heat loss calculation, which works out how much heat each room loses.
Why bigger isn't better
It's tempting to go bigger "just in case", but an oversized boiler costs more to buy and can waste gas. When a boiler is much more powerful than your home needs for heating, it heats the radiators too quickly, switches off, then fires up again shortly after. This is called short cycling, and it reduces efficiency and adds wear.
It's a common problem. A UCL study of 221 combi boilers in UK homes found most were oversized for heating, and many cycled on and off rapidly as a result.
Replacing like for like? Don't assume your old boiler's size was right. Older boilers were often oversized, and if you've insulated your home since it was fitted, you may need less power than before.
Our online quote asks about your home, your bathrooms and showers, and your current boiler, then shows you suitable boilers with a fixed installation price. Get your fixed price to see which sizes we'd recommend for your home.
Boiler size questions
Not sure what size you need? We'll recommend the right boiler for your home
Get my fixed priceReviewed by our technical team. All BoxHeat articles are written by experienced heating professionals and reviewed by our team of Gas Safe registered engineers to ensure accuracy, safety, and compliance with current UK building regulations and manufacturer guidelines.
