Most UK homes need a combi boiler of roughly 24 to 34kW, or a heat-only (regular) or system boiler of around 12 to 18kW, depending on the number of radiators and bathrooms. The right figure for your home comes from its heat loss and hot water demand, and oversizing wastes money and wears the boiler faster.
The tables below give the ballpark ranges manufacturers publish. Treat them as a starting point for a conversation with your installer, not a shopping list.
What the kW number actually means
A boiler's kW rating is its maximum output: how much heat it can produce flat out. On a combi, the headline figure is usually the hot water output, because heating water instantly for a shower takes far more power than keeping radiators warm. The output available to the radiators is often lower than the headline number, and the spec sheet lists both.
That is why combi and system boilers are sized so differently. A system or regular boiler stores hot water in a cylinder, so it only needs enough power to heat the house and reheat the cylinder over time. Viessmann puts the average home heat load at around 10 to 20kW.
Combi boiler sizes: a ballpark guide
These ranges draw on the guides Ideal and Viessmann publish. A combi's size is driven mostly by hot water flow, so bathrooms matter more than radiators.
| Home | Radiators | Bathrooms | Typical combi size |
|---|---|---|---|
| Flat or 1-bed with a shower | Up to 10 | 1 | 24 to 27kW |
| 2 to 3-bed terrace or semi | 10 to 15 | 1 (maybe a downstairs loo) | 28 to 34kW |
| Larger 3 to 4-bed | 15 to 20 | 1 to 2 | 32 to 40kW, or consider a system boiler |
| 4-bed plus, 3 or more bathrooms | 20 plus | 3 or more | A system or regular boiler with a cylinder is usually better |
Viessmann is clear that above four bedrooms and three bathrooms a combi likely will not be the best option, and that bigger homes with high hot water demand suit a system or regular boiler. Our combi vs system boiler guide goes through that choice.
System and regular boiler sizes
Because the cylinder does the hard work on hot water, these boilers are sized on heat loss. Ideal publishes this rough guide for its heat-only boilers, and system boilers are sized on the same principle:
| Home | Radiators | Bathrooms | Ideal's guide for a heat-only boiler |
|---|---|---|---|
| 1 to 2 bedrooms | Up to 10 | 1 | 12kW |
| 3 to 4 bedrooms | 10 plus | 2 to 3 | 15 to 18kW |
| More than 4 bedrooms | 20 plus | More than 3 | 24 to 30kW |
Ideal is clear these are general guidelines, and that a poorly insulated home or a busy household may need more. A draughty solid-wall cottage and a well-insulated 1990s house with the same number of radiators can need very different outputs.
Why oversizing is a problem
It is tempting to go big to be safe. Viessmann warns that choosing a bigger boiler than you need is not recommended and could waste energy, and Ideal makes a similar point. Here is why.
- Short cycling. A boiler that is too powerful heats the radiators so fast it reaches temperature, switches off, cools, then fires again. Lots of short bursts are less efficient than steady running and put extra wear on the ignition, fan and valves.
- Less time condensing. Condensing boilers are most efficient when the water coming back from the radiators is cool. An oversized boiler running in short, hot bursts spends less time in that efficient zone.
- Higher purchase price. Bigger models cost more to buy, for output you will rarely use.
- It does not fix flow. On a combi, a larger kW rating gives a bit more hot water flow, but it cannot beat the limit of your incoming mains. If the water main delivers weak flow, a bigger combi just delivers weak flow faster to heat.
Modern boilers soften this with modulation, which means they can turn down their output. A boiler with a wide modulation range copes better with being slightly oversized. Even so, the best result is a boiler that matches the house.
And undersizing?
Go too small and the problems are obvious. Viessmann notes you can run out of hot water or end up with a house that never properly warms. On a combi, an undersized boiler gives a lukewarm shower in winter when the incoming mains water is coldest. Ideal also points out that if you have extended your home, swapping like for like may leave you short.
How a proper sizing is done
Rules of thumb get you close. A heat loss calculation gets you right. It looks at each room's size, wall and window construction, insulation, and the temperature you want, and works out how many watts of heat the room loses on a cold day. Add the rooms up and you have the heating load.
For a combi, the installer also checks your mains water pressure and flow, and how many hot outlets you use at once. That is what decides whether a combi will cope at all, never mind what size.
In the South West the housing varies so much that this matters. A granite cottage on Bodmin Moor, a Georgian townhouse in Bath and a 1960s estate house in Taunton can all have eight radiators and need very different boilers. When we quote for a new combi boiler, we survey first for exactly that reason.
Do your radiators need upgrading too?
Sometimes. If one or two rooms never get warm, the radiator in that room may simply be too small for its heat loss. A bigger boiler will not fix that, because the radiator can only pass so much heat into the room. Upgrading those radiators is often the cheaper fix. Our radiator installation page covers it.
Larger radiators also let a boiler run at lower flow temperatures, which keeps it condensing more of the time. They make a later move to a heat pump much easier as well. For prices and what affects them, see our new boiler cost guide.
Frequently asked questions
What size combi boiler do I need for a 3-bed house?
Most 3-bed homes with one bathroom and 10 to 15 radiators fall in the 28 to 34kW range, according to Ideal's and Viessmann's sizing guides. Your mains water flow and how many people shower at once matter as much as the bedroom count.
Is a 24kW combi big enough?
For a flat or small house with one shower and up to around 10 radiators, often yes. With a bath and shower that get used together, or a cold winter mains supply, it may feel underpowered on hot water. A survey will tell you.
Does a bigger boiler heat the house faster?
Only up to the point your radiators can pass heat into the rooms. Beyond that, extra kW just makes the boiler cycle on and off. If a room is cold, the radiator is usually the bottleneck, not the boiler.
Can I replace my old boiler with the same size?
Not without checking. Old boilers were often oversized, and many homes have since had insulation, new windows or an extension. Any of those changes the heat loss, so the right size today may be quite different.
What size boiler do I need with two bathrooms?
If both bathrooms get used at the same time, you are often better with a system boiler and a cylinder than a very large combi. If they are rarely used together, a 32 to 40kW combi can work, provided your mains flow supports it.