What Size Boiler Do I Actually Need?
For most homes, boiler size comes down to bathrooms rather than floor area. A one-bathroom house is usually fine on a 24 to 27kW combi. Two bathrooms point to 28 to 34kW. Three or more, or a house that runs two showers at once, usually needs a system boiler and a hot water cylinder instead.
Bigger is not better. An oversized boiler costs more to buy, more to run, and cycles on and off in a way that shortens its life. It's one of the more common mistakes we're asked to put right.
Why bathrooms matter more than square footage
People assume boiler size follows house size. It mostly doesn't.
A combi boiler heats water on demand, so what limits it isn't how big the house is; it's how much hot water you might draw at once. Two showers running together is a much bigger ask than a large house with one bathroom.
Floor area does matter, but it affects the heating side rather than the hot water side, and for most British houses the hot water demand is the number that decides it.
The rough guide
Use this as a starting point, not a specification.
24 to 27kW combi - a one-bathroom house, typically two or three bedrooms with up to about ten radiators. This covers a lot of the housing stock across South East London and North Kent.
28 to 34kW combi - two bathrooms, or one bathroom plus a busy en suite. Three or four bedrooms, up to around fifteen radiators.
System boiler and cylinder - three or more bathrooms, or any house where two showers genuinely run at the same time on a weekday morning. Also worth considering if you've got a large family and the hot water demand comes in one rush.
The Glow-Worm Compact 24kW combi we fit from £1,999 including VAT sits at the smaller end of that range, and suits a large proportion of the homes we work in.
Why oversizing is a real problem
If a boiler is much bigger than the house needs, it heats the water faster than the system can use it, shuts off, then fires again a few minutes later.
That's called short cycling. It wastes gas, it puts wear on the components that fail first, and it can leave the house feeling less comfortable rather than more.
You also pay more up front for a unit you'll never fully use.
The old logic of "get a bigger one to be safe" came from a time when boilers were less efficient, and controls were cruder. It doesn't hold any more.
Why undersizing is also a problem
The opposite happens too, usually when someone replaces like-for-like after an extension or a loft conversion.
An undersized combi will run hot water at a lower flow rate, so the shower drops off when someone runs a tap. In cold weather it may struggle to bring the whole house up to temperature.
If you've added rooms since the last boiler went in, the size needs recalculating rather than copying.
What a proper calculation looks at
The guide above gets you close. A real specification looks at more:
The number and size of radiators, not just the room count
How well insulated the house is, and whether the windows have been done
The incoming mains water flow rate, which for a combi is often the actual limiting factor rather than the boiler itself
Whether you're planning an extension or a loft conversion in the next few years
That last one matters more than people expect. If a loft conversion with an en suite is two years away, it's worth sizing for it now rather than replacing a five-year-old boiler.
What about the mains flow rate?
This catches people out.
A combi can only deliver hot water as fast as the cold mains can supply it. If the incoming flow is poor, fitting a bigger boiler won't improve your shower - the restriction is in the street, not the appliance.
We measure the flow rate before recommending anything. If it's low, that changes the answer, and sometimes it points towards a cylinder rather than a combi, regardless of how many bathrooms you've got.
Frequently asked questions
Is a bigger boiler always better? No. An oversized boiler short-cycles, wastes gas and wears out sooner. Correctly sized is what you want.
What size boiler for a three-bedroom house? Usually a 24 to 30kW combi, depending on whether there's one bathroom or two and how many radiators are on the system.
Do I need a bigger boiler if I've had an extension? Possibly. More radiators means more heating demand. An extra bathroom means more hot water demand. Both are worth recalculating rather than assuming.
Can I move from a cylinder to a combi? Often, but not always. It depends on the number of bathrooms and on the mains water flow. It's worth reading up on the trade-offs before deciding.
How do I know what size I've got now? It's usually printed on the boiler itself or in the manual. The model number often contains the kW rating.
So what size boiler do you need?
Any firm quoting you a boiler without asking how many bathrooms you have and checking your water flow rate is guessing.
We'd be happy to come and take a look, work out what your house actually needs, and give you a straight answer - including if the one you've got is fine. Call 020 8087 0799 or get in touch here.
