Short answer
Most three bedroom homes with one bathroom need a 24kW to 30kW combi. But bedroom count is the wrong thing to size on. What actually decides it is how many bathrooms you have, how many radiators, and your incoming mains water flow rate — and that last one stops more new boilers performing than anything else.
Why Bedrooms Don't Decide It
Nearly every online boiler quote asks how many bedrooms you have. It is a rough proxy, nothing more. Two houses with three bedrooms each can need completely different boilers — one with a single bathroom and eight radiators, the other with two bathrooms, an en-suite and sixteen radiators.
On a combi boiler the headline kW figure mostly refers to hot water output, not heating output. A 30kW combi is not heating a bigger house than a 24kW one. It is delivering hot water faster. That is the bit people get wrong.
The Three Things That Actually Matter
How Many Bathrooms — and Whether They're Used at Once
One bathroom is straightforward. Two bathrooms that are never used simultaneously is still usually fine on a well-sized combi. Two bathrooms in genuine simultaneous use — someone showering while someone else runs a bath — is where combis start to struggle, because a combi heats water on demand and has to split what it has between the outlets.
Radiator Count and Heat Loss
This is the central heating side. More radiators, bigger rooms, solid walls, or an exposed position all increase how much heat the property loses, and therefore how much output it needs. Proper practice is a heat loss calculation for the actual property rather than a rule of thumb — particularly on older solid-walled housing, which we have plenty of round here.
Mains Water Flow Rate — the One Nobody Mentions
This is the big one. A combi can only heat water as fast as your mains delivers it. If your incoming supply gives ten litres a minute, a 35kW combi cannot give you more than ten litres a minute of hot water. It physically cannot. Fitting a bigger boiler onto a poor mains supply is money spent for nothing, and it is the single most common reason someone tells me their expensive new combi is disappointing. We measure flow rate at survey before recommending anything.
Rough Sizing Guide
As a starting point only — the survey decides it:
| Property | Typical size | Our model |
|---|---|---|
| 1–2 bed, 1 bathroom, up to ~10 rads | 24–25kW | Worcester 1000 24kW £1,795 |
| 3 bed, 1 bathroom, 10–15 rads | 28–30kW | Worcester 1000 30kW £1,995 |
| 3–4 bed, 1 bathroom + en-suite | 30kW | Worcester 4000 30kW £2,595 |
| 4+ bed, 2 bathrooms, 15–20 rads | 32kW+ | Worcester 8000 32kW £3,095 |
| 2+ bathrooms in simultaneous use | System boiler + cylinder | Priced at survey |
Why Bigger Isn't Better
People assume oversizing is the safe option. It isn't, for three reasons:
- You pay more up front for output the property will never use.
- It short cycles. A boiler producing far more heat than the house needs fires up, satisfies demand quickly, shuts down, and repeats. That constant on-off cycling causes more wear on components.
- It can run less efficiently. A condensing boiler works best running steadily at lower temperatures, not in short violent bursts.
Undersizing has obvious problems too — the heating never quite gets there on a cold night, and hot water disappoints. The aim is neither. It is matching the boiler to the actual property.
When a Combi Is the Wrong Tool
Combis suit most homes and we fit far more of them than anything else. But there are cases where a system boiler with an unvented cylinder is genuinely the better answer:
- Two or more bathrooms in regular simultaneous use
- Poor mains flow rate that cannot be improved
- Large properties with long pipe runs to the furthest outlet
Stored hot water is not limited by mains flow in the same way, so a cylinder can deliver to several outlets at once where a combi would drop off. It costs more and needs somewhere to put the cylinder, so it is not the default — but where it is right, it is right.
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⚡ Get My Instant Price → 📞 01942 800158How We Size It at Survey
When we come out, we check radiator count and sizes, room dimensions, construction type, how many bathrooms and how they're used, and we measure the incoming mains flow rate. That takes about twenty minutes and it is free.
It is the only way to give you an answer you can rely on. An online form that asks bedrooms and postcode is a guess — a good guess, often close, but a guess.
Frequently Asked Questions
What size boiler do I need for a 3 bedroom house?
Most three bedroom houses with one bathroom and up to around twelve radiators are well served by a 24kW to 30kW combi. Bedroom count alone does not decide it — bathrooms, radiator count and mains flow rate do.
Is a bigger boiler always better?
No. An oversized boiler costs more, tends to short cycle because it produces more heat than the property needs, and that causes more wear and can reduce efficiency. Correct sizing beats maximum output.
Why does my new combi still run out of hot water?
Usually low mains water flow rate rather than an undersized boiler. A combi heats water as it flows through, so it can only deliver what the mains supplies. If your mains gives ten litres a minute, no combi can exceed that.
What does kW mean on a boiler?
Kilowatts — a measure of output. On combis the headline figure usually refers to hot water output rather than heating output, which is why a 30kW combi delivers hot water faster rather than heating a bigger house.
Do I need a combi or a system boiler?
For most one-bathroom homes, a correctly sized combi. Where two or more bathrooms are used at once, or mains flow is poor, a system boiler with an unvented cylinder usually performs considerably better.
The Bottom Line
Get the size right and a mid-range boiler will outperform an expensive one that was never matched to the house. Get it wrong — particularly on mains flow rate — and no amount of kW will fix it.
If you want a straight answer for your property, use the calculator for an instant fixed price, or ring us and we will come and measure it properly.
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⚡ Get My Instant Price → 📞 Call 01942 800158Lewis Heating & Plumbing Ltd · Company No. 16460026 · Gas Safe Registered 928523 · Hindley Green, Wigan · FCA FRN 1055713