What size boiler do I need?

Most homes need a combi between 24 and 42kW. A flat or 1 to 2 bed house is usually 24 to 27kW, a 3 to 4 bed 28 to 34kW and a larger house with two bathrooms 35 to 42kW.

Those are typical bands from trade sites, not rules. The right size for your home comes from how much heat it loses and how much hot water you use at once. Below is what those numbers mean, why the kW on the box isn’t the whole story, and what an installer should check on a survey.

What size boiler for my size of house?

These bands come up again and again on boiler sites. The table below uses MP Moran’s 2026 size guide, which lines up with the others. Use it as a starting point, not an answer.

Notice the table goes by bathrooms as much as bedrooms. That’s because on a combi the hot water is what drives the size. A three bed house where two people shower at the same time each morning needs more from a combi than a bigger house where they don’t.

Don’t just copy the number off your old boiler either. It may have been oversized when it went in. And houses change. New windows, loft insulation or an extension can all change how much heat your home needs. If you’re not sure the old one is worth keeping, our guide to the signs your boiler needs replacing will help.

If you’ve got two bathrooms in regular use, a combi might not be the best fit at all. A system boiler with a hot water cylinder can handle several taps at once in a way a combi can’t. Our guide to combi, system and regular boilers goes through the choice.

How does water flow rate affect combi size?

A combi heats water as it passes through. It can only heat what comes into the house. If the mains coming in is slow, a bigger combi won’t make your shower any stronger. It’ll just heat a weak flow a bit faster.

That’s why the flow rate should be measured at the kitchen tap before anyone picks a combi. It shows how many litres a minute your mains can actually deliver. If it’s low, going up a size is money wasted. A system boiler with a cylinder, or sorting the supply pipe, might be the better fix.

It’s the one thing an online size calculator can’t check. It doesn’t know what’s coming out of your tap. It can’t tell if the stopcock is half shut or the pipe coming into the house is old and narrow. Both of those cap the flow, however big the boiler is.

Why the kW on the box isn’t the heating output

This catches a lot of people out. The number in a combi’s name is usually its hot water output. The heating side is often lower.

Take the Worcester Bosch Greenstar 8000+ combi. On its product page the 32kW model gives 29.6kW to the central heating. The 36kW and 40kW models both give 33.7kW to heating. So the 40kW and the 36kW give you the same heat to the radiators. The difference is in the hot water.

The same page lists the hot water flow rate for each model at a 40°C temperature rise. The 32kW gives 12 litres a minute. The 36kW gives 13.5 and the 40kW gives 15. Those figures only hold if your mains can supply that much water in the first place.

That matters when you compare quotes. The number on the front tells you about the hot water, not how warm the house will get. Two boilers with different numbers on the front might heat the house exactly the same. Ask your installer for both figures, heating and hot water.

Is a bigger boiler better?

No. A boiler that’s much too big heats the radiators fast, switches off and fires again a few minutes later, which is called short cycling and is harder on the parts than running steadily. It also costs more to buy, so go up a size only for a real reason like a big family or a planned extension.

What should an installer check on a survey?

We don’t use a calculator for this. We’d rather look at your house.

Whoever does your survey should check the flow rate at the kitchen tap and how much heat the house loses. They should count the bathrooms and ask how you use hot water. How many people. How many showers in the morning. Whether there’s a bath that gets filled every night. And if you’re planning an extension or a loft conversion. Those answers change the size more than the bedroom count does.

Book a survey for new boiler installation and we’ll measure it properly.

And because we fit heat pumps too, we’ll mention it if your home looks like it could suit one later. Our guide on whether your home suits a heat pump covers what we look at. A heat pump is sized in a different way, from a full heat loss survey. Our guide to heat pump sizing explains how that works.

Frequently asked questions

What size combi for a 3-bed house?

Trade guides usually put a three or four bed house with a bathroom and an en-suite at 28 to 34kW. Many three bed homes sit at the lower end of that. But it depends on your heat loss, your mains flow rate and how much hot water you use at once. A survey will tell you for sure.

Is there a boiler size calculator I can trust?

Online calculators give a rough idea, mostly from bedrooms, bathrooms and radiators. They can’t measure your mains flow or how much heat your house loses. Use one as a starting point, then get an installer to check it on a survey.

What’s the difference between kW for heating and kW for hot water?

On a combi, the headline figure is usually the hot water output. The heating output is often lower. On the Worcester Bosch 8000+ 32kW combi, for example, the heating output is 29.6kW.

Want to know what size boiler your home actually needs? We’ll survey it properly and give you a clear quote.

Sources

• MP Moran, “What size boiler do I need? Expert UK boiler size guide 2026” (published 9 March 2026, updated 16 July 2026), checked 3 November 2026, www.mpmoran.co.uk
• Worcester Bosch, “Greenstar 8000+ Combi” product page (central heating outputs and hot water flow rates at 40°C rise by model), checked 3 November 2026, www.worcester-bosch.co.uk