What size boiler do I need?

Boiler size means power output, measured in kilowatts, not physical size. Get it wrong in either direction and you pay for it.

Too small and the shower goes cold when someone runs a tap. Too big and you've bought a boiler that spends its life cycling on and off, wasting gas and wearing itself out.

A rough guide for combis

Flat or small house, one bathroom, up to 10 radiators: 24 to 27kW. Medium house, one bathroom, 10 to 15 radiators: 28 to 34kW. Larger house, two bathrooms, 15 to 20 radiators: 35kW and up.

For system and regular boilers the numbers are lower, because the cylinder does the hot water work, and the boiler only has to heat the radiators.

Bigger is not better

This is the most common mistake, and it's usually made by the homeowner rather than the engineer. Oversizing feels like a safe bet. It isn't.

A boiler that's too powerful for the house reaches temperature quickly, shuts off, cools, and fires up again. That cycle is the single biggest cause of premature failure, and every one of those starts burns gas.

How a good installer works it out

They count your radiators, ask how many bathrooms you have, check your mains water pressure and flow rate, and look at the insulation. Then they size the boiler to the heat loss of the house, not to a guess.

If an installer quotes you a boiler size over the phone without asking a single one of those questions, that tells you something useful about how they'll do the rest of the job.