What Size Boiler Do I Need? A Plain-English kW Guide

Boiler adverts throw kilowatt numbers around — 24kW, 30kW, 35kW — as if everyone knows what they mean. Here's the plain-English version, and why "get the biggest one" is exactly wrong.

What the kW number means

For a combi boiler, the headline kW figure is mostly about hot water — how fast it can heat water flowing through the tap or shower. The heating side of the same boiler typically needs far less power than the badge number. That's why sizing is really about your hot water demand, not your room count.

Rough sizing by home

• 24–27kW combi: flats and smaller houses, 1 bathroom, 1–2 occupants. Perfectly good shower, one outlet at a time.
• 28–32kW combi: the sweet spot for most 3-bed semis around Preston — a family home with one bathroom or a bathroom plus en-suite used at different times.
• 33–40kW combi: larger homes with higher flow demands — but at this point it's worth asking whether a combi is right at all.
• System boiler + cylinder: 2+ bathrooms in use at the same time, or weaker mains pressure. The cylinder stores hot water so everyone showers happily — see our combi vs system guide.

Why oversizing costs you money

A boiler that's too big for the house "short cycles" — fires up, hits temperature almost instantly, shuts down, repeats. That constant stop-start burns more gas, wears components faster and often leaves rooms less comfortable, not more. Modern boilers modulate (turn themselves down), but only within a range — start too big and even the minimum output is more than your house needs on a mild day.

The thing nobody checks: your mains flow

A 35kW combi can't give you a better shower than your incoming mains can feed it. If your mains flow is weak, the kW number is irrelevant — and it's one of the first things we measure before recommending anything. It's also why online boiler quotes from a postcode and three photos regularly get sizing wrong.

How we size it properly

When we quote, we look at: number of bathrooms and how they're actually used, mains flow and pressure, radiator count and house heat loss, and any plans (loft conversion, new en-suite) that change the picture. Then we recommend a size — honestly, including when the cheaper smaller boiler is the right answer. As a Baxi Approved Installer we can offer extended warranties on select new Baxi boilers at any size.

Get sized properly, free

A ten-minute look beats a guess that lives on your wall for 15 years. Call 07949 075721 or use the form below — free, no-obligation quotes across Preston. More on our boilers page.