Count your radiators — that number, more than floor area or bedroom count, sets the output the boiler needs. Then weigh hot water demand: a family home with a power shower justifies the step up, and two or more bathrooms in simultaneous use points away from a combi. A fifteen-minute survey checks the gas supply, flue routes and water pressure, and the quote names the model in writing.
Radiators first
The heating side of a boiler has to match the radiators it feeds. Count them: that number, more than floor area or bedroom count, sets the output the boiler needs on the heating side. An oversized boiler cycles on and off inefficiently; an undersized one runs flat out and still leaves the far rooms cool.
Then hot water
For a combi, the second question is hot water flow — how many showers and taps you realistically run, and how hard you run them. A family home with a power shower justifies the step up to a Worcester Greenstar 4000 or 8000, which is exactly why they sit where they do on our list. Two or more bathrooms in simultaneous use points away from a combi altogether — see which type you need.
A rough guide by home type
These are indicative classes, not a quote — but they show where most homes land:
| Home | Typical guide |
|---|---|
| Flat or small terrace — up to ~10 radiators, 1 bathroom | Combi in the 24–30kW class |
| 3-bed semi — 10–15 radiators, 1 bathroom | 28–32kW class combi (hot water demand, not heating, drives combi size) |
| 4-bed — 2+ bathrooms or high hot-water demand | 35kW+ combi, or a system boiler with a cylinder |
Why does hot water dominate combi sizing? A combi heats water the instant a tap opens, so its headline kW figure is really its instantaneous water-heating power. The heat-loss of the house — what the radiators actually need — is usually far lower, and modern boilers modulate down to meet it. One more limit worth knowing: the mains flow rate at your property caps what a big combi can deliver. If the incoming main only carries so many litres a minute, a 40kW combi cannot magic up more.
Treat the table as a starting point only. The free survey sizes it properly for your house, and an oversized boiler is not a safety margin — it short-cycles and wastes gas.
What the survey measures
Fifteen minutes at your house settles it: we look at the gas meter and supply pipe, count the radiators, check the flue routes and the incoming water pressure. Those four things — not a postcode calculator — are what size a boiler properly. The quote that follows names the model and output in writing.
Matching size to our list
In practice: the Baxi/Main Eco Compact and Worcester Greenstar 1000 suit flats and small terraces; the Baxi 600 covers similar homes with a longer warranty; the Greenstar 4000 and 8000 carry family homes and power showers; the Vaillant ecoTEC plus is our pick for larger properties. There is no universally best boiler — just the output your radiator count demands and the warranty length that fits how long you are staying.