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.
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 single best boiler — there is the right output for your radiator count and the right warranty for how long you plan to stay.