What's the best boiler for a 3-bed house?

A three-bed semi with one bathroom is the most common heating job in the country, and for most of them the answer is a combi in the 24kW to 30kW range.

But the number that matters is not the one people think.

Two different kW figures

A combi has a heating output and a hot water output, and they are not the same. The heating output for a typical three-bed is 18kW to 24kW - that is all the radiators need.

The hot water output is what determines whether your shower is any good, and that is where the 30kW or 35kW comes from. If you have one bathroom and an en-suite you rarely use together, 30kW is plenty. If two showers run at once, you are into system boiler territory with a cylinder.

The oversizing mistake

Fitting a 35kW combi in a small house because bigger sounds safer makes the boiler cycle on and off constantly, which is less efficient and wears it out faster.

A good installer sizes from a heat loss calculation, not from a rule of thumb. If someone quotes you a kW figure without asking how many radiators you have, they are guessing.

Water pressure decides more than brand

A combi is only as good as your incoming mains. If your cold flow rate is under about 12 litres a minute, a combi will always disappoint you in the shower no matter which one you buy. Any decent installer will measure this before recommending anything. Ask them to.

Brands, honestly

Worcester Bosch and Vaillant have the best reputations and price accordingly. Ideal and Baxi are solid and cheaper. Viessmann is excellent and slightly less common, which sometimes means fewer local engineers who know it.

The truth most installers will tell you privately: fitting quality matters more than badge. A well-fitted Ideal will outlive a badly fitted Worcester.