Guide
What size boiler do I need?
Boiler size is measured in kilowatts, and it is the number most likely to be got wrong, usually upwards. Here is what actually determines it.
Short answer
It comes down to how many bathrooms you have, how many radiators you are heating, and what your hot water demand looks like. Not the floor area of the house, and bigger is not better.
The three things that decide it
Bathrooms. A combi heats water on demand, so the question is how many outlets might run at once. One bathroom is a different boiler to three.
Radiators. The total heat output your radiators need, added up, sets the heating side of the requirement.
What you have now. Whether there is a hot water cylinder in a cupboard, tanks in the loft, or neither, changes both which type of boiler suits and what the install involves.
Why bigger is not better
An oversized boiler does not make the house warmer. It reaches temperature, switches off, cools, and fires again, over and over. That is called short-cycling and it wears the boiler out while burning more gas than a correctly sized one.
It is a common mistake because it feels like the safe option. It costs more to buy, more to run and it fails sooner.
Combi, system or regular
A combi heats water on demand with no cylinder and no tanks. It suits most flats and smaller houses and it frees up a cupboard. Its limit is simultaneous demand: two showers at once is where combis struggle.
A system boiler works with a hot water cylinder, so stored hot water is available to several outlets at the same time. It suits larger homes with more than one bathroom.
A regular boiler uses a cylinder and tanks in the loft. Usually kept where the existing pipework and water pressure make anything else awkward, which in some older Edinburgh properties it genuinely does.
How we actually decide
We size it at the free survey, by counting radiators, looking at the bathrooms and checking your incoming mains flow rate. That last one catches people out: a combi cannot deliver more water than your mains can supply it, no matter how many kilowatts it has.
We will not size a boiler over the phone. Anyone who does is guessing, and the guess is usually rounded up.
Prices shown are guide prices and exclude VAT. Last updated 4 August 2026. The final figure for any job is confirmed after a free survey.