What size boiler do I need?

For most UK homes: a 24 to 27kW combi for a one or two bedroom flat with one bathroom, 28 to 30kW for a typical three bedroom house, and 32 to 35kW for a four or five bedroom house or anywhere with two bathrooms. Those are combi outputs, which are driven by hot water demand rather than heating demand.

Why bigger is not better

An oversized boiler does not heat your house faster. It reaches temperature, shuts off, cools, and fires again. That cycling wastes gas, puts wear on the ignition and heat exchanger, and can make the boiler less efficient than a correctly sized one.

It also costs more to buy. Paying extra for output you cannot use is a common and avoidable mistake.

How combi sizing actually works

A combi has two outputs, and the one quoted on the box is usually the hot water figure. That is the number that matters, because heating a three bedroom house takes far less power than running a shower.

The practical measure is flow rate in litres per minute. A 30kW combi typically delivers around 12 litres a minute; a 35kW around 14. A standard shower uses 9 to 12 litres a minute. Which is why two bathrooms running at once is where combis start to struggle, and where a system boiler with a cylinder often makes more sense.

Your incoming mains flow rate matters too. A combi cannot deliver more water than your mains supplies, so a powerful boiler on a weak mains is money wasted. A good installer will measure it before quoting.

Sizing a system or regular boiler

Different logic. Here the output is about heating the house, because hot water comes from a cylinder that heats up over time rather than on demand.

The rough guide is 1.5kW per radiator plus 3kW for the cylinder, though a proper heat loss calculation accounts for insulation, window area, ceiling heights and whether the property is detached, semi or terraced. A 12 radiator house with a cylinder lands around 21kW, which is why system boilers are often smaller than the combi that would replace them.

What a good installer should do

Count the radiators. Ask how many bathrooms and whether they get used simultaneously. Measure the mains flow rate. Ask about insulation and whether you are planning an extension.

An installer who quotes a size over the phone without any of that is guessing, and the usual guess is oversized, because oversized rarely generates complaints and undersized always does.

Does a bigger boiler heat the house faster?

Barely, and not in a way you would notice. Once a room reaches the temperature set on the thermostat, the boiler stops. A larger boiler gets there marginally sooner and then spends more of its time cycling on and off, which is where the efficiency loss comes from.

What genuinely affects how quickly your house warms up is radiator sizing, insulation and how well the system is balanced. An undersized radiator in a cold room is not fixed by a bigger boiler.

How to check your mains flow rate yourself

It takes two minutes and it is the number that decides whether a combi will satisfy you.

Take a measuring jug of a known size, run the cold tap in the kitchen fully open, and time how long it takes to fill. Multiply up to get litres per minute. Under 10 litres a minute is weak for a combi, 12 to 15 is comfortable, and above 15 will support a powerful one.

If you measure under 10, that is worth raising with every installer who quotes. It may point towards a system boiler with a cylinder rather than a bigger combi.

What about radiators and future extensions?

A rough heating estimate is around 1.5kW per radiator plus 3kW for a hot water cylinder, though a proper heat loss calculation accounts for insulation, window area, ceiling height and whether the property is detached, semi or terraced.

If you are planning an extension or a loft conversion in the next couple of years, say so when you get quotes. There is a difference between sizing generously for known plans and being sold an oversized boiler for a hypothetical one, and a good installer will tell you which you are doing.

Common sizing mistakes

• Matching the old boiler's output without checking. Fifteen year old sizing was often generous, and your insulation may have improved since.
• Sizing for a bathroom you are planning but have not built. Size for the house you have, then discuss whether to allow headroom.
• Ignoring the mains flow rate on a combi. This is the one that causes disappointing showers in brand new boilers.
• Assuming a bigger boiler heats the house quicker. It does not, in any meaningful way.