For most homes with one bathroom, a combi boiler of around 24 to 28 kW is enough. Larger homes with two bathrooms often need something nearer 30 to 36 kW. But with a combi, the right size isn't really set by how many bedrooms you have. It's set mainly by how much hot water you use at once, and by how much cold water your mains supply can deliver.
This post explains what the kW figure on a combi means, why your water supply matters as much as the boiler, and a few simple checks you can make before anyone quotes. Choosing and fitting the boiler is a job for a Gas Safe registered engineer, but knowing the basics helps you spot a recommendation that doesn't add up.
What the kW figure on a combi means
A kW, or kilowatt, is a measure of how much heat a boiler can produce. It has nothing to do with how big the boiler is on the wall.
A combi has two outputs: one for the radiators and one for hot water. They're often different. Baxi's 24 kW combi, for example, gives 24 kW to hot water but only 21 kW to heating. Most homes need far less heat for their radiators than a combi puts into hot water, so for a combi it's usually the hot water side that decides the size.
Why hot water decides the size
A combi has no hot water tank. It heats cold water from the mains as it flows through, the moment you turn on a tap. The more kW it has, the more litres a minute it can warm up.
Manufacturers quote this flow at a 35°C temperature rise, which means warming the water by 35 degrees, for example from 10°C to a comfortable 45°C. Baxi gives these figures for two of its combis:
Around 10 litres a minute is enough for one good shower, but not really for two at once.
Winter matters here. The water coming into your home is colder in the cold months, so the boiler has to warm it further. A combi that copes in summer can give a cooler or weaker shower in January, which is why an undersized boiler often shows itself at this time of year.
Your mains supply sets the limit
A combi can only heat the water it's given. If your mains supply can only bring in 12 litres a minute, Baxi's own guidance is that a combi above 28 kW is no use, because the extra power has no extra water to work on.
Two things matter. Pressure is how hard the water pushes, measured in bar. Flow is how much water actually arrives each minute. A home can have decent pressure but poor flow, often because of an old or narrow supply pipe, and it's the flow that limits a combi most.
Simple checks you can make at home
You can gather most of what an engineer will ask about yourself:
Count your bathrooms and showers, and how often two are used at the same time.
Count your radiators, including any heated towel rails.
Check whether your showers are electric (they heat their own water and don't use the boiler) or mixer showers fed by the boiler.
Do a bucket test. With no other taps running, time how long the kitchen cold tap takes to fill a bucket of known size, such as 10 litres. If it takes a minute, your flow is about 10 litres a minute.
Think about plans, such as an extension or an extra bathroom, that would add to demand.
Don't remove the boiler's casing or change anything inside it. The model name is usually on a label on the front or in the user manual, and that's all you need.
When a combi isn't the right answer
If two showers often run at once, your mains flow is weak, or a big household all wants hot water in the morning, a combi may struggle whatever its size. A system boiler with a hot water cylinder, which stores heated water ready to use, often suits those homes better.
Our guide to the different types of boilers explains how combi, system and conventional boilers compare.
Why bigger isn't always better
It's tempting to pick the biggest combi to be safe, but it doesn't always help. A larger boiler costs more to buy. If your mains flow is the limit, the extra power can't be used. And a boiler much bigger than your home needs for heating may turn on and off more often on mild days, which wastes gas and adds wear.
The aim is a boiler matched to your hot water use, your radiators and your water supply, not the largest one available.
Common questions about combi boiler size
Is a 24 kW combi enough for a three-bedroom house?
Often, yes, if there's one bathroom and you mostly shower. Baxi suggests 24 or 28 kW for a typical two or three-bedroom home with one bathroom. If your household prefers baths, the 28 kW fills one faster.
Can I switch from a tank system to a combi?
Usually, as long as your mains flow is good enough. The loft tank and hot water cylinder come out, which frees up space, but you lose the store of hot water, so a combi suits homes that rarely run two showers at once.
