What size combi boiler do I need for my home?
The right size of combi boiler depends mostly on how much hot water your home uses at once, not on how big the house is. As a rough guide, a 24 to 26kW combi suits a flat or small house with one shower. A 28 to 30kW model suits a typical family home with one bathroom and perhaps an en suite. A 32 to 35kW model suits homes with two bathrooms where hot taps are often running at the same time. Your mains water supply also has to be able to keep up, whatever size you choose.
This post explains what the kW figure on a combi actually means, why winter is the real test of a combi, why a bigger boiler isn't always better, and the details an engineer checks before recommending a size. There's nothing on the boiler itself you need to open or adjust to work this out.
What the kW figure on a combi means
kW stands for kilowatt, a measure of how much heat the boiler can produce. A combi heats water straight from the mains as it flows through, so its kW rating mainly tells you how much hot water it can deliver at once. This is the flow rate, measured in litres per minute.
As a rough guide, when the boiler is raising the water temperature by 35°C:
24kW: about 10 litres per minute, enough for one shower
30kW: about 12 litres per minute, enough for a good shower or a shower and a small tap
35kW: about 14 litres per minute, enough for a shower and a kitchen tap together
Many combis have a lower figure for heating than for hot water. That's normal: most homes need far less heat for the radiators than a combi produces for a shower.
Why winter is the real test
In winter the water coming into your home from the mains is much colder than in summer. The boiler has to raise its temperature further to reach a comfortable shower, and it does that by slowing the flow down. So the same combi gives noticeably less hot water per minute in January than it does in July.
That's why a combi that seemed fine when it was fitted in summer can leave a shower feeling weak once the weather turns cold. A good size recommendation allows for the coldest months, not the warmest.
Why bigger isn't always better
It's tempting to pick the most powerful combi on offer, but a boiler that's much bigger than your home needs can work against you:
Short cycling. When the heating only needs a little warmth, an oversized boiler heats the radiators too quickly, switches off, then fires up again soon after. This constant on and off wastes gas and wears parts.
Higher cost. Larger models usually cost more to buy.
No extra water. A bigger combi can't deliver more water than your mains supply provides. If the pipe coming into the house only brings in 12 litres a minute, a boiler rated for 14 can't give you more.
Modern combis can turn their output down a long way when demand is low, which helps, but sizing it properly in the first place still matters.
What decides the right size for your home
When we survey a home for a new combi, these are the main things we look at:
How many bathrooms and showers you have, and how often more than one is used at the same time.
The flow rate and pressure of your mains water, measured at the kitchen tap or the stop tap.
The size of the incoming water pipe, since older, narrow pipes can limit flow.
The gas supply. A larger combi needs a gas pipe big enough to feed it, and an undersized pipe can stop it reaching its full output.
How much heat the house needs, based on its size, insulation and radiators.
If your home has two or more bathrooms in regular use at the same time, a combi may not be the right type at all. Our guide to combi vs system boilers explains when stored hot water in a cylinder works better.
What you can check yourself before a survey
You don't need any tools to gather useful information:
Look at your current boiler's details. The make, model and kW rating are usually on a label on the front or underneath, or in the user manual. Don't remove the casing to find it.
Think about your busiest time of day. How many showers, baths and hot taps run together in the morning or evening?
Notice how your current hot water behaves. If the shower goes cold or weak when someone opens another tap, write that down for the engineer.
This helps the engineer understand how your household really uses hot water, alongside the measurements they take.
How we size and fit a new combi
As Worcester Bosch Accredited Installers and Gas Safe registered engineers, we start every new boiler with a survey. We check the gas supply and pipework, where the boiler will go and anything extra the job needs, then recommend a size that suits how your household uses hot water. On older systems, we may also recommend a power flush, a deep clean of the radiators and pipework, so sludge doesn't damage the new boiler.
If you're thinking about a new combi, RKL Plumbing & Heating can survey your home and recommend the right size before anything is fitted. Find out more about our boiler installation service across Liverpool, including Worcester Bosch boilers with warranties of up to 12 years.
