If you are replacing a boiler, the kW number on a combi can feel like a guessing game. As a rough answer, most homes with one bathroom are well served by a combi of around 24 to 28 kW, and homes with a bathroom and an en-suite usually need something nearer 30 to 35 kW. What really decides it is how much hot water you use at once, and how much water your mains can deliver.
This post explains what the kW figure means, a simple check you can make at the kitchen tap, and when a combi is not the right boiler at all. There is nothing here you need to take apart: fitting a gas boiler is always a job for a Gas Safe registered engineer.
What the kW figure on a combi means
kW, or kilowatts, is the boiler's maximum output: how much heat it can put out at full power. A combi, short for combination boiler, heats your radiators and also heats water straight from the mains the moment you open a hot tap, with no tank or cylinder.
Most combis list two figures. The higher one is the hot water output, and it is usually the number in the model name, such as a 30 kW combi. The lower one is the most it will put into the radiators. When people talk about the size of a combi, they mean the hot water figure.
Why hot water decides the size, not the radiators
Heating a home takes less power than many people expect. A typical three-bedroom semi often needs well under 15 kW to stay warm on a cold day, which almost any combi can manage.
Hot water is different. A combi has to warm cold mains water by around 35°C as it flows through, instantly. The more kW, the more litres per minute it can heat. A handy rule is to divide the kW by 2.4:
- 24 kW gives roughly 10 litres a minute
- 30 kW gives roughly 12 litres a minute
- 35 kW gives roughly 14 litres a minute
A good mixer shower uses around 8 to 10 litres a minute, so it is easy to see why a small combi struggles when someone runs the kitchen tap mid-shower.
A rough guide by type of home
As a starting point, before anyone has looked at your house:
- A flat or small house with one bathroom: around 24 to 26 kW.
- A family house with one bathroom and busy mornings: around 28 to 30 kW.
- A house with a bathroom and an en-suite, where two showers may run close together: around 32 to 35 kW, and only if the mains can keep up.
These are ranges, not rules. How many people live there, and when they use water, matters as much as size.
Check your water flow before you choose
A combi can only heat the water your mains supplies. A 35 kW combi on a supply that gives 10 litres a minute will still only give you 10 litres a minute at the tap, so the extra power is wasted.
You can get a rough idea yourself, safely. Make sure nothing else in the house is using water, turn the kitchen cold tap fully on, and hold a bucket under it for exactly six seconds. Measure how many litres you collected and multiply by ten. That is your flow in litres per minute.
We measure flow and pressure properly on the survey, but this tells you whether your supply is likely to be the limit.
When a combi is the wrong choice
If you have two or more bathrooms that are often in use at the same time, or a weak mains supply, a bigger combi will not fix it. A combi shares its flow between every open tap, so two showers at once means two weaker showers.
In those homes a system boiler with a stored hot water cylinder usually works better. An unvented cylinder, which holds hot water at mains pressure, can feed several showers at once with strong flow. We fit unvented hot water cylinders as well as combis, so we can talk you through both.
It works the other way too. One of our customers wrote about us swapping the old boiler in their bungalow for a combi and taking out the large water tank, which gave them a whole cupboard back. For a smaller household, that is often the better fit.
Hard water and your new combi
Much of the water in this part of West Sussex comes from the chalk of the South Downs and is classed as hard. Around Chichester, Portsmouth Water's own figures put it well above 200 parts per million of calcium carbonate.
Hard water leaves limescale inside the combi's hot water heat exchanger, the part that passes heat from the boiler into your tap water. Over time that cuts the flow and the temperature you get. Building Regulations guidance expects scale protection, such as a scale reducer on the cold supply, to be fitted with a combi where water is this hard. It is worth checking that any quote includes it.
Why bigger is not always better
It is tempting to buy the biggest combi to be safe. But a larger boiler costs more, and if your mains cannot feed it, you pay for output you never get.
On the heating side, a boiler that is far bigger than the home needs can switch on and off in short bursts, called short cycling, which wastes gas and wears parts. Modern combis can turn their output down a long way to avoid this, which is called modulation.
How we size a boiler on a survey
When we visit to quote for a new boiler, we ask how many people live in the home, how many bathrooms you use and whether showers often run at the same time. We look at the space you have, your radiators and your existing system, and we measure your water pressure and flow rate. Then we recommend the type and size that suit how you live.
For a feel for the options first, our online new boiler estimate asks a few questions about your home and suggests boilers to suit.
If your boiler is due for replacement and you are not sure what size you need, The Sussex Heating Care Company can survey your home and recommend the right combi. You can find out more about our boiler installation service and book a survey when you are ready.
