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How to Pick the Right Size Boiler for Your Home

Unfortunately, there’s no such thing as a one-size-fits-all boiler which can make finding the correct one for your home a daunting task.

When it comes to selecting a new boiler for your home, size really does matter. Get the wrong size and you could end with a boiler that’s incapable of heating your home properly or a boiler that wastes energy and increases your energy costs. This means that getting the right size boiler is crucial to ensuring that your home is comfortable and you don’t have to spend more money than you have to on your bills.

But how are you supposed to know which size is best? As it’s so important to get right, we thought we’d share some advice on finding the perfect size boiler for your home.

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Why Does Size Matter so Much?

If you get a boiler that’s too small, it won’t be able to keep up with your household’s heating and hot water needs, which means you’ll be left with cold and uncomfortable rooms and not enough hot water. And if you get a boiler that’s too big for your home, your energy bills will go through the roof because the boiler will produce more energy than you actually need. Buying and installing an oversized boiler will also lead to unnecessary emissions, so wasting energy this way is not just damaging to your wallet but the environment too.

We should point out that the size of a boiler is not its physical dimensions. Boiler size refers to the output, which is measured in kilowatts (kW). Let’s say you’ve been thinking about purchasing the Baxi 200 Combi for example. This boiler comes in two sizes of 24kW and 28kW, so you need to know which size is best for you before committing to buy.

Factors to Consider When Choosing the Right Size Boiler

Pretty much all combi, system and heat-only boilers come in a range of sizes, no matter which brand you choose. There are, however, some factors that need to be considered to figure out the right boiler size, such as:

  • Size of your current boiler: You can look at your current boiler to find out its size to give you a better idea of what size replacement boiler you need. That’s not to say you should just pick a new boiler that’s the same size as your current system though. Your home might not be the same as it was when you had your current boiler installed and your family may have grown, leading to an increase in heating and hot water demand.
  • Hot water demand: Everyone’s hot water demands differ but it’s essential to choose a boiler that can provide enough hot water for showers, baths and things like washing up. If you use a lot of hot water and you have more than one bathroom and a large family, you might be better off with a system or heat-only boiler. As combi boilers take water from the mains and heat the water when a tap is turned on, they’re better suited to homes with smaller hot water demand. To find the correct size boiler for your hot water needs, you first need to know whether you need a combi, system or heat-only boiler.
  • The number of radiators in your home: If you only have a few radiators in your home, you can get away with installing a small boiler with a lower heating output. Houses with one bathroom and around ten radiators tend to need a small to medium size boiler, and bigger homes with multiple bathrooms and lots of radiators will require one of the larger boiler outputs.

The quickest and easiest way to determine the right size boiler is to call in a qualified Gas Safe registered engineer as they can assess your home and needs before giving you a recommendation on size and type.

What Size kW Combi Boiler Should I Install?

Usually, the larger the home, the larger the boiler needs to be to keep up with the heating and hot water demand. The table below gives you a rough idea of what size combi boiler is needed according to the size of your property.

Size of HomeNumber of
Number of
Boiler Size /
Small 1 to 2 bed
flat or house
1Up to 1024kW to 27kW
Medium 3 to 4
bedroom house
1 to 2Up to 1528kW to 34kW
Large 4 or more
bedroom house
2 or more Up to 2035kW to 42kW
Remember, every home is unique and has different needs so you must only use the table as
a rough guide.

Get Advice from a Gas Safe Registered Engineer

Like we said before, it’s best to get a Gas Safe registered engineer to take a look at your home and help you choose the best size boiler for your flat or house. At least this way you won’t have to stress about whether you’ve got it wrong or right. We also recommend you reach out to more than just one professional engineer and don’t forget to ask them to give you a free quote on a new boiler whilst they’re at your home.

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