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What size boiler do I need?

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How do I know what size boiler my home needs?

For starters, boiler size doesn’t actually mean its physical size. Rather, it refers to the boiler’s output in kilowatts (kW), which measures how much energy the boiler produces in the form of heat. The ‘bigger’ the boiler, the higher the kW.

A general rule of thumb: the bigger your house and the more radiators, hot taps, and showers you have, the more powerful the boiler you’ll need – therefore a bigger boiler will be more suitable.

When choosing the correct boiler size for your home, you should consider how many bedrooms, bathrooms and radiators you have, as well as your existing boiler. You should also think about the type of boiler you want.

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What factors affect boiler size?

There are several factors to think about when picking the right size of boiler for your home, including:

  • 1

    Size of home and household

    How many radiators, bedrooms and bathrooms are in your home, and how many people are living there? The more heat and hot water your household requires, the bigger the boiler you’ll need.

  • 2

    Level of insulation in your home

    If your home is well insulated, you can get a smaller, more energy-efficient boiler – as you won’t need such a high output of heat.

  • 3

    Type of boiler

    Combi boilers are more suitable for smaller properties, while system boilers are better for larger ones with greater demand for hot water. Electric-only boilers are another compact and efficient option, as they only heat up water (no radiators).

  • 4

    Type of fuel available in your home

    Most UK homes use gas boilers (such as combi boilers), which are compact and energy efficient. If your home isn’t connected to a mains gas supply, you may need a bigger oil-fired boiler instead, or a smaller electric-only boiler.

What size combi boiler do I need?

Combi boilers are the most popular boiler type in the UK, due to their smaller physical size (no need for additional units or bits of equipment) and better energy efficiency.

As with deciding on any boiler, you’ll need to think about its size (power output in kW) and the number of radiators, bathrooms and bedrooms in your home.

Smaller houses typically require a 24-30 kW combi boiler, while bigger properties would need a larger 30-35 kW boiler.

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Why it's important to choose the right size boiler

By picking a boiler that best suits your home and your household’s heating needs, you can ensure your home is adequately heated, while also avoiding wasting energy and money.

  • If your boiler is too big…

    • It’ll use more power and produce excess energy, which is bad for the environment

    • Your energy bills will be higher, due to all the excess energy generated

    • It can perform poorly and break down quicker

  • If your boiler is too small…

    • Your radiators won’t feel warm enough

    • You may often run out of hot water

    • Your home might feel too cold in winter

Calculate the boiler size you need

To work out the best boiler size for your home, you should look at the size of your property.

As a rough guide, we’ve listed the number of bedrooms, bathrooms and radiators your home might have, and the recommended combi boiler size to go with it.

Number of bedrooms

Number of bathrooms

Number of radiators

Suitable boiler size

1-2

0-10

1

24-27 kW

3-4

More than 10

2-3

28-34 kW

More than 4

More than 20

More than 3

35-43 kW

System or regular boilers have a smaller power output than combi boilers, as they store hot water ahead of time. If you’re getting either a system or regular boiler, here are the sizes we’d recommend for your home.

Number of bedrooms

Number of bathrooms

Number of radiators

Suitable boiler size

1-2

0-10

1

9-18 kW

3-4

More than 10

2-3

18-26 kW

More than 4

More than 20

More than 3

27-40 kW

If you buy a boiler that’s a much higher capacity than you need, you’ll be wasting money and energy. As well as being more expensive in the first place, an oversized boiler will cost you more in the day-to-day running of it.

You’ll be paying higher energy bills, and your boiler will be producing excess fuel that can be damaging to the environment. Not to mention, an oversized boiler will run ‘on and off’ for short periods of time, which can result in it breaking down quicker.

A boiler that’s too small will struggle meet your household’s needs. You might run out of hot water often, and find that your radiators aren’t warm enough.

For a larger four-bedroom house with more than twenty radiators, consider a combi boiler with an output between 35-43 kW.

The average three-bedroom house in the UK will typically only need a small or medium combi boiler between 29-35 kW.

It also depends on the type of property you’re living in. For example, flats or bungalows only require a boiler size on the lower end of the scale (around 29 kW), while a detached house will likely need something bigger (around 35 kW).