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What Size Boiler Do I Need?

Author: Alyssia Dudley
Updated: Oct 12, 2023
10 minutes read
  • Save hundreds of pounds with the right size boiler
  • Better energy efficiency means lower energy bills and carbon emissions
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Rather than the shape of a boiler, boiler size refers to the power size of your boiler. In other words, the amount of kilowatts (kW) the boiler you install is capable of delivering. Choosing a boiler with the right power capacity is crucial if you want to ensure your boiler isn’t wasting energy or money.

The wrong size boiler could leave you forking out for energy you don’t need if you have too many kilowatts or unable to get all the heating and hot water you want on a daily basis if you don’t have enough. Ultimately, incorrect use will shorten the life of your new boiler.

Having already spent a sizeable amount on purchasing a new boiler, the last thing you want is to end up with the wrong one or be forced to use it incorrectly meaning it will need replacing sooner than it should.

There are, in fact, a number of straightforward ways to calculate which size boiler you need without having to do major mathematics! Read on for all you need to know about how to choose the right size boiler the easy way to ensure your boiler is as energy efficient as possible.

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Why Is Boiler Size Important?

Hot water and heating are basic living requirements we use freely. Running a hot shower or washing up the plates are tasks we expect to be able to do with ease. So much so that we give little thought to the power behind these modern conveniences.

With energy costs rising faster than ever before and guarding the environment against unnecessary greenhouse emissions a responsibility at the forefront of each of our minds, it’s more important than ever before that the boiler size we select for our home is fit for purpose.

How Do I Choose the Right Boiler Size?

The heating needs of different families and properties vary greatly so there are a number of factors to consider when deciding which boiler size will be right for your home. Let’s take a look at the important elements you need to consider within the context of your home.

1. No. of bathrooms and showers: Larger families where numerous people use hot water at the same time require a larger boiler size which can cope with heavy demand.

Hot water and heating for these bigger families is usually managed through a system or heat only boiler system. Similarly, if you have low water pressure in a larger property, you may find a heat only boiler the most suitable option. Compare boiler types.

2. No. of bedrooms: Similarly, more bedrooms mean more radiators to heat. Unlike taps and showers which we turn on and off at different times, radiators are programmed to switch on and off as one so the number of bedrooms in a home are a popular way to assess the boiler size a property requires.

3. Future plans: With your new boiler lasting up to 15 years, when considering how many bedrooms you need to heat and hot water outlets the boiler will have to serve, it’s important to think about how your needs might change in the next few years.

If you plan to grow your family, add more bathrooms or shower rooms or an extension somewhere, incorporate those plans into your calculations because you’ll still have the same boiler for years to come.

5. Boiler type: Official sizing for various types of boilers are calculated differently. When viewing the kilowatts on a combi boiler the measurement will refer to the maximum amount of hot water the boiler can produce. For system and heat only boilers the figure will refer to the amount of heat produced.

Similarly, if you are using an alternative energy source, such as solar panels, this may affect the size of boiler you need.

6. Insulation quality: If you know you have poor or ageing insulation it’s likely your boiler will need to do more work to keep up with the quantities of heat leaking out through the gaps and holes in your property.

More work means more kilowatts required to keep your home warm. Alternatively, you might decide to upgrade your insulation before replacing your old boiler.

Your operative will be able to provide an insulation assessment for you so you can see how much heat you might lose if you stick with your existing insulation. Remember calculations for heat lost through poor insulation include floors and windows.

Your operative will give you a figure based on conditions akin to the coldest day of the year so any insulation update you request provides the robust insulation you need to lower your energy use and purchase the best for your home in all weathers. The best boilers of 2023 can give you an idea on how much you can save.

How Do I Calculate Which Boiler Size I Need?

So how do you match your family’s needs to the stats on displayed on the boilers you see? There are number of options. These are the most popular ways to calculate which size boiler you need.

1. No. of bathrooms and showers: To assess how many kilowatt will best serve your home using the number of bathrooms and showers you have, the following proportions should work as a general rule for contemporary boilers.

2. No. of bedrooms: Stats show that on average most 3 or 4 bedroom houses have about 10 radiators to heat. However, looking solely at the number of bedrooms you have in your home doesn’t take into account how many bathrooms or showers you have or the number of floors in the property.

If we look at the number of kW it takes to power a moderate two-storey home with one bathroom, in comparison to a larger home with the same number of bedrooms but an additional bathroom, we can get a clearer idea of how our power needs change with the size of a property.

3. Boiler type: If you remember, earlier in the article we said for combi boilers kilowatts are calculated in terms of hot water produced but for system and heat only boilers the number of kWs refers to the amount of heat produced.

As such, the previous table would give a better reference point for combination boilers although combi boilers aren’t usually suitable for larger homes and families.

For system boilers and heat only boilers the following data, which uses the quantity of radiators in the property, may be more useful.

If you calculate the kilowatts you’ll need by multiplying the number of radiators you have by 1.5kW plus 3kW for every water storage cylinder you possess, (no. of radiators x 1.5kW + 3kW x no. of water cylinders), the minimum boiler sizes you’ll need for your home look like this.

Want to know more? Learn about the major differences between combi, system and heat only boilers in our guide to boiler types or talk to our experienced operatives today about the best type and size boiler for your home.

Combi Boilers: The Ultimate Guide gives more information. Alternatively, get a bespoke quote for a boiler from our expert team and learn which size you’ll need.