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What Size Boiler Do I Need?

Becky Mckay
By: Becky Mckay
Updated: 21st March 2022

What boiler size do I need?

If you're looking for a new, or replacement boiler, you may need to do a lot of research to find out what's best for your home. One crucial factor you have to consider, however, is what size boiler you will need.

Not all boilers are made equal, and neither are our homes. To ensure your home and hot water are perfectly heated, it's crucial to know what boiler size is all about.

In this article we'll be looking at what boiler size means, why it's so important and how you can ensure you pick the right size for your home.

What we mean by boiler size

When we talk about boiler size, it might be surprising to know that we're not actually referring to the physical dimensions of the boiler, but rather how powerful it is. The 'size' of a boiler is in fact an output rating, measured in kilowatts (kW).

A similar example to this would be the way we refer to battery 'sizes', they are classed in terms of power rather than dimensions (e.g. AA, AAA or 9 volts etc.) and this is how we know what size we need.

So for boilers, it's very similar and 'sizes' come in output ratings such as 23kW, 45kW etc. and each home will need its own unique 'size' of boiler in order to be powerful enough for the property's individual needs.

Why is this important to know?

You might think this is something only for an engineer to worry about, but if you're doing research for a new boiler this is a really important bit of info. If you don't get the right size of boiler for your home, it could either be running too little power; meaning your radiators will struggle to heat up fully and your hot water won't get warm enough. However, if you get a boiler that's far too powerful, you're going to be wasting a lot of money on high energy bills for no reason.

Higher output doesn't equal a better boiler, and low output doesn't mean it's cheaper (as you'll be working it much harder). So to get the very best out of a boiler you need to choose the best size for your circumstances.

One other very useful tip:

Don't assume the size of your existing boiler is what your new boiler should be!

It's very likely that if your household has changed since you got your last boiler – for example, you have new family members, fitted a new bathroom or you're considering an extension – you're going to need a more powerful boiler to serve the increased demand for hot water. Equally, you may have had a much too powerful boiler to begin with, and could see some savings on your bills if a smaller size can fulfil your heating needs.

Does boiler type matter?

In a word, yes! This is because different types of boilers use different levels of power.

Combi boilers are the most common type of boiler found in many homes, but for larger homes with multiple bathrooms regular and system boilers are usually fitted. These two boilers work alongside a hot water storage tank, so can easily provide higher quantities of hot water on demand than a combi.

Surprisingly, combis actually use more power than a system or regular boiler. That's because a combi is an all-in-one unit, having to provide heating and hot running water in one place, whereas the storage tank which works with the system and regular boilers, picks up a lot of the slack and reduces the amount of power the boiler needs to run on.

So it's important that you consider what type of boiler you have, or need to install, when you're considering the best size. You can read more about how each boiler type works, and the key differences in our helpful guide 'Boiler Types Explained'.

What size boiler do I need?

Now we've considered the boiler type, we can look at the best size option for your circumstances.

As a rule of thumb, working out the most suitable output rating (or size) for your boiler can be done with a very simple estimation. For every radiator you have in your home, allow for 1.5 to 2kW of power. For example, if your house has 10 radiators, you will need a boiler with at least 20kW of power.

This estimate is just a starting point, because, as we've discussed, different types of boiler might require an adjustment of this. It's not an exact figure, but more of a guideline to help you determine what is too low and what may be far too high. To help with this, we've created the simple table below for reference. Based on boiler type and how many radiators you have, you can see the recommended size range for your home.

No. of Radiators Combi System Regular
Up to 10 24-27 kW 9-18 kW 9-18 kW
Up to 15 28-34 kW 18-26 kW 18-26 kW
Up to 20 (and over) 35-42 kW 27-40 kW 27-40 kW

What if you don't fit in this chart?

We know that not all homes are the same, and perhaps this table doesn't reflect your exact circumstances. The good news is though, there are many different boiler manufacturers and most of them will offer a great range of sizes in all boiler types.

If you would like to get advice on the best size and type of boiler for you, you can contact an engineer to get a recommendation from those in the know. To find qualified engineers, who can offer professional advice and quotes for an installation, you can fill out our simple form below and get up to 3 free no-obligation quotes.



If you would like to compare boiler options, you can use our boiler comparison tool. It allows you to choose up to 4 boiler models and compare them by multiple features, including available size ranges.

If you would like to learn more about boilers and how to choose the right one for your home, we have some other articles we think you'll find useful:

Becky Mckay

About the author

Becky Mckay

Becky is one of our home heating and renewable energy experts and has a wealth of experience writing about the world of heating.

approved by engineer This article has been checked and approved by a Gas Safe registered engineer.

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