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Last Updated on January 22, 2021

What Size Boiler Do I Need? Residential Sizing Calculator & Rule of Thumb

Zac Houghton

Zac Houghton

Construction expert and founder of Loftera.

They say that size isn’t everything, and when it comes to boiler size, that might well be true because the boiler you need is likely to be much smaller than you might think.

Every home has one these days, but if you are in the process of changing it or in need of a new one, then you might be asking yourself “What boiler do I need?”.

what size boiler do I need

Table of Contents

How to size a boiler

In a nutshell, the size of your boiler links to the output rating of your unit and not the size it actually measures in centimetres or inches.

The output rating relates to your boiler’s power and what it gives to your heating process throughout your property. In reality, the boiler size rule of thumb is the more radiators you have, the higher output rating you need with A++ being the best that is currently on the market.

So if you’re looking to find out the answer to “what size boiler do I need for 16 radiatiors?”, then the answer is one with the best possible rating that you can find.

Which boiler do I need?

As well as deciding on how big a boiler you need, you’ll first need to work out which type of boiler you actually want or require.

Here in Britain, most our boilers are fuelled by gas, but depending on where you live, you might not be connected to the National Grid and will therefore need oil or LPG fuels to make your boiler work.

All the fuels run the three most popular types of boiler in the UK, which includes the combi boiler, system boiler and the regular boiler. Here are a few points on how they all differ:

Combi Boiler

Arguably the most popular boiler around, the combi boiler is not only smaller in size, but it’s also much better on your bank balance with some of the best energy efficiency ratings around. They work best in small to medium homes that ideally don’t have more than two toilet or bathroom areas, although they can still work well with slightly more demand.

They operate by taking the water straight from the mains and send hot water through your pipe system and to source, as soon as you need it. There’s no need to wait for it warming up, and nor is there a need for any cylinder or tank in your home to hold the water.

System Boiler

There are many similarities between a combi boiler and a system boiler, including the fact that they can take water straight from the main source, and that they are able to power your heating system as well.

Where they differ, is that the system boiler relies on a water tank to store the water that is used. Of course, this requires extra space in your home, with many properties installing them into a upstairs cupboard, often known as an airing cupboard, or even in their loft.

Although this might be a negative for some people, it does mean that they are pretty useful when it comes to homes that require a lot more hot water than the norm.

Regular Boiler

You might be able to guess what to expect from a regular boiler by its name. These tend to be the traditional type of system that has been installed around the UK, and if you’re currently living in an older or traditional home, then you’ll like to have one of these.

They work as part of a wider system, which requires support from a tank and feed in your attic, as well as a cylinder to hold the hot water. Basically, colder water will enter into the boiler through the loft tank, before being heated up and put in either your radiators or cylinder.

As a general rule, you wouldn’t want to be installing this type of boiler if you are looking to replace your current one. The modern boilers contain all the latest tech, so unless you already have a regular boiler, then we wouldn’t advise making this your next choice. With so much required to install the system, you’d be creating a lot more work for very little overall benefit. 

Boiler Size Calculator

Now that you have an idea on what the size means and the types of boilers that are available, we can now get cracking with working out exactly what you need with our boiler calculator.

Before deciding on what you buy, there are a few things to consider as they will all impact on the size you will require.

  • Boiler Type
  • Amount of radiators in your property
  • Total amount of bedrooms
  • Total amount of bathrooms (new bathroom cost)
  • How many people currently live in your home

Of course, the key here is all to do with the size of your home, as ultimately that will decide how big your boiler needs to be.

Depending on which type of boiler you go for, there are also a few other things to consider:

What size combi boiler do I need?

Normally, you’ll find that the actual combi boiler is much bigger than the boiler that would come with a general or system boiler set up. This is because they need to generate hot water around the clock, so that whenever you turn on your taps, the hot water is automatically there.

With this, they also have the capability to heat up your water and your heat system, so because of this they have two types of ratings for their output.

The first is CH, which stands for central heating output, while the other is DHW, which refers to your domestic hot water output.

Because systems and regular boilers keep their hot water in a cylinder, they only have a CH rating for their central heating output. As for the combi boiler, their CH output can be split into three different sections, which are outlined below. This should help you to be better informed when it comes to deciding on which boiler you need. 

Radiator Amount

Size of Combi Boiler We Would Advise (kW)

10 or less

24 to 27 CH output

Up to 15

28 to 32 CH output

Up to 20

35 to 42 CH output

What size system boiler do I need?

With system boilers (1) and regular boilers using a hot water tank to help with storing your hot water, they don’t need to generate as much power as a combi boiler normally would.

However, like the combi boiler, the amount of radiators in your home will help you to work out what size system you need.

Radiator Amount

Size of System Boiler We Would Advise (kW)

Between 10 and 15

18 to 26 CH output

Up to 20

27 to 40 CH output

What regular boiler do I need?

Again, the rules for a system boiler are similar to that of the system boiler when it comes to sizing. The best way to work it out is on radiators, with our handy guide giving you some extra information of what choice might be best for you.

Radiator Amount

Size of Regular Boiler We Would Advise (kW)

Between 10 and 15

20 to 27 CH output

Up to 20

30 to 50 CH output

When to replace your boiler?

When changing over your boiler, it’s likely that you’ll be without any heating or hot water for at least a few hours, if not a couple of days. Because of this, we’d advise trying to switch over your systems in the summer months, so at least you aren’t in desperate need of getting warmed up from your radiators.

Heating professionals also tend to be a bit quieter in the summer, you won’t be waiting around for somebody to come and get the work done, plus you might actually get them at a slightly cheaper price with their services not being as much in demand.

As we mentioned earlier, we wouldn’t recommend switching to a regular boiler, unless you already have one, so either go like for like, or upgrade your system to include a combi boiler. They really are the choice to make in the modern era, with better all round quality and less chance of breaking down, they’ll also stand the test of time. 

Professional Boiler Sizing Help

If you still aren’t sure on which boiler you will need on your home, then it’s best to get some professional advice on exactly what you need.

Here at Loftera, we can help you along with that and even offer you some price comparisons so that you’ll know exactly what you’ll expect to pay.

Comparing the different prices is always a worthwhile task as not only will it help you to get a better price but it will also give you a good idea of what different boilers are on the market and the sizes you can get.

Don’t forget, we can also help with a whole host of home improvements from kitchen upgrades (new kitchen cost) to loft conversions cost, we really do have the experience to deal with your every request.

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