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What Size Boiler Do I Need? Complete Guide

Boilers are not all alike, as you already know if you have one in your home.

What some people don’t know is that they come in different sizes, and by that we mean different outputs.

Choosing the right size is important because above all else, it affects the efficiency and the cost-effectiveness of your boiler.

And if you think it’s complicated to determine what size boiler you need for your home or office, think again.

The truth is that there are only a few things you need to do in order to determine the size that is right for you.

Choosing the Right Boiler

Boiler “sizes” or outputs are measured in kilowatts, written this way: kW. The larger your home, the higher that kW number needs to be.

If you end up not choosing the right boiler, two things can happen:

  • It may not meet your heating needs. This means that your home will not be as comfortable as you want it to be.
  • It can cost you a lot more money on your utility bill. This is due to the fact that the system will be overloaded; therefore, you’ll end up burning extra gas.

One problem a lot of people make is getting a boiler that is too small in an effort to save some money.

But it ends up costing them more in the long run. When your boiler is too small, it has to work extra hard to do its job, and as a result more gas is burned in the end.

This always results in a higher utility bill, which is something that won’t happen if you choose a boiler that is the correct size.

That being said, getting a boiler that is bigger than what you need is smart if you’re planning an expansion of your home.

Otherwise, you should simply stick to the size that meets and doesn’t exceed your needs.

Deciding on the Right Size

When you’re researching the size of your next boiler, you’ll often find charts or even calculators that help you get the right one.

For instance, here is a list of recommended boiler sizes according to how many bathrooms or showers are in your home:

  • One bathroom/shower: 26kW
  • Two bathrooms/showers: 30kW
  • Three bathrooms/showers: 35kW

If you have an unusually large home or more than three bathrooms, a combi boiler may not be for you. Instead, shoot for a conventional boiler that you use with a hot water tank for the best results.

For combi boilers, here are the minimum kW sizes recommended:

  • Two bedrooms, one bath/shower: 26kW
  • Two bedrooms, two baths/showers: 29kW
  • Three bedrooms, one bath/shower: 29kW
  • Three bedrooms, two baths/showers: 35kW
  • Four bedrooms, one bath/shower: 29kW
  • Four bedrooms, two baths/showers: 35kW
  • Five bedrooms, one bath/shower: 35kW
  • Five bedrooms, two baths/showers: 35kW

In addition, regular/conventional and system boilers, often called “heat only” boilers, have another set of rules.

When it comes to these types of boilers, you can look at the number of radiators you have in your home to determine the size boiler you need.

With heat only boilers, bathrooms essentially don’t matter.

In general, you’ll need 1.5kW for every radiator you have in your home, plus 3kW for your water cylinder.

To demonstrate, here are some recommendations based on the number of radiators found in your home:

  • Six radiators: minimum 12kW (6 x 1.5kW + 3kW)
  • Eight radiators: minimum 15kW (8 x 1.5kW + 3kW)
  • Twelve radiators: minimum 21kW (12 x 1.5kW + 3kW)
  • Fifteen radiators: minimum 26kW (15 x 1.5kW + 3kW)
  • Twenty radiators: minimum 33kW (20 x 1.5kW + 3kW)

Choosing brands such as Ideal Logic or Veissmann Vitodens is always a great idea because you get a lot of dependability for the money you spend.

Thinking About Converting to a Combi Boiler?

Combi boilers are very efficient and inexpensive to run. They also provide instant hot water from your shower or baths.

If you’re going to convert to a combi boiler, just know that it won’t be inexpensive. In fact, the price for this service usually starts at just under £2500.

Nevertheless, it saves you tons of money over time and is always worth the price you pay.

Here are some recommended sizes for combi boilers based on how many showers or bathrooms you have:

  • One bath/shower: 26kW
  • Two baths/showers: 30kW
  • Three baths/showers: 35kW

You don’t have to let all of these numbers confuse you because deciding on the right boiler size is much easier than you think.

The right company will have experts on staff who can help you decide which boiler type and which boiler size will work best for you.

Choose the Right Type of Boiler

As you can see from the examples already mentioned, the type of boiler you choose will affect what size you need to purchase for your home or office.

Here is a brief recap of the four main types of boilers:

  • Back boiler units. These are rarely used because they are not very efficient, even though they last around 25 years.
  • Combination (combi) boilers. These are the most common boilers chosen in the UK.
  • Heat-only boilers. These are also called regular, traditional, conventional, or vent boilers.
  • System boilers. These are best suited for homes with a lot of demand for hot water.

The type of boiler you choose can affect the kilowatt size you need for your home or office.

Fortunately, there are sizes that can accommodate anyone; therefore, you shouldn’t have any problems finding the right boiler for you.


Boiler outputs count when it comes to finding one that meets your needs.

Getting one that is neither too small nor too large is your smartest option, and that’s easy to do with some help from the professionals.

The boiler in your home is a big investment, so finding the one that best suits your needs is crucial.

And when you consider that the average boiler only costs around £2000 or less and lasts for 10 to 15 years, it’s easy to understand that this is not an expensive investment at all.

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