What KW Size Boiler Do I Need?

Does size matter? When it comes to replacing a boiler, or getting a new one, yes.

he size of a boiler is also known as output, and is measured in kW (kilowatt). This is going to be a guide for all those who are looking for a boiler size that’s perfect for their home.

Importance of Finding the Right Size Boiler

The importance of getting the right boiler system depending on the size of your home cannot be stressed enough. That’s because if you end up getting a boiler system that’s too small, it will not be able to provide you with the heat required to live comfortably in your home. On the other hand, getting a boiler system that’s larger than the size you require only means that you will have to fork out more money in your monthly utility bills, not to mention, the large boiler will be responsible for not only wasting energy, but also for building up unnecessary emissions, which will not be good for the environment.

How to Get the Right Size Boiler?

Most boilers come in output ranging between 24kW and 42kW. To find out the right size boiler for your home it is important to keep the following in mind:

Size of Current Boiler

You will have to find out the size of your current boiler and use that as a starting point, but do not assume that you will need the same size. That’s because a number of things may have changed in your home since the last boiler system was installed.

Demand for Hot Water

Another important factor for deciding a boiler size is the amount of hot water that is required in your home. If you have an old boiler and want to change it to a combi boiler, you will first have to ensure that the boiler size you choose matches the main’s pressure. For instance, if you currently have two bathrooms, and a cylinder that came along with the older conventional boiler, getting the right size system will allow you to work the showers in both bathrooms at the same time, and have a steady stream of hot water flowing. But, if you are thinking of removing the cylinder to fit the combination boiler, you are probably not going to be able to run both showers at the same flow rate simultaneously.

Home Size

The following are some general combi boiler options according to the size of your home:

  • 24-27kW – 1 to 2 bedrooms, 1 bathroom and up to 10 radiators
  • 28-34kW – 4 bedrooms, 1-2 bathrooms and up to 15 radiators
  • 35-42kW – 4 or more bedrooms, with 2 or more bathrooms and up to 20 radiators.


One of the most common mistakes that people make is to choose a boiler based solely on the square footage of their home. For instance, a common rule of thumb is to use 23 Btu per sq ft to determine the size of a boiler, which is wrong. A more accurate determination would also include the amount of heat loss through the windows and walls of your home. However it is best to sign up here for advice and a quote from the Hassle Free Boiler expert or see how much you can save with the right boiler using our savings calculator.

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