What Size Boiler Do I Need?

By | 16th March 2017

There are literally thousands of boilers on the market. And specification sheets rarely make deciding what size boiler you need any easier.

Although you should consult a Gas Safe engineer to calculate the correct BTU output and therefore, decide on the correct boiler, this guide will help you understand some of the popular options.

Looking to compare quotes, and have some advise what size boiler you need? You can grab a free boiler installation quote here.

If you already know what brand of boiler you want, skip straight over to manufacturer’s boiler calculators.

Boiler Size Calculators

Glow-Worm Boiler Size Calculator

Worcester Bosch Combi Boiler Size Calculator

Baxi Boiler Size Calculator

Vaillant Boiler Size Calculator

Biasi Boiler Size Calculator

Understanding What Size Boiler You Need

Unfortunately, not all boiler manufacturers have their own boiler calculator.

Ideal Boilers is a prime example.

There are three major groups of combi boiler sizes:

  1. 24kw-27kw
  2. 28kw-34kw
  3. 35kw-42kw

And typically, you can estimate the kw boiler size needed using this as a ball park.

Property type



Size of boiler (kw)

Smaller properties. Typically, apartments and 2-bedroom terrace houses. 10 or less 1 24kw-27kw
Medium sized properties. 2-4 bedrooms. 10-15 radiators 2 28kw-34kw
Large properties 15-20 radiators 2 35kw-42kw


This all assumes that there is “reasonable” demand for hot water, and nothing excessive. Combination boilers heat hot water as and when it’s needed. So, no hot water is stored.

Priority is always given to your hot water. So, if you jump in the shower for 10 minutes, some of the output directed at the central heating will likely be pushed towards the shower. If this isn’t a problem (which it isn’t for most), a combi boiler will fit your needs perfectly.

Also, it assumes you have average sized radiators.

If you have large radiators (such as a 500mmx2000mm double convector) as part of your central heating, you’ll likely need a higher output than the one mentioned in the above table.

Here are a couple of examples of Worcester Bosch and Ideal Boilers suitable for a range of properties.

Boiler size for a small 1-2 property with up to 10 radiators (including apartments and flats)

Worcester Bosch 25SI Compact

Ideal Logic Plus 24

Boiler size for a 3-bedroom property with 10 radiators

Worcester Bosch Greenstar 25i or 30i

Ideal Logic Plus 30

Boiler size for a 4-bedroom property with 14 radiators

Worcester Bosch Greenstar 42CDI Classic

Ideal Logic Plus 35

Boiler size for properties with 5 or more bedrooms, with more than 16 radiators

It is possible to use a combination boiler for larger properties with this number of radiators, but calculations need to be made before hand. Typically, properties of this size will have a system boiler feeding a hot water cylinder that stores water.


All of the above is a guideline for the size boiler your property is likely to need. Realistically, a Gas Safe engineer that specialises in boiler installation should visit the property and match the right boiler with the BTU rating of your property.

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