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What Size Boiler Do I Need?

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.

A cool way to figure out what size boiler you need, is to use a company’s fixed price quotation tool.

HEATABLE have a 90-second form that asks you questions about your property to determine which is the most suitable boiler for your heating system.

Worcester Bosch Greenstar i from Heatable

They then list a range of boilers with the cheapest and most suitable first. After a few questions being answered, here’s the the boiler they think is ideal for a 2-bedroom flat (or, just go and fill out the web form yourself here)

Viessmann Vitodens 050-W from Heatable

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.

Want an exact price?

Use this boiler quote calculator

Boiler Sizing Chart

Property Type Radiators Bathrooms Boiler Sizing
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 a “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 500mm x 2000mm 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, Viessmann 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 (prices start at £1,770 with HEATABLE)

Viessmann Vitodens 050 (prices start at £1,670 with HEATABLE)

Ideal Logic Plus 24

Boiler size for a 3-bedroom property with 10 radiators

Worcester Bosch Greenstar 25i or 30i

Viessmann Vitodens 050

Ideal Logic Plus 30

Boiler size for a 4-bedroom property with 14 radiators

Viessmann Vitodens 100

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 beforehand. Typically, properties of this size will have a system boiler feeding a hot water cylinder that stores water.

Boiler Size Calculators

  • HEATABLE boiler size and price calculator (the one we recommend)
  • BOXT boiler size guide and price calculator
  • Glow-Worm boiler size calculator
  • Worcester Bosch combi boiler size calculator
  • Baxi boiler size calculator
  • Vaillant boiler size calculator
  • Biasi boiler size calculator


All of the above is a guideline for the size boiler your property is likely to need. You can get your boiler spec’d correctly (as well as getting a fixed online price) using this online boiler calculator, in less than 90 seconds.

Still got questions relating to boiler sizing? Drop us a line and we’ll do our best to help you out.

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  1. have large house 3400 sq foot under floor and about 22 rad how many kw would i need

    1. Hi Damien,

      You’ll need a heating engineer to spec your property based on BTU.

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