Boiler Broken Down?

Save £1000+ on a brand new Worcester Bosch.

Lock In Your Fixed Price

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.

BOXT (part owned by Bosch), have a 90-second form. That asks you questions about your property to determine which is the most suitable boiler for your heating system.

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 form yourself here):

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.

Boiler Sizing Chart

Property Type Radiators Bathrooms Boiler Sizing
Smaller properties. Typically, apartments and 2-bedroom terrace houses. 10 or less 1 24kw-27kw – Check price
Medium sized properties. 2-4 bedrooms. 10-15 radiators 2 28kw-34kw – Check price
Large properties 15-20 radiators 2 35kw-42kw – Check price


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.

Boiler Size Calculators

BOXT Boiler Size Guide & 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.

Join the conversation

  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.

Leave a comment

Your email address will not be published. Required fields are marked *

Busted Boiler?

Get a new Worcester Bosch fitted as early as tomorrow

Get your fixed price

Boxt is the No. 1 rated Heating company on TrustPilot.

Ways to pay for your new boiler

Do it in full, monthly chunks or even next year

Boiler on Finance

The best pay monthly packages available

Read guide

Boxt Boiler Replacement

Considering buying a new boiler from BOXT?

Read guide

New Boiler Costs

Confused about boiler installation costs?

Read guide