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.
Another 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):
If you already know what brand of boiler you want, skip straight over to manufacturer’s boiler calculators.
Glow-Worm Boiler Size Calculator
Worcester Bosch Combi Boiler Size Calculator
Vaillant Boiler Size Calculator
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:
And typically, you can estimate the kw boiler size needed using this as a ball park.
Property type | Radiators | Bathrooms | 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.
Worcester Bosch 25SI Compact
Ideal Logic Plus 24
Worcester Bosch Greenstar 25i or 30i
Ideal Logic Plus 30
Worcester Bosch Greenstar 42CDI Classic
Ideal Logic Plus 35
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.
Hi Damien,
You’ll need a heating engineer to spec your property based on BTU.
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