What Size Boiler Do I Need For My Home?

Posted by mprice 08/08/2017 2 Comment(s) Heating,

For the boiler, this is not referring to the physical dimensions of the boiler but to it's power output. This is measured in kW (kilowatts). Simply put, the higher the kW rating, the more hot water it can supply.


For your home, this is more to do with the physical size i.e. the number of rooms, but also the size of these rooms, the number of radiators and the number of bathrooms and showers that the boiler needs to supply. Please note that electric showers make no demand on the boiler so do not need to be considered.


Modern combi boilers are compact and discreet and can be mounted in cupboards if required


Choice of Boiler


In this guide we are focusing on Combi (short for combination) boilers. This type of boiler heats water on demand - the higher the hot water demand, the higher kW boiler required. This hot water is fed to the radiators for heating your home and supplies hot water to your taps / shower.

There is no requirement for storage cylinders or loft header tanks and, as the water supply to the boiler is fed directly from the mains, a better water pressure is generally offered.

Should your home have multiple bathrooms where hot taps or showers will often be run simultaneously, a combi boiler may not be suitable. We would recommend gaining advice from a professional for a more suitable boiler system if this the case.


A summary of the pros and cons of a combi boiler



  • Hot water on demand

  • Compact - no requirement for storage tanks

  • Efficient - it only heats the water you require

  • Improved water pressure as mains fed

  • Cleaner – less sludge build up as no holding tanks

  • Cheaper – generally less expensive to buy as less complex

  • Different fuel types available – Natural Gas, LPG, Oil



  • No stored hot water in the event of a breakdown

  • Cannot feed multiple hot taps / showers simultaneously

  • Cannot be used with power showers as reliant on mains water pressure


What Size Boiler?

The following is a rough guide to discuss with your installer:

  • 24-27kW - Up to 10 radiators + 1 bathroom – Small house, flats & apartments (1-2 bed)

  • 28-34kW - Up to 15 radiators + 1/2 bathrooms – Medium house (3-4 bed)

  • 35-42kW - Up to 20 radiators + 2 plus bathrooms – Large house (4+ bed)


Should you be replacing an old boiler, check its output rating and use this as a good starting point!


Check out our full range HERE and don't forget we have a dedicated team waiting to help, just drop us a line or call Freephone 0800 9709 779  


Fitting a new boiler – for better warmth and economy, consider upgrading your radiators aswell

2 Comment(s)

Amrik Parmar:
12/08/2017, 05:13:32 PM

I am looking for a price on the following boiler. Is it suitable for a house as below? 35-42kW - Up to 20 radiators + 2 plus bathrooms – Large house (4+ bed) I have a single story house with 16 radiators and 3 bathrooms. Currently have a 10 year old boiler/tank system. I am looking to replace it, too complicated.

Service Team:
15/08/2017, 07:27:29 AM

Thank you for your inquiry. We will be in touch shortly. Kind Regards

lucien boudier:
08/10/2017, 06:56:34 PM

10yrs is young for a boiler, 3 bathrooms need large storage so a tank system is a must, a thermal storage system would possibly be most cost effective due to heating load. best to get someone out to do a proper survey before spending any money.

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