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