By Mohammed Miah, Gas Safe registered heating engineer
When people talk about boiler 'size', they mean output, measured in kilowatts (kW), not the physical size of the unit. Getting the right output matters: too small and it'll struggle to keep up with demand; too large and you're paying more than you need to, both upfront and on running costs.
What affects the output you need
- Number of bedrooms and bathrooms in the property.
- Number of radiators the boiler needs to heat.
- How many taps or showers might run hot water at the same time (for a combi boiler in particular).
- How well insulated the property is.
As a very rough guide, a 24–27kW combi boiler suits a smaller property with one bathroom, while a larger home with two or more bathrooms often needs 30kW or more, or may suit a system boiler with a cylinder better than a combi. This is exactly the kind of thing worth getting properly assessed rather than guessed, during a free estimate or site survey, we'll size it correctly based on your actual property rather than a generic rule of thumb.
