Fitting a boiler that's too small means your home won't heat properly. Too large, and the boiler will short-cycle (turn on and off rapidly), reducing efficiency and shortening its life. Getting the size right matters.
What does boiler output mean?
Boiler output is measured in kilowatts (kW). For combi boilers, there are two relevant figures:
How is heating output calculated?
A proper heat loss calculation considers:
For a typical 3-bedroom semi with loft insulation and double glazing, a 25–28kW combi boiler is usually sufficient.
Hot water output for combi boilers
The DHW output determines how quickly the boiler can heat water for showers and taps. The key factor is how many hot water outlets will be used simultaneously:
When a combi might not be right
Combi boilers can struggle to meet demand in large homes with high hot water usage. In these cases, a **system boiler** with an unvented hot water cylinder provides better performance — hot water is stored and instantly available regardless of flow rate.
Don't just match the old boiler size
Older boilers were often oversized by installers who erred on the side of caution. Modern boilers are more efficient and better matched to actual heat loss. During our free home survey, we calculate the correct output for your property rather than simply matching the old boiler.
Book a free boiler survey — **07507 133204**.