How to Choose the Right Boiler Size
Boiler size is about output in kW, not the number on the box that sounds impressive.
Boiler size is about output in kW, not the number on the box that sounds impressive. Oversizing wastes money and causes short-cycling; undersizing leaves you cold on the coldest days.
What actually drives the number
Radiator count and size, insulation levels, and hot water demand if it is a combi. A proper heat loss calculation beats a rule of thumb every time.
Why bigger is not automatically better
An oversized boiler cycles on and off more often, which wears components faster and rarely improves comfort.
Combi output is a different question
For a combi, the kW rating also determines hot water flow rate, so bathroom count matters as much as radiator count.
Questions we get asked
Can I just match the kW of my old boiler?
It's a reasonable starting point but not a substitute for checking radiator sizes and hot water demand properly.
Does a bigger boiler heat the house faster?
Not meaningfully, and it increases the risk of short-cycling rather than delivering faster comfort.
Do combi boilers and system boilers get sized the same way?
No, a combi's size also has to cover hot water flow rate, which a system boiler's cylinder handles separately.