How to Choose the Right Boiler Size
Getting the boiler output (kW) right is just as important as picking the right brand. Too small and you'll be cold; too big and you waste gas every cycle.
Boiler output is measured in kilowatts (kW). Choosing the wrong size is one of the most common mistakes homeowners make — and one we see regularly when visiting properties for upgrades.
Why sizing matters
An oversized boiler "short cycles" — it fires up, heats the system quickly, then turns off before it has run efficiently. This wastes gas and causes premature wear. An undersized boiler runs continuously and still can't reach target temperature on cold days.
Quick sizing guide
- 1-bed flat or studio: 18–24 kW combi
- 2-bed house: 24–28 kW combi
- 3-bed semi: 28–32 kW combi
- 4-bed detached: 32–40 kW combi, or system boiler
- 5+ bed: System or heat-only boiler with large cylinder
Hot water demand matters too
For combis, the kW rating also affects hot water delivery speed. A 28 kW combi produces around 10.5 L/min. Two showers running simultaneously need roughly 18–20 L/min — more than most combis can deliver. For larger families with two bathrooms in regular use, a system boiler with an unvented cylinder is often the better long-term solution.
Let us size it for you
The most accurate approach is a heat loss calculation — we measure your home's insulation, window area, ceiling height, and radiator sizes to calculate exactly the right output. We do this as part of our free survey. Book yours here.
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