Choosing the wrong boiler size is a surprisingly common mistake. Too small and your home never heats up properly. Too large and the boiler short-cycles constantly, wearing out faster and wasting gas.
How boiler size is measured
Boiler output is measured in kilowatts (kW). A 24 kW boiler produces 24 kW of heat. For combi boilers, there's also a hot water (DHW) output figure — typically 10–12 kW higher than the central heating output.
Quick guide by property type
| Property Type | Recommended kW |
|---|---|
| 1–2 bed flat or small house | 24–28 kW combi |
| 3 bed semi-detached | 28–32 kW combi |
| 4 bed detached | 32–35 kW combi |
| 5+ bed large house | 35+ kW combi, or system boiler |
The accurate way: heat loss calculation
The most accurate method involves calculating the heat loss of each room based on its dimensions, insulation levels, and number of windows. This is called a heat loss calculation. We carry this out for all new boiler installations.
Why bigger isn't better
An oversized boiler fires up, heats the water quickly, and then switches off — only to fire up again a few minutes later. This "short cycling" wears out the heat exchanger and circulator pump prematurely. Always spec for what you actually need.