The short version
- Hot water demand usually sets the kW, not the size of the house
- An oversized boiler short-cycles, wearing components faster
- Radiator output and heat loss set the heating requirement
- The mains flow rate limits what any combi can actually deliver
Two different numbers
A combi has a central heating output and a domestic hot water output. The heating side for a typical three-bed semi is modest. The hot water side is what pushes the headline kW up, because it determines shower flow and how quickly a bath fills.
Why oversizing costs you
A boiler that's too big for the heat loss reaches temperature quickly and shuts off, then re-fires — short cycling. That increases wear on the fan, ignition and pump, and hurts efficiency because the boiler spends less time condensing.
How we size
Heat loss for the heating side, measured mains flow rate and bathroom count for the hot water side.
- Measure incoming mains flow in litres per minute
- Count outlets and realistic simultaneous use
- Calculate heat loss rather than using rules of thumb
- Check existing radiator capacity can absorb the output
- Match to a model whose modulation range fits the low-demand periods too
Common questions
Is a 24kW boiler enough for a three-bed house?+
For heating, usually comfortably. Hot water performance is the reason people go higher.
What flow rate do I need for a good shower?+
The higher the litres per minute, the better the shower — but no combi can exceed your incoming mains supply.
Does a bigger boiler heat the house faster?+
Marginally, and at the cost of cycling and efficiency the rest of the time.
Do you calculate heat loss?+
Yes — we size from measurements, not from the number of bedrooms.
Get the size right first time
We measure flow and heat loss before we recommend a model.