The two numbers, and why hot water usually wins
Space heating load is calculated room by room from wall, window, roof and floor areas, their construction, and the temperature you want to hold on a cold day. A well-insulated three-bedroom semi frequently comes out somewhere between 7kW and 12kW — a long way below the 24kW or 30kW combi commonly fitted to it.
Hot water demand is the second number, and on a combi it is the one that sets the output, because heating water instantly takes far more power than warming a house slowly. That is the honest reason combis are rated as high as they are — not the radiators.
Where a cylinder stores hot water, that instant demand disappears. A system boiler can then be sized much closer to the actual heat loss, which is why cylinder-based systems often run a smaller, gentler appliance.
Rough guide to output selection
Indicative only — a heat loss calculation replaces this, it does not confirm it.
| Property | Typical space heating need | What normally drives the combi rating |
|---|---|---|
| One or two bed flat | Around 4–7kW | Hot water: a modest combi is usually ample |
| Three bed semi, insulated | Around 7–12kW | Hot water flow to the shower and bath |
| Four bed detached | Around 12–18kW | Bath filling speed, or a cylinder if demand overlaps |
| Older solid wall property | Higher, and worth calculating properly | Space heating can genuinely rival hot water demand |
What oversizing actually does to you
- Short-cycling: the boiler fires, satisfies the demand in minutes, stops, and repeats all day
- More wear on the components that fail most — fan, ignition electrode, diverter valve
- Poorer condensing performance, because return temperatures never settle low
- Noticeable temperature swings instead of steady, quiet background heat
- Wasted purchase price on capacity the house cannot use
How sizing should be done at survey
- 1
Room-by-room heat loss
Measured or calculated, not estimated from the number of bedrooms.
- 2
Radiator capability check
Existing radiators are assessed against those room losses, especially if lower flow temperatures are the goal.
- 3
Mains flow measurement
Sets the realistic ceiling for combi hot water output.
- 4
Demand pattern discussion
Baths versus showers, and whether outlets overlap, decides combi versus cylinder.
- 5
Output chosen last
The nearest sensible model above the calculated requirement — not two sizes above it for comfort.
The sizing question people ask is "how big?" The useful question is "how little can this house get away with?" — because the answer to the second one runs quieter, lasts longer and costs less to feed.