Sizing
What size combi boiler do I need?
Output is measured in kilowatts (kW). The right size is set by hot-water demand first, radiator count second. Get this wrong and you either pay for capacity you do not use, or you find the shower goes lukewarm when someone runs a tap.
Quick reference
Sizing by property type
The shortcut. If your home matches a row here, this is the kW band most fitters will quote you, with indicative supply-and-fit ranges.
| Property | Beds | Baths | Radiators | Recommended | Supply & fit |
|---|---|---|---|---|---|
| 1-bed flat | 1 | 1 | 4 to 6 | 24 kW | £1,800 to £2,500 |
| 2-bed flat | 2 | 1 | 6 to 8 | 24 to 28 kW | £1,800 to £2,800 |
| 2-bed terrace | 2 | 1 | 7 to 9 | 28 kW | £2,000 to £3,000 |
| 3-bed semi | 3 | 1 to 2 | 9 to 12 | 28 to 30 kW | £2,200 to £3,200 |
| 3-bed detached | 3 | 2 | 10 to 14 | 30 to 35 kW | £2,400 to £3,400 |
| 4-bed detached | 4 | 2 | 12 to 16 | 35 to 38 kW | £2,500 to £3,500 |
| 5-bed detached | 5 | 3+ | 16 to 20 | 38 to 40 kW+ | £3,000 to £4,000+ |
Tool
Sizing tool
Step the values to match your home. We build the kW band from radiator count, bathroom count, and whether multiple outlets run at the same time.
Sizing tool
Recommended output
28 to 30 kW
Sized for 9 to 11 radiators
Spec talk
What kW actually means
Central heating output
The kW available to push hot water round your radiators. A 30 kW combi typically hands about 25 kW to central heating, the remainder is reserved for the hot-water circuit when a tap or shower is running.
Domestic hot water (DHW)
The kW directed at the plate exchanger when you open a hot tap. It is the number that decides whether the shower stays hot. A 30 kW combi delivers around 12 to 13 litres per minute at a 35C rise.
Flow rate
What each kW band actually delivers
Read off realistic litres per minute for each output, at a typical 35C temperature rise. Two simultaneous showers need at least 15 LPM.
Fig.02 / DHW flow rate
Litres / min @ 35C rise
24 kW
28 kW
30 kW
35 kW
40 kW
A comfortable shower wants 9 to 12 LPM. Two simultaneous showers need 15 LPM and up. Winter mains are colder, so add roughly 15 percent to your output target to keep flow strong from December to February.
The Goldilocks zone
Oversize vs undersize
Too big
- !Higher purchase price for capacity you never use
- !Short cycling: the boiler fires up and shuts down rapidly, wasting gas
- !Lower seasonal efficiency at part-load
- !More wear on the diverter valve and fan
Too small
- !Lukewarm showers in winter when mains is colder
- !Slow heating, struggles to bring all radiators up to temperature
- !Boiler runs flat-out, accelerating component wear
- !Hot water drops off when a second tap opens
When a combi is not the right answer
Three signals to switch to a system boiler
Three or more bathrooms with simultaneous use. A combi cannot store hot water, so simultaneous demand drops the temperature at every outlet. A system boiler with an unvented cylinder handles this without complaint.
Very large properties (5+ beds). The radiator circuit becomes too big to heat efficiently from a combi. A system boiler distributes heat more evenly.
Low mains pressure (under 1 bar). Combis are mains-fed. If your area has consistently low pressure, a system boiler with a pressurised cylinder is more reliable.
FAQs
Common questions
What size combi boiler do I need for a 3 bed house?v
Most 3 bed semi-detached homes work well on a 28 to 30 kW combi. A 3 bed detached with two bathrooms typically wants 30 to 35 kW. Radiator count matters: if you have more than 12 radiators, lean to the higher band.
Will a 24 kW combi heat a 3 bed house?v
It can heat the radiators, but the hot-water flow rate of around 10 LPM is borderline for a household with two regular shower users. Most fitters recommend 28 kW or 30 kW for 3 beds with one bathroom.
Is a 40 kW combi too big for a 3 bed?v
Usually yes. A 40 kW combi short-cycles in a small home, costs more upfront, and offers no benefit if you only have one bathroom. Match the kW to actual hot-water demand, not just floor area.
What is the largest combi available in the UK?v
Combis run up to about 42 kW for domestic use. Worcester, Vaillant, and Viessmann all make a 38 to 42 kW unit. Above that, Gas Safe fitters typically specify a system boiler with a stored cylinder.
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