Size the boiler
A guide for the conversation with the customer. Combis are sized for hot water flow; system and regular boilers for the heat loss plus the cylinder.
A rule-of-thumb guide, not a design. Size the heating from a proper heat loss calculation and check the boiler's heating and hot water outputs on the manufacturer's data sheet.
Boiler size guide tables
Typical figures quoted by manufacturers and merchants, plus the physics for hot water flow and cylinder reheat.
| Radiators | Bedrooms | Bathrooms | Boiler size |
|---|---|---|---|
| 0–5 | 1 | 1 | 9–18 kW |
| 5–10 | 2 | 2 | 24–27 kW |
| 10–20 | 3 | 3 | 28–34 kW |
| 20+ | 4+ | 4+ | 35–43 kW |
A typical manufacturer rule of thumb. The average UK home needs roughly 10–20 kW for heating; a combi's rating is usually set by hot water, not heating.
| Home | Combi size |
|---|---|
| Up to about 10 radiators, 1 bathroom | 24–30 kW |
| 10–20 radiators or 2 bathrooms | 30–35 kW |
| About 20 radiators or more, or 3+ bathrooms | 35–43 kW |
| Combi rating | Flow (litres/min) |
|---|---|
| 24 kW | 9.8 |
| 26 kW | 10.6 |
| 28 kW | 11.5 |
| 30 kW | 12.3 |
| 32 kW | 13.1 |
| 35 kW | 14.3 |
| 38 kW | 15.6 |
| 40 kW | 16.4 |
| 43 kW | 17.6 |
Theoretical flow: kW × 60 ÷ (4.186 × 35). Real flow depends on the boiler, the incoming main and the cold water temperature — winter mains water is colder, so flow at the tap drops.
| Cylinder | in 30 min | in 45 min | in 60 min |
|---|---|---|---|
| 120 L | 14.0 kW | 9.3 kW | 7.0 kW |
| 150 L | 17.4 kW | 11.6 kW | 8.7 kW |
| 180 L | 20.9 kW | 14.0 kW | 10.5 kW |
| 210 L | 24.4 kW | 16.3 kW | 12.2 kW |
| 250 L | 29.1 kW | 19.4 kW | 14.5 kW |
| 300 L | 34.9 kW | 23.3 kW | 17.4 kW |
How to choose the boiler size
Combi: sized for hot water
A combi heats mains water as it flows, so its kW rating mostly decides how fast the hot water comes out. At a 35 °C rise a 24 kW combi gives about 10 litres a minute, 30 kW about 12 and 35 kW about 14. That is enough for one shower at a time; two outlets at once will share the flow. Homes with three or more bathrooms are usually better with a system or regular boiler and a cylinder.
The heating side of a combi
Many combis have a lower central heating output than their hot water rating. Check the data sheet: the heating output must cover the house's heat loss, and a boiler that modulates down well copes better with a small, well-insulated house.
System and regular boilers: heat loss plus cylinder
Size for the house's heat loss (the total of the room calculations), then check the cylinder. Reheating 150 litres from 10 °C to 60 °C takes about 8.7 kW for an hour. Most modern systems give hot water priority, so the boiler only needs to cover the larger of the two; if heating and cylinder run together, it needs the sum.
Why not just go bigger?
An oversized boiler costs more and short-cycles in mild weather, which hurts efficiency. A heat loss calculation is the best way to avoid it.
Questions heating engineers ask
What size combi boiler do I need?
As a typical guide, 24–30 kW for a home with up to about 10 radiators and one bathroom, 30–35 kW for 10–20 radiators or two bathrooms, and 35–43 kW for about 20 radiators or more. The size mostly sets hot water flow: about 10 litres a minute from 24 kW and 14 from 35 kW at a 35 °C rise.
What size boiler for a 3 bedroom house?
A combi of around 24–30 kW is common for a 3 bedroom, one bathroom house with about 10 radiators. For a system or regular boiler, size it from the house's heat loss, then check the power needed to reheat the cylinder.
How many litres per minute does a combi give?
Roughly kW × 60 ÷ (4.186 × 35) at a 35 °C rise: 24 kW gives about 9.8 L/min, 30 kW about 12.3 L/min, 35 kW about 14.3 L/min. In winter the mains water is colder, so the rise needed is bigger and the flow drops.
Is a bigger boiler better?
No. Too big a boiler short-cycles in mild weather and runs less efficiently. Size the heating side from a heat loss calculation and choose the combi rating for the hot water flow the household needs.
More free tools for plumbers & heating engineers
BTU calculator — room heat loss and radiator size, ΔT50 to ΔT30› All free calculators for the trades›Quotes, jobs and certificates from the van
Quote on site, dispatch callouts, keep CP12s and installation certificates against each job, and give the crew a 4-button app for timesheets.
See Graftum for plumbing & heatingA rule-of-thumb guide; size the heating from a heat loss calculation and the manufacturer's data.