What size boiler for a 3-bedroom house? (UK: 28-34 kW combi, 18-26 kW system)
A 3-bedroom house needs 28-34 kW from a combi boiler or 18-26 kW from a system boiler with one or two bathrooms, on the UK installers' tables. A third bathroom moves it to 35-42 kW combi or 27-40 kW system. The heating alone is smaller: a 90 m² house loses 7.20 kW to 11.70 kW depending on insulation. The combi figure is higher because it heats the hot water on demand. Below: the kW by bathrooms, by radiator count and by floor area.
Open the boiler size calculator
1. 3-bedroom house: combi and system kW by bathrooms
The installers' tables (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes) read the row from bedrooms and bathrooms and take the higher of the two. Three bedrooms select the middle row, so one or two bathrooms give the same answer.
| Bathrooms | Combi | System or regular |
|---|---|---|
| 1 bathroom | 28-34 kW | 18-26 kW |
| 2 bathrooms | 28-34 kW | 18-26 kW |
| 3 bathrooms | 35-42 kW | 27-40 kW |
A 4-bedroom house sits in the same row; the 4-bedroom boiler guide covers what changes above it.
2. By number of radiators
The same tables have a radiator column (Heatable, What Size Boiler Do I Need?, radiator column of both tables): up to 10 radiators in the first row, 10 to 20 in the second, 20 or more in the third. With one bathroom, the radiator count alone gives the rows below. Three bedrooms still select the 28-34 kW row, so a low radiator count does not take a 3-bed house below it on the tables; 20 or more radiators take it above.
| Radiators | Combi row | System row | Combi by count |
|---|---|---|---|
| 6 | 24-27 kW | 12-18 kW | 24 kW |
| 8 | 24-27 kW | 12-18 kW | 24 kW |
| 10 | 28-34 kW | 18-26 kW | 28 kW |
| 12 | 28-34 kW | 18-26 kW | 28 kW |
| 15 | 28-34 kW | 18-26 kW | 32 kW |
| 18 | 28-34 kW | 18-26 kW | 32 kW |
| 20 | 35-42 kW | 27-40 kW | beyond the guide |
| 25 | 35-42 kW | 27-40 kW | beyond the guide |
The last column is the manufacturer's guide (Ideal Heating, Combi Boiler Size Guide, radiators a combi output serves). More on one output: how many radiators a 28 kW boiler can run. To size each radiator, use the radiator size guide.
3. Heat loss of a 3-bedroom house by floor area
Bedrooms do not tell you the floor area, and the heating load follows the floor area: floor area x 130 W/m² poor, 100 W/m² average, 80 W/m² good (TradeCalculator.co.uk, Radiator Sizing Calculator, rule of thumb for whole-building heat loss per m² of floor area). Find your floor area in the table.
| Floor area | poor | average | good |
|---|---|---|---|
| 50 m² | 6.50 kW | 5.00 kW | 4.00 kW |
| 70 m² | 9.10 kW | 7.00 kW | 5.60 kW |
| 90 m² | 11.70 kW | 9.00 kW | 7.20 kW |
| 120 m² | 15.60 kW | 12.00 kW | 9.60 kW |
| 150 m² | 19.50 kW | 15.00 kW | 12.00 kW |
| 200 m² | 26.00 kW | 20.00 kW | 16.00 kW |
Every row is below the 28-34 kW combi range, which is why the combi figure is set by the hot water. A system boiler (18-26 kW) sits closer to the heating load. The combi boiler size calculator puts your floor area, bedrooms, bathrooms and radiators side by side; a Gas Safe installer's heat loss calculation decides.
Frequently asked questions
- What size combi boiler for a 3-bed house?
- 28-34 kW with one or two bathrooms (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes). The tables put 3 and 4 bedrooms in the same row, so a 3-bed house with a family bathroom and an en-suite is still 28-34 kW; a third bathroom moves it to 35-42 kW.
- Is a 24 kW combi big enough for a 3-bedroom house?
- Not on the installers' tables: 24-27 kW is the row for one or two bedrooms with one bathroom, and a 3-bed house starts at 28-34 kW. The manufacturer's radiator guide puts a 24 kW combi at up to 10 radiators (Ideal Heating, Combi Boiler Size Guide, radiators a combi output serves), so the radiator count alone would allow one in a house with few radiators; the bedrooms row does not. The installer's heat loss and hot water check decide.
- Combi or system boiler for a 3-bed house?
- Both are on the tables: 28-34 kW combi, which heats water on demand with no cylinder, or 18-26 kW system or regular, which heats a hot water cylinder. The sources suggest considering a system boiler at the top row (three or more bathrooms, 20+ radiators), where a 3-bed house with three bathrooms sits: 35-42 kW combi against 27-40 kW system.
- How many kW does it take to heat a 3-bedroom house?
- Floor area x 80 W/m² to 130 W/m² depending on insulation (TradeCalculator.co.uk, Radiator Sizing Calculator, rule of thumb for whole-building heat loss per m² of floor area). A 90 m² house: 7.20 kW well insulated, 9.00 kW average, 11.70 kW poorly insulated. Table 3 has other floor areas; measure yours rather than guessing it from the bedrooms.
- What size boiler for a 3-bed house with 2 bathrooms?
- 28-34 kW combi or 18-26 kW system: the same row as one bathroom, because 3 bedrooms already select it (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes).
More guides
- Combi boiler size calculator (UK): kW by bedrooms, radiators and bathrooms
Combi and system boiler kW by bedrooms, bathrooms or radiator count from the UK installers' tables, whole house heat loss, kW to radiators, rules of thumb.
- What size radiator do I need? (BTU and watts by room size and type, UK)
BTU/h and watts for UK room sizes by type from the merchants' ft³ x factor rule, each adjustment, and the flat x4 rule. A 4 x 4 m living room: 7,500 BTU/h.
- What size boiler for a 4-bedroom house? (UK: 28-34 kW combi, 35-42 kW with 3 bathrooms)
A 4-bedroom house is 28-34 kW combi or 18-26 kW system with one or two bathrooms, 35-42 kW combi with three. Combi or system, heat loss, radiators.
- 28kW boiler: how many radiators can it run? (10 to 15, UK)
The manufacturer's guide puts a 28 kW combi at 10 to 15 radiators. Radiators and bathrooms to kW, 24 and 32 kW for comparison, and heat loss.
Last updated 2026-09-29. Sizing estimates from published charts and rules of thumb (ENERGY STAR, manufacturer guides), each with its source and date: a Manual J load calculation by an installer is the final word.