What size boiler for a 4-bedroom house? (UK: 28-34 kW combi, 35-42 kW with 3 bathrooms)
A 4-bedroom house with one or two bathrooms needs a 28-34 kW combi or an 18-26 kW system boiler on the UK installers' tables, the same row as a 3-bedroom house. Three bathrooms, or 20 or more radiators, move it to the top row: 35-42 kW combi or 27-40 kW system, where the sources suggest looking at a system boiler with a cylinder. Below: kW by bathrooms, by radiator count, and the heat loss by floor area.
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1. 4-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) put 3 and 4 bedrooms in the middle row, and read the bathrooms as a second condition: the higher of the two rows applies. For four bedrooms that means the bathrooms only matter from the third one.
| 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 |
The same answer for one or two bathrooms applies to a 3-bedroom house; see the 3-bedroom boiler guide for the smaller house.
2. When a 4-bed house moves to the top row
Three things move it: a third bathroom, 20 or more radiators (Heatable, What Size Boiler Do I Need?, radiator column of both tables), or a fifth bedroom. The radiator counts below reach the top row on their own: 20 and 25 radiators give 35-42 kW even with one bathroom.
| Radiators | Combi, 1 bathroom | Combi, 2 bathrooms | Combi, 3 bathrooms | System, 1 bathroom |
|---|---|---|---|---|
| 6 | 24-27 kW | 28-34 kW | 35-42 kW | 12-18 kW |
| 8 | 24-27 kW | 28-34 kW | 35-42 kW | 12-18 kW |
| 10 | 28-34 kW | 28-34 kW | 35-42 kW | 18-26 kW |
| 12 | 28-34 kW | 28-34 kW | 35-42 kW | 18-26 kW |
| 15 | 28-34 kW | 28-34 kW | 35-42 kW | 18-26 kW |
| 18 | 28-34 kW | 28-34 kW | 35-42 kW | 18-26 kW |
| 20 | 35-42 kW | 35-42 kW | 35-42 kW | 27-40 kW |
| 25 | 35-42 kW | 35-42 kW | 35-42 kW | 27-40 kW |
These rows are the radiator reading alone. Four bedrooms already select the 28-34 kW row, so for a 4-bed house the table only adds information where it shows the top row. For one output in detail: how many radiators a 28 kW boiler can run.
3. Heat loss of a 4-bedroom house by floor area
The heating load follows floor area and insulation, not bedrooms: 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).
| 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 |
A 200 m² house with poor insulation loses 26.00 kW; set that against 18-26 kW for a system boiler, which heats the radiators and a cylinder rather than water on demand. The combi boiler size calculator shows your floor area, bedrooms, bathrooms and radiators side by side; a Gas Safe installer's heat loss calculation and hot water check decide the boiler.
Frequently asked questions
- What size combi boiler for a 4-bed house?
- 28-34 kW with one or two bathrooms, 35-42 kW with three or more (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes). A 4-bed house with 20 or more radiators is also in the 35-42 kW row on the radiator column of the same tables.
- What size boiler for a 4-bed house with 2 bathrooms?
- 28-34 kW combi or 18-26 kW system or regular. Two bathrooms select the middle row, the same row four bedrooms already select, so the second bathroom does not change the answer; a third one does (35-42 kW combi).
- Combi or system boiler for a 4-bedroom house?
- The tables give both: 28-34 kW combi or 18-26 kW system with one or two bathrooms. For the top row (three or more bathrooms, 20+ radiators) the source adds "often consider a system boiler" (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes); the system row there is 27-40 kW.
- Is a 28 kW combi enough for a 4-bed house?
- 28 kW is the bottom of the 28-34 kW row that covers a 4-bed house with one or two bathrooms, and the manufacturer's guide puts it at 10 to 15 radiators (Ideal Heating, Combi Boiler Size Guide, radiators a combi output serves). With three bathrooms or 20+ radiators the tables move to 35-42 kW.
- What size boiler for a 5-bedroom house?
- The top row whatever the bathrooms: 35-42 kW combi or 27-40 kW system (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 3-bedroom house? (UK: 28-34 kW combi, 18-26 kW system)
A 3-bedroom house with one or two bathrooms needs 28-34 kW from a combi or 18-26 kW from a system boiler on the UK tables; heat loss by floor area.
- 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.