28kW boiler: how many radiators can it run? (10 to 15, UK)
A 28 kW combi boiler runs 10 to 15 radiators on the manufacturer's sizing guide, between the 24 kW (up to 10) and the 32 kW (15 to 20). On the UK installers' tables 28 kW falls in the 28-34 kW combi row, the row for 10 to 20 radiators, two bathrooms or three to four bedrooms. Below: the radiator groups, the row for every radiator count and bathroom count, and the floor area whose heat loss reaches 28 kW.
Open the boiler size calculator
1. Radiators per combi output
The manufacturer's guide (Ideal Heating, Combi Boiler Size Guide, radiators a combi output serves) groups combi outputs by the number of radiators they serve:
| Combi output | Radiators | Installers' table row |
|---|---|---|
| 24 kW | up to 10 | 24-27 kW |
| 28 kW | 10 to 15 | 28-34 kW |
| 32 kW | 15 to 20 | 28-34 kW |
The installers' tables use wider groups (Heatable, What Size Boiler Do I Need?, radiator column of both tables): up to 10 radiators for 24-27 kW combi or 12-18 kW system; 10 to 20 radiators for 28-34 kW combi or 18-26 kW system; 20 or more radiators for 35-42 kW combi or 27-40 kW system. As a system boiler, 28 kW sits in the 27-40 kW system row, the one for 20 or more radiators.
2. Radiators and bathrooms: which row, and is 28 kW in it?
The row is the larger of the radiator row and the bathroom row, because a combi is sized for the hot water too. The last column says whether a 28 kW combi is inside the one-bathroom row.
| Radiators | 1 bathroom | 2 bathrooms | 3 bathrooms | 28 kW in row? |
|---|---|---|---|---|
| 6 | 24-27 kW | 28-34 kW | 35-42 kW | no |
| 8 | 24-27 kW | 28-34 kW | 35-42 kW | no |
| 10 | 28-34 kW | 28-34 kW | 35-42 kW | yes |
| 12 | 28-34 kW | 28-34 kW | 35-42 kW | yes |
| 15 | 28-34 kW | 28-34 kW | 35-42 kW | yes |
| 18 | 28-34 kW | 28-34 kW | 35-42 kW | yes |
| 20 | 35-42 kW | 35-42 kW | 35-42 kW | no |
| 25 | 35-42 kW | 35-42 kW | 35-42 kW | no |
The combi boiler size calculator takes your radiator count and bathrooms and shows this row beside the bedrooms row. By house size: 3-bedroom house, 4-bedroom house.
3. Floor area whose heat loss equals the boiler output
Heat loss = floor area x a W/m² figure for the insulation level (TradeCalculator.co.uk, Radiator Sizing Calculator, rule of thumb for whole-building heat loss per m² of floor area). Turned around, this is the floor area at which the heating alone would need the full output.
| Output | poor (130 W/m²) | average (100 W/m²) | good (80 W/m²) |
|---|---|---|---|
| 24 kW | 185 m² | 240 m² | 300 m² |
| 28 kW | 215 m² | 280 m² | 350 m² |
| 32 kW | 246 m² | 320 m² | 400 m² |
A combi also heats hot water on demand and is sized for that, which is why the tables pick it from bathrooms and radiators rather than from floor area alone. To size each radiator in watts and BTU, use the radiator size guide.
Frequently asked questions
- How many radiators can a 28 kW combi boiler run?
- 10 to 15 (Ideal Heating, Combi Boiler Size Guide, radiators a combi output serves). The guide is a rule of thumb: it counts radiators, not their size, so 15 small bedroom radiators and 15 large living room radiators count the same.
- Can a 28 kW boiler run 15 radiators?
- 15 is the top of the 28 kW group and the bottom of the 32 kW group (15 to 20) in the manufacturer's guide. The calculator takes the larger boiler on a boundary, so it shows 32 kW for 15 radiators; the installers' tables keep it in the 28-34 kW row.
- Can a 28 kW combi run 20 radiators?
- Not on the tables: 20 or more radiators is the top row, 35-42 kW for a combi or 27-40 kW for a system boiler (Heatable, What Size Boiler Do I Need?, radiator column of both tables), where the source suggests considering a system boiler.
- How many bathrooms can a 28 kW combi supply?
- The 28-34 kW row is for two bathrooms; a third bathroom moves the tables to 35-42 kW (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes). A combi heats water on demand, so the bathrooms used at the same time are what the kW has to cover.
- Is 28 kW enough to heat my house?
- For the heating side, a house would need to lose 28 kW to use all of it: about 215 m² at 130 W/m² (poor insulation), 280 m² at 100 W/m², 350 m² at 80 W/m² (TradeCalculator.co.uk, Radiator Sizing Calculator, rule of thumb for whole-building heat loss per m² of floor area). Check the central heating output on the model's data sheet, and let an installer do the heat loss calculation.
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.
- 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.
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.