What Size Boiler Do I Need?
Quick Answer
24–27kW combi for 1 bathroom, 28–34kW for 2
Based on: hot water demand (number of bathrooms and outlets). Heating demand is usually lower than hot water demand for combi boilers.
Combi Boiler Sizing Guide
- 1 bathroom, up to 10 radiators: 24–27kW
- 1–2 bathrooms, 10–15 radiators: 28–34kW
- 2–3 bathrooms, 15–20 radiators: 35–42kW
- 3+ bathrooms: Consider a system boiler with unvented cylinder instead
System/Regular Boiler Sizing
System and regular (heat-only) boilers are sized primarily on heating demand, as the cylinder handles hot water storage:
- Small house (up to 10 radiators): 12–18kW
- Medium house (10–15 radiators): 18–24kW
- Large house (15–20+ radiators): 24–35kW
Key Considerations
- Incoming mains flow rate: Combi boilers need at least 10–12 litres/min mains flow. Low flow = consider a system boiler
- Simultaneous demand: Running 2+ showers at once? You need a system boiler + cylinder, not a combi
- Insulation level: Well-insulated homes need smaller boilers for heating
- Future expansion: Allow headroom if you plan to add radiators or a bathroom
Costs (2026 UK)
- Combi boiler supply + install: £1,800–3,500
- System boiler supply + install: £2,500–4,500
- Annual service: £80–120
Last updated: April 2026. Boiler installation must be carried out by a Gas Safe registered engineer. Always get a proper heat loss calculation for accurate sizing.