Boiler Sizing Calculator UK
What size boiler do I need?
Calculate the right kW output for your home. Get recommendations for combi or system boilers based on your property size, type, and hot water demand.
Quick Boiler Sizing Guide
- • 1-2 bed flat: 24-27kW combi
- • 3 bed semi (1 bath): 24-30kW combi
- • 3-4 bed (2 bath): 30-35kW combi or system
- • 4+ bed (3+ bath): 35kW+ system with cylinder recommended
Calculate Your Boiler Size
Include en-suites
Standalone showers, utility room taps etc.
🔥 Complete Heating Design Workflow
For accurate boiler sizing, calculate your radiator requirements first:
Understanding Boiler Sizing
Choosing the right boiler size is crucial for comfort, efficiency, and running costs. An undersized boiler won't heat your home properly or deliver enough hot water. An oversized boiler wastes energy through frequent cycling and costs more upfront.
Boiler size is measured in kilowatts (kW) - this represents the heat output, not electricity consumption. A 30kW combi boiler can deliver 30kW of heat for central heating OR hot water (not simultaneously at full power).
Example Calculation
A 3-bed semi-detached house (1980s build) with 8 radiators totalling 9,500 Watts (9.5kW) and 1 bathroom:
- • Heating demand: 9.5kW × 1.15 (buffer) = 10.9kW
- • Hot water demand: 10 L/min × 2.1 = 21kW
- • Combi sized for hot water (higher): 24kW minimum
- • Recommended: 24-28kW combi boiler
Combi vs System vs Regular Boilers
Combi Boiler
Heats water on demand. No cylinder or tank needed. Most popular choice for UK homes.
Best for: 1-4 bed, 1-2 bathrooms, limited space
System Boiler
Works with hot water cylinder. Better for high demand. Can run multiple showers.
Best for: 4+ bed, 2+ bathrooms, high hot water demand
Regular Boiler
Traditional system with cylinder AND cold water tank. Being phased out.
Best for: Replacing like-for-like, low mains pressure
Boiler Size Guide by Property
| Property | Bathrooms | Combi Size | System Size |
|---|---|---|---|
| 1-2 bed flat | 1 | 24-27kW | Not needed |
| 2-3 bed terrace | 1 | 24-28kW | 18-24kW |
| 3 bed semi | 1-2 | 28-32kW | 18-25kW |
| 4 bed detached | 2 | 32-38kW | 25-30kW |
| 5+ bed | 3+ | May struggle | 30-40kW + cylinder |
* Adjust up for older/poorly insulated properties, down for modern well-insulated homes
Hot Water Flow Rates
For combi boilers, the kW rating determines hot water flow rate. Higher kW = more hot water per minute:
Flow Rate by Boiler Size
- 24kW combi: 9-10 L/min
- 28kW combi: 11-12 L/min
- 32kW combi: 12-13 L/min
- 35kW combi: 14-15 L/min
- 40kW combi: 16-17 L/min
What You Need
- Basic shower: 8-10 L/min
- Power shower: 12-15 L/min
- Bath fill: 15-20 L/min
- 2 showers at once: 18-22 L/min
Gas Safe Registered Engineers Only
By law, only Gas Safe registered engineers can install, service, or repair gas boilers in the UK. Always check the engineer's Gas Safe ID card and verify at gasssaferegister.co.uk before any work.
This calculator provides guidance for sizing - always get a professional survey and quote from a qualified engineer who can assess your specific property.
Frequently Asked Questions
What size boiler do I need for a 3 bedroom house?
A typical 3-bedroom semi-detached house with 1 bathroom usually needs a 24-30kW combi boiler. If you have 2 bathrooms or poor insulation, you may need 30-35kW. Use our calculator above for a precise recommendation based on your specific property.
What is the difference between a combi and system boiler?
A combi (combination) boiler heats water on demand with no storage cylinder - compact and efficient for smaller properties. A system boiler works with a hot water cylinder, storing hot water for higher demand. System boilers are better for homes with 2+ bathrooms where multiple outlets may run simultaneously.
How do I calculate boiler size from radiator output?
Add up the total watt or BTU output of all your radiators (from heat loss calculations), add 10-20% for warm-up time, then match to a boiler with at least that heating capacity. For hot water, you also need to consider flow rate requirements based on bathrooms and showers.
What flow rate do I need for a combi boiler?
For one shower/bath at a time, 10-12 litres/min is adequate (24-28kW boiler). For two simultaneous outlets, you need 15+ litres/min (30-35kW). For three or more outlets running together, consider a system boiler with cylinder instead.
Is a bigger boiler always better?
No. An oversized boiler cycles on/off more frequently, reducing efficiency and lifespan. It also costs more upfront. The ideal boiler matches your actual heating and hot water demand. Modern condensing boilers are most efficient when properly sized.
How does property age affect boiler size?
Older properties (pre-1930) typically have solid walls and poor insulation, losing heat faster and requiring larger boilers. Modern properties (post-2000) with cavity wall insulation, double glazing, and loft insulation retain heat better and can use smaller boilers.
What size boiler for a 4 bedroom detached house?
A 4-bedroom detached house typically needs 30-40kW depending on insulation quality and number of bathrooms. Detached properties lose more heat than terraced or semi-detached homes. With 2+ bathrooms, a system boiler with cylinder is often recommended.
Can I replace a regular boiler with a combi?
Yes, in most cases. Switching from a regular (conventional) boiler to a combi removes the need for hot water cylinder and cold water tank, freeing up space. However, if you have high hot water demand (3+ bathrooms), staying with a system boiler may be better.
How long does a boiler last?
A quality boiler typically lasts 10-15 years with annual servicing. Premium brands like Worcester Bosch, Vaillant, and Viessmann often come with 10-year warranties. Budget boilers may last 8-12 years. Efficiency decreases with age, so replacement after 15 years often makes economic sense.
What affects hot water flow rate?
Flow rate depends on boiler output, incoming mains pressure, and pipe diameter. A 24kW combi typically delivers 9-10 litres/min, while a 35kW delivers 14-15 litres/min. Low mains pressure limits flow regardless of boiler size - check your pressure before sizing.
Do I need a new boiler for a heat pump?
Heat pumps replace boilers entirely for heating, though some hybrid systems use both. If considering a heat pump, you may not need a new gas boiler. Heat pumps work best with underfloor heating or oversized radiators due to lower flow temperatures.
How much does a new boiler cost in the UK?
A new combi boiler installed typically costs £1,800-3,500 depending on brand and complexity. Premium brands (Worcester, Vaillant) cost more but offer longer warranties. System boilers with cylinder installation range £2,500-4,500. Always get 3 quotes from Gas Safe registered engineers.