How to Choose the Right Size Boiler for Your Home
Choosing the correct boiler size keeps your home warm, water hot, and energy bills low. At ARA Services Ltd, our Gas Safe engineers calculate the ideal boiler output (kW) for every property across London — so you get efficiency without waste. You can also explore the best boiler brands in 2025 to find the right match for your needs and budget.
What Determines Boiler Size?
- Property size: bedrooms, bathrooms, and number of radiators.
- Hot-water demand: how many showers or taps are used simultaneously.
- Water pressure: affects whether you need a combi, system, or regular boiler.
- Insulation & heat loss: well-insulated homes may need smaller output.
Boiler Size Recommendations (2025 Guide)
| Home Type | Typical Bedrooms | Recommended Output (kW) | Ideal Boiler Type |
|---|---|---|---|
| Flat / Small Home | 1 – 2 | 18 – 27 kW | Combi Boiler |
| Medium Home | 3 – 4 | 24 – 34 kW | Combi / System Boiler |
| Large Property | 4 + | 35 – 42 kW + | System / Regular Boiler |
How to Choose Based on Your Needs
🏠 Small Homes (1–2 Bedrooms)
Choose a combi boiler (18–27 kW). It heats water instantly and saves space — ideal for apartments or smaller houses. Ensure good mains water pressure for best results. Check our gas vs electric boilers comparison to explore greener alternatives.
🏡 Medium Homes (3–4 Bedrooms)
A 24–34 kW system or combi boiler usually suits this size. If multiple showers or taps run at once, a system boiler with a cylinder provides steadier hot-water flow. See our boiler installation cost guide for 2025 for full pricing.
🏘️ Large Homes (4 + Bedrooms)
Go for a 35–42 kW + system or regular boiler. These store hot water in a cylinder, perfect for households with high demand or several bathrooms. You might also qualify for government boiler installation grants to offset the upgrade cost.
System & Heat-Only Boilers
Both heat large cylinders of water for use across multiple outlets. System boilers suit homes with good mains pressure, while heat-only boilers are ideal in areas with low water pressure or existing gravity-fed systems. Discover when to replace an older boiler in our Signs You Need a New Boiler guide.
Other Important Factors
- Energy Efficiency: Choose A-rated models (≈ 94 % +) to save on bills. Learn more in Are High-Efficiency Boilers Worth the Investment?
- Fuel Type: Gas is most common; electric boilers are emission-free alternatives for off-gas homes.
- Heat-Loss Calculations: A professional survey measures insulation, windows, and floor area to match the exact kW output.
For expert help, book a home heating assessment with our certified engineers.
Related Boiler Guides
FAQs
What size boiler do I need for a 3-bedroom house?
A 24–30 kW combi or system boiler usually provides enough heating and hot water. See our boiler installation cost guide for price examples.
Is it bad to oversize a boiler?
Yes — oversizing wastes fuel and shortens lifespan due to frequent cycling. Always consult a Gas Safe engineer for accurate sizing.
How do professionals calculate boiler size?
Through a heat-loss calculation that considers insulation, radiators, and water-flow rates.
This ensures your system runs efficiently year-round.