Boiler Size Calculator
Calculate the optimal boiler size for your property based on type, age, and room requirements
Calculate Your Boiler Size
Combi Boiler
Heating + Hot Water
Heat Only
Heating Only
System Boiler
Built-in Components
Provides heating and instant hot water in one unit. Ideal for smaller homes with limited space.
Excellent insulation - affects heat loss calculations
Include all heated bedrooms
Include full bathrooms and en-suites
Boiler Size Results
Heating Load: 10kW
Hot Water Demand: 24kW
Property Type: Excellent insulation
Recommendation: Choose a boiler between 27kW and 37kW for optimal efficiency
Size Analysis
Example Calculation
Modern 3-Bedroom House Example
Property: Built after 2000 (excellent insulation)
Layout: 3 bedrooms, 2 bathrooms
Boiler Type: Combi boiler
Insulation Factor: 0.9 (modern property)
Calculation Breakdown
Heating Load: (3 × 1.5kW + 2 × 2.0kW + 2.5kW) × 0.9 = 8.55kW
Hot Water Demand: 2 bathrooms × 12kW = 24kW
Required Size: max(8.55kW, 24kW) + 3kW safety = 27kW
Recommended: 18-19kW combi boiler
Boiler Types
Combi Boiler
Heating and hot water in one unit
Best for: Small to medium homes
Heat Only
Separate hot water cylinder needed
Best for: Large homes with high demand
System Boiler
Built-in pump and expansion vessel
Best for: Medium to large homes
Sizing Tips
Don't over-size - it reduces efficiency
Consider future room additions
Older properties need larger boilers
Combi boilers need higher output for hot water
Understanding Boiler Sizing
Why Correct Sizing Matters
Choosing the right boiler size is crucial for efficient heating and hot water supply. An undersized boiler will struggle to heat your home adequately, while an oversized boiler will be inefficient and cost more to run.
Key Factors
- •Property size and number of rooms
- •Insulation quality and property age
- •Hot water demand (bathrooms and usage)
- •Boiler type and efficiency rating
Calculation Method
Heating Load = (Bedrooms × 1.5kW + Bathrooms × 2.0kW + 2.5kW) × Age Factor
Combi: Max(Heating Load, Hot Water Demand) + Safety Margin
- Age Factors:
- • Pre-1900: 2.2 (very poor insulation)
- • 1900-1950: 1.8 (poor insulation)
- • 1950-1980: 1.4 (basic insulation)
- • 1980-2000: 1.1 (good insulation)
- • Modern: 0.9 (excellent insulation)
Note: This calculator provides estimates. Consult a heating engineer for precise sizing.