Boiler / Heating Capacity Calculator

Heat Load

Building or process heat requirement
Residential: 1.1-1.2 | Commercial: 1.15-1.25

Boiler Configuration

LHV basis. Condensing can exceed 100%

Hydraulic Parameters

Radiators: 75-80°C | Fan Coils: 60°C
Must be < 55°C for real condensing

Fuel & Economics

Per m³ for Gas, per kWh for Electric
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Boiler / Heating Capacity Selection

Boiler sizing is critical for reliable heating system performance. Oversizing leads to short cycling, reduced efficiency, and component wear. Undersizing results in inadequate heating during peak demand. This calculator applies a safety factor to the calculated heat load, then determines the water flow rate required to transport that heat through the distribution system. Fuel consumption and operating costs are calculated based on the selected fuel type, boiler efficiency, and annual operating hours. CO2 emissions help assess environmental impact and regulatory compliance.

Key Points:
  • Condensing boilers: 90-98% efficiency, require return temp < 57°C for condensation
  • Non-condensing boilers: 80-88% efficiency, higher flue gas temperatures (120-200°C)
  • Electric boilers: 100% efficiency, zero direct emissions, highest operating cost
  • Safety factor: 1.15 typical for commercial, 1.2 for industrial applications
  • Flow/return ΔT: 20°C standard (80/60°C), 30°C for district heating (90/60°C)
  • Natural gas CV: ~10.5 kWh/m³ | Diesel CV: ~11.9 kWh/kg | LPG CV: ~12.8 kWh/kg
  • Annual efficiency degrades 1-2% per year without proper maintenance
  • Modulating burners improve part-load efficiency and reduce cycling
Calculation Steps:
  1. Enter all required parameters
  2. Click "Update Calculation"
  3. Review results and analysis
Formula Variables:
Q_b Boiler Capacity (kW)
Q_load Heat Load (kW)
SF Safety Factor (-)
ṁ Water Mass Flow (kg/s)
V̇ Water Volume Flow (m³/h)
ΔT Temperature Difference (K)
F Fuel Consumption (unit/h)
η Efficiency (%)
CV Calorific Value (kWh/unit)
C_annual Annual Cost (€/year)