Boiler Efficiency Calculator
Calculate your boiler's thermal efficiency from steam flow, pressure, feedwater temperature, and fuel consumption. Understand the direct method efficiency formula and identify where your energy is going.
Inputs
Results
23.3%
Direct method (input-output)
6,648.6 kW
2,393.5 kJ/kg enthalpy gain
28,500 kW
Based on 38 MJ/Nm³ calorific value
How It's Calculated
The direct method (also called the input-output method) calculates efficiency as the ratio of energy absorbed by the steam to energy supplied by the fuel:
Where: h_steam = enthalpy of steam at operating pressure, h_feedwater = enthalpy of feedwater, CV = calorific value of fuel
Typical efficiency ranges: Natural gas boilers: 85–92%. Fuel oil boilers: 84–90%. Coal boilers: 75–85%. If your efficiency is below 80%, investigate tube fouling, excess combustion air, blowdown rate, and insulation.
Factors Affecting Efficiency
- Stack temperature — every 22°C above design wastes approximately 1% efficiency. High stack temperature indicates fouled tubes.
- Excess combustion air — too much air carries heat up the stack. Target 3–5% O₂ in flue gas.
- Blowdown rate — every kg of blowdown is hot water leaving the boiler. Recover heat from blowdown.
- Feedwater temperature — every 6°C increase in feedwater temperature saves approximately 1% fuel.
- Radiation losses — uninsulated boiler surfaces radiate heat. Insulate all hot surfaces.
- Fuel quality — moisture in fuel (especially coal and biomass) reduces efficiency.