How do I calculate boiler efficiency by the direct and indirect methods?
Direct method: efficiency = steam output x (steam enthalpy minus feedwater enthalpy) / (fuel burnt x fuel GCV) x 100. Indirect method: efficiency = 100 minus the sum of measured heat losses, such as dry flue gas, moisture, unburnt carbon and radiation. The indirect method shows where the losses are.
Direct method, step by step
It is quick and needs few instruments. It gives no clue why efficiency is low, and a 2% error in fuel weight is a 2% error in the result.
- Run the boiler at steady load for a fixed period, such as one shift.
- Measure steam generated (kg), steam pressure and temperature, and feedwater temperature.
- Weigh the fuel burnt and get its GCV from a lab test on samples from the same lot.
- Read enthalpies from steam tables, in kcal/kg or kJ/kg.
- Efficiency = Q x (hg minus hf) / (q x GCV) x 100, with Q steam and q fuel in kg per hour.
Indirect method, the losses to measure
The BEE Energy Auditor Guide Book 4 lists these losses as a share of fuel GCV:
- L1 dry flue gas loss, from flue gas O2 or CO2, exit gas temperature and ambient temperature.
- L2 loss from hydrogen in fuel, which forms water vapour.
- L3 loss from moisture in fuel.
- L4 loss from moisture in combustion air.
- L5 loss from partial combustion, from CO in the flue gas.
- L6 surface radiation and convection loss.
- L7 and L8 unburnt carbon in fly ash and bottom ash, for solid fuels.
Readings you need for the indirect method
- Fuel ultimate analysis: carbon, hydrogen, sulphur, nitrogen, oxygen, moisture and ash, with GCV.
- Flue gas O2 and CO at the boiler exit, and flue gas temperature.
- Ambient temperature and humidity.
- Ash samples tested for unburnt carbon.
- Excess air, worked out as O2 / (21 minus O2) x 100.
Which method to use
Use the direct method for a routine check on fuel per tonne of steam. Use the indirect method for an energy audit, because each loss points to a fix: high L1 means excess air or a fouled economiser, high L7 means poor combustion or grate settings. The two results should agree within a few points; a large gap usually means a metering problem. IS 8753 and ASME PTC 4 give the full test procedures.
Worked example: a coal-fired boiler over one shift
- Direct: 40,000 kg steam, hg 665 kcal/kg, feedwater 85 kcal/kg; 8,000 kg coal at GCV 4,000 kcal/kg
- 40,000 x (665 minus 85) / (8,000 x 4,000) = 23,200,000 / 32,000,000 = 72.5%
- Indirect losses (illustrative): L1 12.0, L2 4.5, L3 3.0, L4 0.3, L5 0.7, L6 1.5, L7 3.0, L8 2.5 = 27.5%
- Indirect efficiency: 100 minus 27.5 = 72.5%
- If efficiency rises to 78% at the same steam: coal = 8,000 x 72.5 / 78 = 7,436 kg
- Saving: 564 kg coal x 2.42 kg CO2 per kg = 1,365 kg, about 1.4 t CO2 per shift
Loss figures are illustrative. The 2.42 kg CO2 per kg coal factor is the IPCC 2006 default; Indian coal with a lower GCV emits less per kg.
What Senseible does here
Senseible works out Scope 1 from your actual fuel invoices, so a more efficient boiler shows up as fewer tonnes of coal or litres of oil billed. It does not calculate boiler efficiency or read flue gas analyser data.
Sources: BEE Energy Auditor Guide Book 4 (Energy Performance Assessment), Chapter 1: Boilers, 2006 IPCC Guidelines, Volume 2: Energy
Last reviewed 2026-10-16.