# How do I calculate boiler efficiency by the direct and indirect methods?

Canonical: https://senseible.earth/answers/boiler-efficiency-direct-and-indirect-method

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.

1. Run the boiler at steady load for a fixed period, such as one shift.
2. Measure steam generated (kg), steam pressure and temperature, and feedwater temperature.
3. Weigh the fuel burnt and get its GCV from a lab test on samples from the same lot.
4. Read enthalpies from steam tables, in kcal/kg or kJ/kg.
5. 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.

- [CO2 per tonne of steam](https://senseible.earth/answers/co2-per-tonne-of-steam)
- [Coal, PNG or biomass boiler](https://senseible.earth/answers/coal-vs-png-vs-biomass-boiler-emissions)
- [Indian coal emissions by GCV](https://senseible.earth/answers/indian-coal-emissions-by-gcv)
- [Coal emission factor](https://senseible.earth/emission-factors/coal)

Sources: [BEE Energy Auditor Guide Book 4 (Energy Performance Assessment), Chapter 1: Boilers](https://beeindia.gov.in), [2006 IPCC Guidelines, Volume 2: Energy](https://www.ipcc-nggip.iges.or.jp/public/2006gl/vol2.html)

Last reviewed 2026-10-16.