Stack Losses – General Methodology

Calculating boiler stack losses consists of six steps. The calculations described below form the basis of the Boiler Efficiency Calculator. Since stack losses depend on fuel composition‚ firing conditions and flue gas temperature more detailed descriptions of the steps are available for five fuel types.

Step 1: Obtain and convert measurements

Determine the maximum capacity rating of the boiler unit in lb/h for steam or millions of Btu/h for high temperature water generators.


  • gas temperature (FGT), °F or °C
  • combustion air temperature (CAT)‚ F or °C
  • O2 in flue gas, % by volume
  • actual unit output, lb/h or millions of Btu/h

If FGT and CAT are measured in °C, convert to °F.

If O2 is measured on a wet gas basis, convert to dry gas basis as described for each fuel.

Determine the CO2 in flue gas, % by volume, dry gas basis, from the relationship to measured O2‚ as described for each fuel. Express the actual unit output as a percentage of the maximum capacity rating

Step 2: Calculate dry flue gas (DG), lb/lb fuel

The ASME formula for wt of dry flue gas is:

DG‚ lb/lb fuel‚ = 11CO2 + 8O2 + 7(N2 + CO) x [( lb C burned) + 0.375S]
    3(CO2 + CO) ( lb fuel )  

Step 3: Calculate excess air

The value of excess air is not needed for calculation of stack losses‚ but it is the most common parameter of burner performance. In the following calculations CO is assumed to be zero‚ because the maximum amount allowable under emissions regulations has a very small impact on efficiency calculations. For the same reason‚ it is assumed that all the carbon in the fuel is burned.

The ASME formula for excess air‚ (EA)‚ % by volume‚ when CO is negligible‚ is:

EA‚ % by volume  =  100 x O2dry    
0.2682N 2 – O2dry    

Step 4. Calculate dry flue gas loss (LDG )‚ % of fuel input

Dry flue gas loss is the (sensible) heat energy in the flue gases due to the flue gas temperature.

The ASME formula for LDG% of fuel input‚ is:

LDG% of fuel input  =  [DG x Cp x (FGTCAT)]   x 100

Step 5: Calculate latent heat loss (LH)‚ % of fuel input

Flue gas loss due to moisture represents the (latent) energy in the steam in the flue gas stream due to the water produced by the combustion reaction being vaporized from the high flue gas temperature. In the process of combustion the hydrogen content of the fuel is converted to H2O‚ which normally leaves the stack as water vapor‚ carrying with it the latent heat of vaporization‚ as well as sensible heat proportional to the stack temperature.

The ASME formula for LH‚ % of fuel input is:

LH% of fuel input=9H2 x [(enthalpy of vapor at 1 psia & FGT) – (enthalpy of liquid at CAT)]  x 100

Step 6: Calculate stack losses (Ls )‚ % of fuel input

This is the sum of Steps 4 and 5.

LS% of fuel input = LDG + LH