Boiler efficiency is a combined result of efficiencies of different components of a boiler. A boiler has many sub systems whose efficiency affects the overall boiler efficiency. Couple of efficiencies which finally decide the boiler efficiency are –

1. Combustion efficiency
2. Thermal efficiency

Boiler efficiency may be indicated by

  • Combustion Efficiency – indicates a burners ability to burn fuel measured by unburned fuel and excess air in the exhaust
  • Thermal Efficiency – indicates the heat exchangers effectiveness to transfer heat from the combustion process to the water or steam in the boiler, exclusive radiation and convection losses
  • Fuel to Fluid Efficiency – indicates the overall efficiency of the boiler inclusive thermal efficiency of the heat exchanger, radiation and convection losses – output divided by input.

Boiler Efficiency is in general indicated by either Thermal Efficiency or Fuel to Fluid Efficiency depending the context.


Boiler Efficiency related to the boilers energy output to the boilers energy input can be expressed as:

Boiler efficiency (%) = 100 (heat exported by the fluid (water, steam ..) / heat provided by the fuel)     

For measuring the efficiency of an existing boiler there are two methods:

1. The Input-Output (Direct) method is the ratio of output (i.e. steam) to input (fuel + other inputs if appropriate) times 100%. This method is done by taking measurements of actual streams and can be highly accurate if done right and as long as there is no doubt about the heat content of the fuel.

2. The Heat Loss (Indirect) method is done by subtracting all of the losses (as percentages) from 100% to arrive at % efficiency. This method can also be used to estimate the efficiency of a boiler that has not yet been built or not yet gone into service (effectively from the heat and mass balance + other assumptions).

These two methods both result in a gross efficiency rather than a net efficiency (which would take account of all of the power supplied to auxiliary equipment such as fans, pumps and so on).

If you are considering purchasing a boiler based on quoted boiler efficiency, a word of caution:

While most boiler manufacturers quote their ‘guaranteed’ efficiencies as accurately as is possible, some, manufactures do not. I will leave it to you to decide whether they do it out of incompetence or unscrupulousness. Boiler efficiency can be ‘cheated’ in a variety of ways, by subtle variations in the operating parameters, some of which are listed below.

  • Flue gas temperature
  • Fuel specification
  • Excess air
  • Ambient air temperature
  • Radiant and convective heat losses

By far your greatest loss will be from hot flue gasses. Unfortunately, little can be done to reduce this as cool flue gasses will condense several species of wet acids which will corrode the boiler internally. Make sure that you have at least two manufacturers that you believe in, compare their quoted efficiencies and, if they are similar, seek out anyone who is quoting efficiencies greater than that and ask yourself ‘why? First thing to check: flue gas outlet temperature.

Make sure that the quoted efficiency is guaranteed, accurate, believable and repeatable.

You pay for the fuel that you use, not the fuel that the manufacturer ‘guaranteed’ ! Once a boiler is installed there is no going back to change its efficiency.