Flue gas temperatures depend on steam temperature, excess air and heat transfer efficiency. Monitoring this parameter gives a good indication of the boiler operating conditions.

Accurate control of the amount of air is essential to boiler efficiency:

  • Too much air will cool the furnace, and carry away useful heat.
  • Too little air and combustion will be incomplete, unburned fuel will be carried over and dark smoke is produced.

High flue gas temperature indicates a solution where there is excess of air. By rule of thumb, the maximum acceptable stack gas temperature is 30oC over the steam temperature.

Every 20oC deviation from optimum flue gas temperatures increases the fuel consumption by 1%. By monitoring the stack temperatures, it is possible to identify anomalies if any in the boiler operation.


The high flue gas temperature may point towards one of the following anomalies: