Excess Air in Gas Fired Boiler

A small amount of excess air may help to ensure full, clean combustion. Too much air lowers the temperature, thereby diminishing heat transfer to the boiling water.

Higher than usual stack (exhaust) temperatures indicate fouling of the boiler tubes, which could be either soot on the fire side, or scale deposits on the water side.

Monitoring methods include a stack thermometer (or thermocouple or RTD) and a flue gas analyzer. Classically this would be an Orsat apparatus, but nowadays is more likely to be an electronic instrument. Possibly also a pH analyzer, a dissolved oxygen analyzer, and a dissolved solids analyzer for the feedwater.