Complete combustion of fuel oil in a furnace requires oxygen greater than the theoretically calculated value. An excess of oxygen is required to drive the reaction. The more favorable the conditions for combustion are, the less the excess oxygen is needed for a driving force.

Favorable conditions include good fuel quality, poor fuel atomization, and proper burner design.

As conditions become less favorable a greater driving force or more excess oxygen is needed.

The driving force or excess oxygen needed is usually measured as excess air. Combustion air is provided to the boiler by suitable dampers or air inlet ports. Often the fireman sets the proper air-fuel ratio by reducing the air inlet until a smoking condition is apparent from the stack, then raising it slightly. However, more suitable means for measuring excess air can be found in the use of air meters or an Orsat Analyzer.

The use of an Orsat Analyzer is the most common method used for measuring excess air. In this method a sample of flue gas is drawn into a gas measuring burette, allowed to come to constant temperature, and passed successively into absorption pipettes, each containing solutions which selectively absorb carbon dioxide, oxygen, and carbon monoxide. The percentage by volume of each constituent of the flue gas is calculated from the decrease in volume of gas after passing through each absorbing pipette. The percent excess air can then be found from a chart such as that found below.