Operating your boiler with an optimum amount of excess air will minimize heat loss up the stack and improve combustion efficiency. Combustion efficiency is a measure of how effectively the heat content of a fuel is transferred into a usable heat. The stack temperature and flue gas oxygen (or carbon dioxide) concentrations are primary indicators of combustion efficiency. Given complete mixing, a precise or stoichiometric amount of air is required to completely react with a given quantity of fuel. In practice, combustion conditions are never ideal and additional or “excess” air must be supplied to completely burn the fuel.

Flue Gas Analyzers
The percentage of oxygen in the flue gas can be measured by inexpensive gas absorbing test kits. More expensive ($500 – $1,000) hand-held, computer-based analyzers display percent oxygen, stack gas temperature, and boiler efficiency. They are a recommended investment for any boiler system with annual fuel costs exceeding $50,000.

Oxygen Trim Systems
When fuel composition is highly variable (such as refinery gas, hog fuel, or multi-fuel boilers) or where steam flows are highly variable, an on-line oxygen analyzer should be considered. The oxygen “trim” system provides feedback to the burner controls to automatically minimize excess combustion air and optimize the air-to-fuel ratio.

Source: Office of Industrial Technologies, Energy Efficiency and Renewable Energy, U.S. Department of Energy