Excess air in boiler 

Excess air means that amount of air supplied in addition to the theoretical quantity necessary for complete combustion of all fuel or combustible waste material present.

Excess air ensures that there is enough air for complete combustion. 

Excess air is expressed as a percentage of theoretical air required. Thus, 10% excess air indicates that 110% total air is being supplied. 

In boiler operation, excess air represents a heat loss. This loss must be balanced against losses from incomplete combustion.

Boiler efficiency is highly dependent on the excess air rate. So operators should optimize excess air to increase system efficiency. To ensure combustion is complete, they also should provide more combustion air than theoretically is required for boilers. This tactic helps ensure safe boiler operation.

Technicians also should keep excess air levels as low as possible about 15 percent excess air, equivalent to 3 percent oxygen to reduce the quantity of air to be heated and exhausted at the stack. As a rule of thumb, boiler efficiency increases 1 percent for every 15 percent reduction in excess air.

To correct excess air levels, operators should perform a tune-up at least annually, and in some cases they should do so more often. The tune-up frequency depends on how quickly the controls require adjustment. Oxygen-trim and carbon-monoxide trim controls can adjust automatically and continuously to compensate for variables inherent in the combustion process and improve the control and efficiency of excess air levels.