Primary air provides a percentage of the combustion air, but more importantly, controls the amount of fuel that can be burned.

Secondary air improves combustion efficiency by promoting the fuel to burn completely.  Power burners generally do not require secondary air.  However, air leaking in through access/clean out doors, burner mounting flanges, boiler sections, etc., dilutes the flame and flue gas temperatures, reducing operating efficiencies as well as our ability to accurately monitor combustion conditions.

Excess air is supplied to the combustion process to ensure each fuel molecule is completely surrounded by sufficient combustion air.  As a burner tune-up improves the rate at which mixing occurs, the amount of excess air required can be reduced.

Dilution air does not participate directly in the combustion process and is primarily required to attempt to control stack draft and reduce the likelihood that moisture in the flue gases will condense in the vent system which directly influences combustion air intake, safety and efficiency.