Was Googlin' this morning on boiler air control, came up with this site and, although it's focus is on natural gas boilers, it has lots of info on boilers, and you might find this specific link, as well as the other links, helpful in better understanding and operating your boiler.
http://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Combustion_Air_Control.asp
http://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Combustion_Air_Control.asp