Inspection and Operating and Maintenance Guidelines for ...
Furnace capacity is usually limited by the gas handling capacity of the control equipment attached to the furnace. If the gas han- dling capacity is exceeded excess emissions occur through leaks in the system; e.g. leaks from the top flanges covers or doors.
Purging Airflow Before Combustion: What Is Required? - BDC
Each part of a furnace from the burner up to the stack must be purged. The stack however does not have to be purged. A basic rule of thumb: for every cubic foot of the heating chamber there should be four cubic feet of fresh air or inert passing through the purge