I was a young apprentice working with a journeyman technician in a boiler room and he explained how to determine the combustion air required for a boiler room.
He measured the existing louvers and calculated the free area of the opening using a tablet and a pencil. The free area of an opening determines how much combustion air can be delivered to the boiler room. After completing his calculations, he announced the existing grill was too small for the boiler room. He pulled out a hammer from his metal tool box and broke one of the glass panes from the window.