I inspected a house that has a 32-year old 100K BTU furnace in a closet. There is a single 10x20 louvered opening above the door. I understand the need for an additional vent in the lower 12 inches of the door. My question is; Why do I need two openings? I’m thinking that one opening should provide enough air, but with only one opening there could be a CO build up, maybe? Or that not enough air would be supplied which could cause improper combustion which could lead to something bad happening? Just need a little more info as the “why” question.