I am replacing a louvered door with a standard 6 panel door for a closet that contains a natural gas hot water heater. The louvered door is the only source of incoming air for the room. I am planning to install a vent in the wall to allow air to enter the closet (which is roughly 4' x 10') but not sure how large of a vent I will need to ensure that proper air flow is maintained. Any advice would be appreciated. Thanks.