Combustion air
The fresh air a gas appliance needs to burn fuel safely.
Furnaces, water heaters, and other fuel-burning appliances need a supply of fresh 'combustion air' to burn cleanly and vent properly. In tight, sealed rooms or closets, starving an appliance of air causes incomplete combustion and dangerous carbon monoxide. Never block the vents or louvers on a mechanical-room door, and keep the area around fuel-burning appliances clear.