Vents in a water heater closet help to remove harmful gases and improve the air circulation. These are most often used with natural gas water heaters that have pilot lights. Burning the pilot light generates carbon monoxide and depletes oxygen from the closet space.
The vents on the bottom of the closet pull air in to be used by the pilot light. These can be in the closet door or on an outside wall. As the pilot light burns, the carbon monoxide gas is channeled into a flue on the top of the water heater. The gas travels up the ductwork of the flue to the outside.
The flue has a wide hood that is attached to the water heater using metal legs. It sits a few inches from the surface to encourage air flow. The vents on the bottom of the closet help create a draft, which makes air flow more efficient.
If the draft is working properly, air can be felt as it flows into the flue. If the air moves out from the bottom of the flue, there is a down draft, and the gas to the water heater must be turned off to let the flue be repaired.