Venting a water heater closet is not the same as venting the combustion gasses.
Construction practice and codes have long allowed water combustion vents -- both to and from the appliance -- to be located inside buildings. For this reason, louvers in the top and bottom of water heater closet doors are commonplace. The practice is no longer considered desirable, because the design potentially allows carbon monoxide from gas-fired appliances to accumulate in the occupied parts of the building. Conduits that pass into loft voids are taking the place of , and for this system to work a ceiling must be installed in the water heater closet.
Venting of Combustion Gasses
Combustion gasses from gas-fired water heaters should always be entirely vented through the water heater's flue; they should never enter the closet. Malfunctions can occur, however, that result in combustion gasses entering the closet. In old-style installations these gasses could enter the building through the door vents. In new-style installations, they are vented to the loft void.
Attic Vents
The experts at Marler Inspections in Texas note that "modern building codes allow combustion air to be provided by the attic; vents can be installed in the closet ceiling." Such vents normally take the form of two open 4-inch pipes that pass from the closet into the loft void; one pipe drops down almost to the floor, the other extends into the closet to only just below the ceiling. The longer pipe takes the place of the lower vent in older style doors, allowing combustion air to enter the closet. The shorter pipe takes the place of the upper vent in older style doors, allowing heated air -- not combustion gasses -- to vent.
Condensation
Condensation affects water heaters fueled by electricity as well as gas. Because heated air rises, condensation typically occurs above the heat source. Moisture condensates on cooler walls, so keeping the moisture-laden air away from the occupied areas of a building -- which are likely to be cooler than an enclosure in which a heating appliance is located -- is important. Installing a closet ceiling and loft vents makes this possible. Water heaters do not necessarily have to leak to cause condensation. If the pipes are not well insulated they will "sweat." Because water entering the heater is colder than the inside of the appliance -- properly called the heat exchanger -- moisture can condensate there. This sometimes happens in such amounts that drips beneath the appliance are wrongly diagnosed as leaks.
Other Considerations
A ceiling in a water heater closet is an asset in other ways: It prevents the closet from being a conduit that equalizes warmed or cooled air in the occupied parts of a building with the untreated loft atmosphere, a process that wastes money. Further, pressure equalizes in buildings; an extractor fan in a bathroom could potentially create a degree of negative pressure that restricts the availability of combustion air to a gas-fired water heater served by door vents. The ceiling vent system also avoids this scenario.
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