Combustion Air for a Confined Space
A 50,000 BTU/h natural gas furnace and a 40,000 BTU/h natural gas water heater are installed in a utility closet that is 8 feet long, 6 feet wide, and 8 feet high. The closet has a solid door and no openings to the interior. The walls and door are weather-stripped. The total input rating is 90,000 BTU/h. The local code requires that combustion air be provided by one of the methods in the code book: two permanent openings, one within 12 inches of the top and one within 12 inches of the bottom, each sized at 1 square inch per 1,000 BTU/h if directly communicating with outdoors, or 1 square inch per 2,000 BTU/h if communicating with the outdoors through vertical ducts. The closet is on an exterior wall, and the installer plans to install two openings directly to the outdoors, each 4 inches by 6 inches (24 square inches).
1. Determine if the proposed combustion air openings are adequate. Show your calculations.
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The total input rating is 90,000 BTU/h. For two permanent openings directly communicating with outdoors, each opening must be sized at 1 square inch per 1,000 BTU/h. Therefore, each opening must be at least 90,000/1,000 = 90 square inches. The proposed openings are 4 inches by 6 inches = 24 square inches each, which is far less than the required 90 square inches. Thus, the openings are inadequate. Additionally, the openings must be located within 12 inches of the top and bottom of the enclosure. The installer must increase the size of each opening to at least 90 square inches (e.g., 10 inches by 9 inches). Alternatively, if using vertical ducts, the required size would be 90,000/2,000 = 45 square inches each, but the proposed 24 square inches is still insufficient.
2. What other factors should be considered to ensure proper combustion air and venting?
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The openings must be properly located: one within 12 inches of the top and one within 12 inches of the bottom of the enclosure. The openings should not be obstructed by the appliances or piping. The combustion air must be drawn from a clean source, not from a contaminated area. Also, the furnace and water heater must be properly vented to prevent backdrafting. A draft hood or induced draft fan must be checked. Consider installing a CO alarm as a safety measure. If the closet is sealed with weather-stripping, ensure the door has louvers or provide a permanent opening. Finally, verify that the combustion air openings are sized per the code and that they are protected with corrosion-resistant screens (minimum 1/4-inch mesh) to prevent insect intrusion.