(All Air From Outdoors)
The confined space shall be provided with two permanent openings, one located within 12 inches of the top and one located within 12 inches of the bottom of the enclosure. The openings shall communicate directly, or by ducts with the outdoors or spaces (properly ventilated crawl and attic) that freely communicate with the outdoors as shown below. (NFPA 54, 9.3.3.1).
When directly communicating through an outside wall, each opening shall have a minimum free area of 1 square inch per 4,000 Btu per hour of total input rating of all equipment in the enclosure.
When communicating with the outdoors through vertical ducts, each opening shall have a minimum free area of 1 square inch per 4,000 Btu per hour of total input rating of all equipment in the enclosure as shown to the right. (NFPA 54, 9.3.3.1(1)*)
When communicating with the outdoors through horizontal ducts, each opening shall have a minimum free area of 1 square inch per 2,000 Btu per hour of total input rating of all equipment in the enclosure. See to the left. (NFPA 54, 9.3.3.1(2)*).
