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B. All Air from Outdoors: The confined space shall be provided with two permanent openings, one
commencing within 12 inches of the top and commencing within 12 inches of the bottom of the
enclosure. The openings shall communicate directly, or by ducts, with the outdoors or spaces (crawl or
attic) that freely communicate with the outdoors.
1. When directly communicating with the outdoors, each opening shall have a minimum free area of
one square inch per 4,000 Btu per hour of total input rating of all equipment in the enclosure.
2. When communicating with the outdoors through vertical ducts, each opening shall have a minimum
flee, area of one square inch per 4,000 Btu per hour of total input rating of all equipment in the
enclosure.
3. When communicating with the outdoors through horizontal ducts, each opening shall have a
minimum free area of one square inch per 2,000 Btu per hour of total input rating of all equipment in
the enclosure.
4. When ducts are used, they shall be of the same cross-sectional area as the free area of the openings to
which they connect. The minimum dimension of rectangular air ducts shall be not less than three
inches.
FRESH
AIR
FRESH AIRDUCTCAPACITIESFORDUCTSSUPPLYINGFRESH _RTOBOILERINTIGHTLY
CONSTRUCTED HOUSES
Fresh Air
Duct Size
3J/2 '" x 12"
8" x 8"
8" x 12"
8"x 16"
¼" Mesh Screen
(Btuh)*
144,000
256,000
384,000
512,000
Wood Louvers
(Btuh)*
36,000
64,000
96,000
128,000
Metal Louvers
(Btuh)*
108,000
192,000
288,000
384,000
*Btuh = British Thermal Units per hour based on opening covered by ¼" mesh
screen, wood louvers, or metal louvers.