Manual

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 free 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.
FRES
D/C
FRESH AIR DUCT CAPACITIES FOR DUCTS SUPPLYING FRESH AIR TO BOILER IN
TIGHTLY CONSTRUCTED HOUSES
Fresh Air 1/4"Mesh Screen Wood Louvers Metal Louvers
Duct Size (Btuh)* (Btuh)* (Btuh)*
3-½" x 12" 144,000 36,000 108,000
8" x 8" 256,000 64,000 192,000
8"x 12" 384,000 96,000 288,000
8"x 16" 512,000 128,000 384,000
*Btuh = British Thermal Units per hour based on opening covered by _¼"mesh
screen, wood louvers, or metal louvers.
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