9
B. All Air from Outdoors:
Confined space shall
be provided with two permanent openings,
one commencing within 12 inches of top and
another commencing within 12 inches of bottom
of enclosure. Openings shall communicate
directly, or by ducts, with outdoors or spaces
(crawl or attic) that freely communicate with
outdoors.
1.
When directly communicating with outdoors, each
opening shall have 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 outdoors through
horizontal ducts, each opening shall have
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 same cross
sectional area as free area of openings to which
they connect. Minimum dimension of rectangular
air ducts shall be not less than three inches.
FRESH AIR DUCT CAPACITIES
THROUGH LOUVERS
Fresh Air
Duct Size
¼” Mesh
Screen
Wood
Louvers
Metal
Louvers
(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.
FRESH AIR FOR COMBUSTION
Figure 5 - Fresh Air Duct Capacities For Ducts Supplying Fresh Air To Boiler In
Tightly Constructed Houses
PN 240013023, Rev. B [12/31/2020]
