A.O. Smith 3400 - Manual (Page 24)

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The confined space shall be provided with two permanent vertical
ducts, one commencing within 12 inches (300 mm) of the top and
one commencing within 12 inches (300 mm) of the bottom of the
enclosure. The vertical ducts shall communicate directly with the
outdoors. See Figure 20.
Each duct opening shall have a minimum free area of 1 square
inch per 4,000 Btu/hr (550 mm 2 per kW) of the aggregate input
rating of all appliances installed in the enclosure.
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 3 inches.
AIR FROM OTHER INDOOR SPACES
The confined space shall be provided with two permanent
openings, one commencing within 12 inches (300 mm) of the top
and one commencing within 12 inches (300 mm) of the bottom of
the enclosure. See Figure 21.
Each opening shall communicate directly with an additional
room(s) of sufficient volume so that the combined volume of all
spaces meets the criteria for an Unconfined Space.
Each opening shall have a minimum free area of 1 square inch
per 1,000 Btu/hr (1100 mm 2 per kW) of the aggregate input rating
of all appliances installed in the enclosure. Each opening shall
not be less than 100 square inches (645 cm 2 ).
OUTDOOR AIR THROUGH TWO HORIZONTAL DUCTS
The confined space shall be provided with two permanent
horizontal ducts, one commencing within 12 inches (300 mm) of
the top and one commencing within 12 inches (300 mm) of the
bottom of the enclosure. The horizontal ducts shall communicate
directly with the outdoors. See Figure 19.
Each duct opening shall have a minimum free area of 1 square
inch per 2,000 Btu/hr (1100 mm 2 per kW) of the aggregate input
rating of all appliances installed in the enclosure.
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 3 inches.
OUTDOOR AIR THROUGH TWO VERTICAL DUCTS
The illustrations shown in this section of the manual are a
reference for the openings that provide fresh air into confined
spaces only.
Do not refer to these illustrations for the purpose of vent
installation.
FIGURE 19. OUTDOOR AIR THROUGH TWO HORIZONTAL
DUCTS
FIGURE 20. OUTDOOR AIR THROUGH TWO VERTICAL
DUCTS
FIGURE 21. AIR FROM OTHER INDOOR SPACES