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506471-01Page 8 of 41 Issue 1034
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 no less than 3 inches (75 mm). In calculating
free area, the blocking effect of louvers, grilles, or screens
must be considered. If the design and free area of protective
covering is not known for calculating the size opening
required, it may be assumed that wood louvers will have 20
to 25 percent free area and metal louvers and grilles will
have 60 to 75 percent free area. Louvers and grilles must
be fixed in the open position or interlocked with the
equipment so that they are opened automatically during
equipment operation.
EQUIPMENT IN CONFINED SPACE
(ALL AIR FROM OUTSIDE)
NOTE: Each air duct opening shall have a free area of at least one
square inch per 2,000 Btu (645 mm² per .59 kW) per hour of the
total input rating of all equipment in the enclosure. If the equipment
room is located against an outside wall and the air openings
communicate directly with the outdoors, each opening shall have a
free area of at least 1 square inch per 4,000 Btu (645 mm² per 1.17
kW) per hour of the total input rating of all other equipment in the
enclosure.
Figure 6
EQUIPMENT IN CONFINED SPACE - ALL AIR FROM OUTSIDE
(All Air through Ventilated Attic)
NOTE: The inlet and outlet air openings shall each have a free area
of at least one square inch per 4,000 Btu (645 mm² per 1.17 kW) per
hour of the total input rating of all equipment in the enclosure.
Figure 5
Figure 4
EQUIPMENT IN CONFINED SPACE - ALL AIR FROM OUTSIDE
NOTE: The inlet and outlet air openings shall each have a free area
of at least one square inch per 4,000 Btu (645 mm² per 1.17 kW) per
hour of the total input rating of all equipment in the enclosure.
(Inlet Air from Crawlspace &
Outlet Air to Ventilated Attic)