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
(Inlet Air from Crawlspace & Outlet Air to 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 4
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
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
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