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506501-01Page 8 of 48 Issue 1031
Air from Outside
If air from outside is brought in for combustion and ventilation,
the confined space shall be provided with two permanent
openings. One opening shall be within 12” (305 mm) of the
top of the enclosure and one within 12” (305 mm) of the
bottom. These openings must communicate directly or by
ducts with the outdoors or spaces (crawl or attic) that freely
communicate with the outdoors or indirectly through vertical
ducts. Each opening shall have a minimum free area of 1
square inch per 4,000 Btu (645 mm² per .59 kW) per total
input rating of all equipment in the enclosure (See Figure 5).
If air from outside is brought in for combustion and ventilation,
the confined space must have two permanent openings. One
opening shall be within 12 inches (305 mm) of the top of the
enclosure and one opening within 12 inches (305 mm) of
the bottom. These openings must communicate directly or
by ducts with the outdoors or spaces (crawl or attic) that
freely communicate with the outdoors or indirectly through
vertical ducts. Each opening shall have a minimum free
area of 1 square inch (645 mm²) per 4,000 Btu (1.17 kW)
per hour of total input rating of all equipment in the enclosure.
See Figures 5 and 6. When communicating with the
outdoors through horizontal ducts, each opening shall have
a minimum free area of 1 square inch (645 mm²) per 2,000
Btu (.56 kW) per total input rating of all equipment in the
enclosure. See Figure 7.
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 Crawl Space and 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 5
EQUIPMENT IN CONFINED SPACE - ALL AIR FROM OUTSIDE
(Inlet 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 6
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 7