507276-03Page 8 of 59 Issue 1621
Air from Outside
If air from outside is brought in for combustion and
ventilation, the conned 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 hour of the total input rating of all equipment in the
enclosure (See Figures 6 and 7). It is also permissible to
bring in air for combustion from a ventilated attic (Figure 9)
or ventilated crawl space (Figure 10).
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 hour of
the total input rating of all equipment in the enclosure. See
Figure 8.
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
xed in the open position or interlocked with the equipment
so that they are opened automatically during equipment
operation.
Figure 6
Equipment in Conned Space - All Air from Outside
(Inlet Air from Crawl Space and Outlet Air to Ventilated Attic)
Figure 7
Equipment in Conned Space - All Air from Outside
(All Air through Ventilated Attic)
Figure 8
Equipment in Conned Space - All Air from Outside
OUTLET AIR
INLET AIR
NOTE−Each air duct opening shall have a free area of at least one
square inch per 2,000 Btu (645mm
2
per .59kW) 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 communi-
cate directly with the outdoors, each opening shall have a free area
of at least 1 square inch per 4,000 Btu (645mm
2
per 1.17kW) per
hour of the total input rating of all other equipment in the enclosure.
ROOF TERMINATED
EXHAUST PIPE
SIDE WALL
TERMINATED
EXHAUST PIPE
(ALTERNATE
LOCATION)
FURNACE
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