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Air from Inside
If the confined space that houses the furnace adjoins a
space categorized as unconfined, air can be brought in
by providing two permanent openings between the two
spaces. Each opening must have a minimum free area of 1
square inch (645 mm²) per 1,000 Btu (.29 kW) per hour of
total input rating of all gas-fired equipment in the confined
space. Each opening must be at least 100 square inches
(64516 mm²). 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. See Figure 5.
Figure 5. Equipment in Confined Space - All Air from
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 hour of the total input rating of all
equipment in the enclosure (See Figure 6 and Figure 7).
It is also permissible to bring air for combustion from a
ventilated attic (Figure 8) or ventilated crawl space (Figure
9).
Page 8 of 52
Inside
Figure 6. Equipment in Confined Space - All Air from
(Inlet Air from Crawl Space and Outlet Air to
Figure 7. Equipment in Confined Space - All Air from
(All Air Through Ventilated Attic)
Roof Terminated
Exhaust Pipe
* See Maximum Vent Lengths table
NOTE-The inlet and outlet air openings shall each have a free area
of at least one square inch per 4,000 Btu (645mm
hour of the total input rating of all equipment in the enclosure.
Figure 8. Equipment in Confined Space
(Inlet Air from Ventilated Attic & Outlet Air to Outside)
Issue 2327
Outside
Ventilated Attic)
Outside
Ventilation Louvers
Inlet Air
(Minimum
12 in.(305mm) Above
attic floor)
*Intake Debris
Screen
(Provided)
Furnace
2
per 1.17kW) per
508391-01
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