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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 1.17kW) per hour of total input rating of all
equipment in the enclosure (see Figure 6 and Figure 9).
It is also permissible to bring in air for combustion from a
ventilated attic (Figure 7) or ventilated crawl space (Figure
8).
Figure 5. Equipment in Confined Space - All Air from
Figure 6. Equipment in Confined Space - All Air from
Page 8 of 51
Inside
Outside
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 10.
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 20to 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.
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 of all equipment in the enclosure.
Figure 7. Equipment in Confined Space
(Inlet Air from Ventilated Attic and Outlet Air to
If this unit is being installed in an application with
combustion air coming in from a space serviced by an
exhaust fan, power exhaust fan, or other device which
may create a negative pressure in the space, take care
when sizing the inlet air opening. The inlet air opening
must be sized to accommodate the maximum volume
of exhaust air as well as the maximum volume of
combustion air required for all gas appliances serviced
by this space.
Issue 1917
Ventilation Louvers
Inlet Air
(Minimum 12 in.
(305mm) above
Attic Floor)
*Intake Debris
Screen
(Provided)
Furnace
per 1.17kW) per
2
Outside)
WARNING
507271-02
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