507266-04B
Page 8 of 56
Issue 1904
terminate outside the space containing the furnace. This
is especially important when the furnace is mounted on
a platform in a confined space such as a closet or small
equipment room. Even a small leak around the base of the
unit at the platform or at the return air duct connection can
cause a potentially dangerous negative pressure condition.
Air for combustion and ventilation can be brought into the
confined space either from inside the building or from
outside.
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
Inside
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 9) or ventilated crawl space (Figure
Figure 6.
Equipment in Confined Space - All Air from
Outside
(Inlet Air from Crawl Space and Outlet Air to
Ventilated Attic)
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 7.
Equipment in Confined Space - All Air from
Outside
(All Air through Ventilated Attic)
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.
WARNING
