507867-02
Page 8 of 54
Issue 2128
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 total input rating of all equipment in the
enclosure (see Figure 6 and Figure 7). It is also permissible
to bring in air for combustion from a ventilated attic (Figure
10) or ventilated crawl space (Figure 9).
Figure 6.
Equipment in Confined Space - All Air from
Outside
(Inlet Air from Crawl Space and Outlet Air to
Ventilated Attic)
Figure 7.
Equipment in Confined Space - All Air from
Outside
(All Air through 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 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 fixed in the open position or interlocked
with the equipment so that they are opened automatically
during equipment operation.
