507330-01B
Page 6 of 33
Issue 1809
for combustion if the structure does not provide enough air
by infiltration. If the furnace is located in a building of tight
construction with weather stripping and caulking around
the windows and doors, follow the procedures in the air
from outside section.
Confined Space
A confined space is an area with a volume less than 50
cubic feet (1.42 m3) per 1,000 Btu (.29 kW) per hour of
the combined input rating of all appliances installed in that
space. This definition includes furnace closets or small
equipment rooms.
When the furnace is installed so that supply ducts carry
air circulated by the furnace to areas outside the space
containing the furnace, the return air must be handled by
ducts which are sealed to the furnace casing and which
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.
Figure 2.
Equipment in Confined Space - All Air From
Inside
NOTE:
Each opening shall have a free area of at least one square
inch per 1,000 Btu (645 mm² per .29 kW) per hour of the total
input rating of all equipment in the enclosure, but not less than 100
square inches (64546 mm²).
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 2.
Air from Outside
If air from outside is brought in for combustion and
ventilation, the confined space must have two permanent
openings. 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. 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 (645 mm2)
per 4,000 Btu (1.17 kW) per hour of total input rating of all
equipment in the enclosure. See Figure 3 and Figure 4.
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 5.
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.
Figure 3.
Equipment in Confined Space - All Air from
Outside
(Inlet Air from Crawl Space & Outlet Air to Ventilated
Attic)
NOTE:
The inlet and outlet air openings shall each have a free
area of at least one square inch per 4,000 Btu (645 mm² per
1.17 kW) per hour of the total input rating of all equipment in the
enclosure.
