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Though an area may appear to be unconfined, it might
be necessary to bring in outdoor air 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 sec-
tion.
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 plat-
form or at the return air duct connection can cause a po-
tentially dangerous negative pressure condition. Air for
combustion and ventilation can be brought into the con-
fined space either from inside the building or from outside
.
CHIMNEY
OR GAS
VENT
FURNACE
AIR FLOW
NOTE - Each opening shall have a free area of at least one square
2
inch (645 mm
ing of all equipment in the enclosure, but not less than 100 square
2
inches (64516 mm
).
FIGURE 2
EQUIPMENT IN CONFINED
SPACE ALL AIR FROM INSIDE
WATER
HEATER
OPENINGS
(To Adjacent
Room)
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 spac-
es. 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 ventila-
tion, the confined space must have two permanent open-
ings. 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 commu-
nicate 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 FIG-
URE 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-sec-
tional 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 protec-
tive 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.
Page 6
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