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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 con-
dition. Air for combustion and ventilation can be brought
into the confined 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
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.
EQUIPMENT IN CONFINED
SPACE ALL AIR FROM INSIDE
WATER
HEATER
OPENINGS
(To Adjacent
Room)
Each opening must have a minimum free area of 1 square
inch (645 mm2) per 1,000 Btu (.29 kW) per hour of total in-
put 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 communi-
cate directly or by ducts with the outdoors or spaces (crawl
or attic) that freely communicate with the outdoors or in-
directly through vertical ducts. Each opening shall have a
minimum free area of 1 square inch (645 mm2) per 4,000
Btu (1.17kW) per hour of total input rating of all equipment
in the enclosure. See figures 3 and 4. When communi-
cating 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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