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When the furnace is installed so that supply ducts carry air
circulated by the furnace to areas outside the space con
taining 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.
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 mm
2
) 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 mm
2
). 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).
EQUIPMENT IN CONFINED SPACE
ALL AIR FROM INSIDE
CHIMNEY OR
GAS VENT
WATER
HEATER
OPENINGS
(To Adjacent
Room)
FIGURE 2
NOTE-Each opening shall have a free area of at least one square
inch per 1,000 Btu per hour of the total input rating of all equipment in
the enclosure, but not less than 100 square inches.
G23
FURNACE
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 indirectly through vertical ducts. Each opening shall
have a minimum free area of 1 square inch (645 mm
2
) per
4,000 Btu (1.17 kW) per hour of total input rating of all
equipment in the enclosure. (See figures 3 and 4). When
communicating with the outdoors through horizontal
ducts, each opening shall have a minimum free area of 1
square inch (645 mm
2
) 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 calculat
ing 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.
EQUIPMENT IN CONFINED SPACE
ALL AIR FROM OUTSIDE
(Inlet Air from Crawl Space and
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 per hour of the total input
rating of all equipment in the enclosure.
VENTILATION LOUVERS
(Each end of attic)
OUTLET
AIR
WATER
HEATER
INLET
AIR
CHIMNEY OR
GAS VENT
VENTILATION
LOUVERS
(For unheated crawl
space)
FIGURE 3
G23
FURNACE
EQUIPMENT IN CONFINED SPACE
ALL AIR FROM OUTSIDE
(All Air Through 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 per hour of the total input
rating of all equipment in the enclosure.
CHIMNEY
OR GAS
VENT
WATER
HEATER
OUTLET
AIR
VENTILATION LOUVERS
(Each end of attic)
INLET AIR
(Ends 12 in. above
bottom)
FIGURE 4
G23
FURNACE
