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CAUTION
Do not install the furnace in a corrosive or contami
nated atmosphere. Meet all combustion and ventila
tion air requirements, as well as all local codes.
All gas‐fired appliances require air for the combustion pro
cess. If sufficient combustion air is not available, the fur
nace or other appliance will operate inefficiently and un
safely. Enough air must be provided to meet the needs of
all fuel-burning appliances and appliances such as ex
haust fans which force air out of the house. When fire
places, exhaust fans, or clothes dryers are used at the
same time as the furnace, much more air is required to en
sure proper combustion and to prevent a downdraft. Insuf
ficient air causes incomplete combustion which can result
in carbon monoxide.
In addition to providing combustion air, fresh outdoor air di
lutes contaminants in the indoor air. These contaminants
may include bleaches, adhesives, detergents, solvents
and other contaminants which can corrode furnace compo
nents.
The requirements for providing air for combustion and ven
tilation depend largely on whether the furnace is installed in
an unconfined or a confined space.
Unconfined Space
An unconfined space is an area such as a basement or
large equipment room with a volume greater than 50 cubic
feet (1.42 m
3
) per 1,000 Btu (.29 kW) per hour of the com
bined input rating of all appliances installed in that space.
This space also includes adjacent rooms which are not
separated by a door. Though an area may appear to be un
confined, 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 section.
Confined Space
A confined space is an area with a volume less than 50 cu
bic feet (1.42 m
3
) per 1,000 Btu (.29 kW) per hour of the
com-bined 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 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 ei
ther 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 5.
FIGURE 5
EQUIPMENT IN CONFINED SPACE - ALL AIR FROM INSIDE
OPENINGS
(To Adjacent
Unconfined
Space)
NOTE
- Each opening shall have a free area of at least one square inch
per 1,000 Btu (645mm
2
per .29kW) per hour of the total input rating of
all equipment in the enclosure, but not less than 100 square inches
(64516mm.
2).
ROOF TERMINATED
EXHAUST PIPE
SIDE WALL
TERMINATED
EXHAUST PIPE
(ALTERNATE
LOCATION)
Air from Outside
If air from outside is brought in for combustion and ventila
tion, the confined space shall be provided with two perma
nent openings. One opening shall be within 12” (305mm) of
the top of the enclosure and one within 12” (305mm) 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 verti
cal ducts. Each opening shall have a minimum free area of
1 square inch per 4,000 Btu (645mm
2
per 1.17kW) per hour
of total input rating of all equipment in the enclosure. When
communicating with the outdoors through horizontal ducts,
each opening shall have a minimum free area of 1 square
inch per 2,000 Btu (645mm
2
per .59kW) per total input rat
ing of all equipment in the enclosure (See figure 6). It is also
permissible to bring in air for combustion from a ventilated
attic (figure 7) or ventilated crawl space (figure 8).
