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CAUTION
Insufficient combustion air can cause headaches,
nausea, dizziness or asphyxiation. It will also cause
excess water in the heat exchanger resulting in rust-
ing and premature heat exchanger failure. Excessive
exposure to contaminated combustion air will result
in safety and performance related problems. Avoid
exposure to the following substances in the combus-
tion air supply:
Permanent wave solutions
Chlorinated waxes and cleaners
Chlorine base swimming pool chemicals
Water softening chemicals
De-icing salts or chemicals
Carbon tetrachloride
Halogen type refrigerants
Cleaning solvents (such as perchloroethylene
Printing inks, paint removers, varnishes, etc.
Hydrochloric acid
Cements and glues
Antistatic fabric softeners for clothes dryers
Masonry acid washing materials
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
unsafely. 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
dilutes contaminants in the indoor air. These contami-
nants may include bleaches, adhesives, detergents, sol-
vents and other contaminants which can corrode furnace
components.
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
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 contain-
ing 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 1.
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 communicate di-
rectly or by ducts with the outdoors or spaces (crawl or at-
tic) 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 en-
closure. See
figures 2 and 3. 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 4.
