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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. It can also
cause property damage.
When fireplaces, exhaust fans, or clothes dryers are used
at the same time as the furnace, much more air is required
to ensure proper combustion and to prevent a down-draft
situation. Insufficient amounts of air also cause incomplete
combustion which can result in sooting. Requirements for
providing air for combustion and ventilation depend largely
on whether the furnace is installed in an unconfined or con-
fined 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 (1.4 cubic meters) per 1,000 Btu (293 W) per hour of
the combined 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
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 outlined for
using air from the outside for combustion and ventilation.
Confined Space
A confined space is an area with volume less than 50 cubic
feet (1.4 cubic meters) per 1,000 Btu (293 W) 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 con-
taining the furnace, the return air must be handled by ducts
which are sealed to the furnace casing and which termi-
nate outside the space containing the furnace. This is es-
pecially important when the furnace is mounted on a plat-
form in a confined space such as a closet or small
equipment room.
Small leaks around the base of the unit at the platform or at
the return air duct connection can cause a potentially dan-
gerous 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 an Adjacent Space
If the confined space housing the furnace adjoins space
categorized as unconfined, air can be brought in by provid-
ing two permanent openings between the two spaces.
Each opening must have a minimum free area of 1 square
inch
(6.4 square centimeters) per 1,000 Btu (293 W) per
hour of the total input rating of all fuel-fired equipment in the
confined space. Each opening must be at least 100 square
inches (614.5 square centimeters). One opening shall be
within 12" (305 mm) of the top of the enclosure and one
opening within 12" (305 mm) of the bottom (See figure 10).
Equipment In Confined Space −
All Air From Inside
CHIMNEY OR
OIL VENT
WATER
HEATER
OPENINGS
(TO ADJACENT
ROOM)
Figure 10
NOTE−EACH OPENING SHALL HAVE A FREE AREA OF AT LEAST 1 SQUARE
INCH
(6.4 SQUARE CENTIMETERS) PER 1,000 BTU (293 W) PER HOUR OF THE
TOTAL INPUT RATING OF ALL EQUIPMENT IN THE ENCLOSURE, BUT NOT LESS
THAN 100 SQUARE INCHES
(614.5 SQUARE CENTIMETERS).
OIL
FURNACE
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" (305 mm)
of the top of the enclosure and one within 12" (305 mm) 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
vertical ducts.
Each opening shall have a minimum free area of 1 square
inch (6.4 square centimeters) per 4,000 Btu (1172 W) per
hour of total input rating of all equipment in the enclosure
(See figures 11 and 12). When communicating with the
outdoors through horizontal ducts, each opening shall
have a minimum free area of 1 square inch (6.4 square
centimeters) per 2,000 Btu (586 W) per total input rating of
all equipment in the enclosure (See figure 13).
