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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 cuĆ
bic feet per 1,000 Btu per hour of the combined input ratĆ
ing of all appliances installed in that space. This definiĆ
tion 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
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 furĆ
nace. 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 reĆ
turn air duct connection can cause a potentially danĆ
gerous negative pressure condition. Air for combusĆ
tion and ventilation can be brought into the confined
space either from inside the building or from outside.
Air from Inside
If the confined space housing the furnace adjoins space
categorized as unconfined, air can be brought in by proĆ
viding two permanent openings between the two
spaces. Each opening must have a minimum free area of
1 square inch per 1,000 Btu per hour of total input rating
of all gas-fired equipment in the confined space. Each
opening must be at least 100 square inches. One opening
shall be within 12 inches of the top of the enclosure and
one opening within 12 inches of the bottom. See figure 3.
EQUIPMENT IN CONFINED SPACE
ALL AIR FROM INSIDE
WATER
HEATER
OPENINGS
(To Adjacent
Room)
FIGURE 3
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.
G20R
FURNACE
CHIMĆ
NEY OR
GAS
VENT
Air from Outside
If air from outside is brought in for combustion and venĆ
tilation, the confined space must have two permanent
openings. One opening shall be within 12 inches of the
top of the enclosure and one within 12 inches of the botĆ
tom. These openings must communicate directly or by
ducts with the outdoors or spaces (crawl or attic) that freeĆ
ly communicate with the outdoors or indirectly through
vertical ducts. Each opening shall have a minimum free
area of 1 square inch per 4,000 Btu per hour of total input
rating of all equipment in the enclosure. See figures 4 and
5. When communicating with the outdoors through horiĆ
zontal ducts, each opening shall have a minimum free
area of 1 square inch per 2,000 Btu per total input rating of
all equipment in the enclosure. See figure 6.
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
FIGURE 4
G20R
FURNACE
VENTILATION
LOUVERS
(For unheated crawl space)
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" above
bottom)
FIGURE 5
G20R
FURNACE
