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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 (e.g., 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 con-
fined space either 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 9).
Equipment In Confined Space −
All Air From Inside
CHIMNEY OR
OIL VENT
OIL
FURNACE
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).
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
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WATER
HEAT-
ER
OPENINGS
(To Adjacent
Room)
Figure 9
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Btu (1172 W) per hour of total input rating of all equipment
in the enclosure (See figures 10 and 12). When communi-
cating 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
12).
Equipment In Confined Space − All Air
From Outside (Inlet Air From Crawl Space
& Outlet Air To Ventilated Attic)
CHIMNEY
OR OIL
VENT
OIL
FURNACE
VENTILATION
LOUVERS
(For unheated
crawl space)
NOTE − The inlet and outlet air openings shall each have a free
area of at least one square inch (6.4 square centimeters) per
4,000 Btu (1172 W) per hour of the total input rating of all equip-
ment in the enclosure.
Equipment In Confined Space − All
Air From Outside (All Air Through
Ventilated Attic)
CHIMNEY
OR OIL
VENT
OUTLET
OIL
FURNACE
NOTE−The inlet and outlet air openings shall each have a free area of
at least one square inch (6.4 square centimeters) per 4,000 Btu (1172
W) per hour of the total input rating of all equipment in the enclosure.
Page 8
VENTILATION
LOUVERS (Each
end of attic)
OUTLET
AIR
WATER
HEATER
INLET AIR
Figure
10
VENTILATION LOUVERS
(Each end of attic)
AIR
INLET AIR
(Ends 12" above
bottom)
WATER
HEATER
Figure 11
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