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12
Each opening
to outside must
be at least
1 sq. in. per
4000 Btuh of
total input
rating.
12" Max
Total Input
Rating (Btuh)
40,000
60,000
80,000
100,000
120,000
140,000
160,000
Minimum
Free Area
(Each Opening)
10 sq. in.
15 sq. in.
20 sq. in.
25 sq. in.
30 sq. in.
35 sq. in.
40 sq. in.
Round Duct
Diameter
4"
5"
5"
6"
6"
7"
8"
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Furnace
Water Heater
Vent or
Chimney
12"
Max
Figure 11. Equipment in a Confined Space
with all Combustion Air drawn from
Outdoors through Vertical Ducts – from
Ventilated Attic
Air Duct must be
at least 1 sq. in.
per 4,000 Btuh of
total input rating.
Ducts must
extend above
attic insulation.
Air Duct must be
at least 1 sq. in.
per 4,000 Btuh of
total input rating.
Ventilation Louvers at each end of attic
Attic
Insulation
12" Max
Total Input
Rating (Btuh)
40,000
60,000
80,000
100,000
120,000
140,000
160,000
Minimum
Free Area
(Each Opening)
10 sq. in.
15 sq. in.
20 sq. in.
25 sq. in.
30 sq. in.
35 sq. in.
40 sq. in.
Round Duct
Diameter
4"
5"
5"
6"
6"
7"
8"
Furnace
Water Heater
Vent or
Chimney
Figure 10. Equipment in a Confined Space
with all Combustion Air drawn from
Outdoors through Exterior Wall
Water
Heater
Vent or
Chimney
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Furnace
Ventilation Louvers For
Unheated Crawl Space
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Inlet Air
Ventilation Louvers
(each end of attic)
NOTE: Air open-
ings shall each
have a free area
of not less than
one square inch
per 4,000 Btuh of
the total input rat-
ing of all equipment
in the enclosure.
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Alternate
Air Inlet
Outlet
Air
Total Input
Rating (Btuh)
40,000
60,000
80,000
Round Duct
Diameter
12"
12"
12"
Minimum
Free Area
(Each Opening)
100 sq. in.
100 sq. in.
1
00
sq
. in.
Furnace
12" Max.
12" Max.
Water Heater
Vent or
Chimney
Each opening must
be at least 100 sq. in.
or 1 sq. in. per 1000
Btuh of total input
rating, whichever is
greater. See minimum
area per table.
Figure 9. Equipment in a Confined Space
with all Combustion Air drawn from Inside
Figure 12. Equipment in a Confined Space
with all Combustion Air drawn from
Outdoors through Ventilated Crawl Space
and Ventilated Attic