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15
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 13. Equipment in a Confi ned
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 12. Equipment in a Confi ned Space
with all Combustion Air drawn from Out-
doors through Exterior Wall
Figure 14. Equipment in a Confi ned Space
with all Combustion Air drawn from Out-
doors through Ventilated Crawl Space and
Ventilated Attic
Total Input
Rating (Btuh)
40,000
60,000
80,000
100,000
120,000
140,000
160,000
Minimum
Free Area
(Each Opening)
20 sq. in.
30 sq. in.
40 sq. in.
50 sq. in.
60 sq. in.
70 sq. in.
80 sq. in.
Round Duct
Diameter
5"
6"
7"
8"
9"
10"
10"
Furnace
Water Heater
Air Duct
must be
at least
1 sq. in.
per 2000
Btuh of
total input
rating.
Vent or
Chimney
Air Duct
Air Duct
Figure 15. Equipment in a Confi ned Space
with all Combustion Air Drawn from the
Outside through Horizontal Ducts
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