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Vent or
Chimney
Furnace
Water Heater
Minimum
Total Input
Free Area
Rating (Btuh)
(Each Opening)
40,000
100 sq. in.
60,000
100 sq. in.
80,000
100 sq. in.
100,000
100 sq. in.
120,000
120 sq. in.
140,000
140 sq. in.
160,000
160 sq. in.
Figure 5. Equipment in a
Confi ned Space with all
Combustion Air drawn from Inside
Ventilation Louvers at each end of attic
Vent or
Chimney
Attic
Insulation
Furnace
Water Heater
Minimum
Total Input
Free Area
Rating (Btuh)
(Each Opening)
10 sq. in.
40,000
60,000
15 sq. in.
80,000
20 sq. in.
25 sq. in.
100,000
120,000
30 sq. in.
140,000
35 sq. in.
40 sq. in.
160,000
Figure 7. Equipment in a Confi ned Space
with all Combustion Air drawn from
Outdoors through Vertical Ducts – from
Ventilated Attic
12
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.
12" Max.
12" Max.
Round Duct
Diameter
12"
12"
12"
12"
13"
14"
15"
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.
12" Max
Alternate
Round Duct
Diameter
4"
5"
5"
6"
6"
7"
8"
Vent or
Chimney
- - -
- - -
12"
- - -
Max
- - -
- - -
- - -
Water Heater
12" Max
Minimum
Total Input
Free Area
Rating (Btuh)
(Each Opening)
40,000
10 sq. in.
60,000
15 sq. in.
80,000
20 sq. in.
100,000
25 sq. in.
120,000
30 sq. in.
140,000
35 sq. in.
160,000
40 sq. in.
Figure 6. Equipment in a Confi ned Space
with all Combustion Air drawn from Out-
doors through Exterior Wall
Vent or
Chimney
Water
Furnace
Heater
Inlet Air
Air Inlet
Ventilation Louvers For
Unheated Crawl Space
Figure 8. Equipment in a Confi ned Space
with all Combustion Air drawn from Out-
doors through Ventilated Crawl Space and
Ventilated Attic
Furnace
Round Duct
Diameter
4"
5"
5"
6"
6"
7"
8"
Ventilation Louvers
(each end of attic)
Outlet
NOTE: Air open-
Air
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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