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Ventilation louvers
at each end of attic
Vent or
Chimney
Attic
Insulation
Water
Heater
Furnace
Crawl Space
Figure 2. Combustion Air Drawn from a Crawl
Space or Vented Attic
Outdoor Air Using Vertical Ducts
If combustion air is taken from outdoors through vertical
ducts, the openings and ducts must have a minimum
free area of one square inch per 4,000 Btuh of total
appliance input. In installations drawing combustion air
from a ventilated attic, both air ducts must extend above
the attic insulation (Figure 3).
Vent or
Attic
Chimney
Insulation
Water
Furnace
Heater
Total Input Rating
Minimum Free Area
(Btuh)
(Each Opening)
40,000
60,000
80,000
100,000
120,000
140,000
160,000
Figure 3. Combustion Air Drawn from Outside
Through Vertical Ducts
Outdoor Air Using Horizontal Ducts
If combustion air is taken from outdoors through horizontal
ducts, the openings and ducts must have a minimum free
area of one square inch per 2,000 Btuh of total appliance
input (Figure 4).
8
Outlet air duct must be
at least 1 sq. in. per
4,000 Btuh of total
input rating. Must
extend above attic
insulation
Inlet air duct must be at
least 1 sq. in. per 4,000
Btuh of total input rating.
Ventilated Louvers for
unheated crawl space
Ventilation Louvers at
each end of attic
Outlet Air Duct must
be at least 1 sq. in.
per 4,000 Btuh of
total input rating.
Inlet and Outlet
Ducts must
extend above
attic insulation.
Inlet Air Duct must
be at least 1 sq. in.
per 4,000 Btuh of
total input rating.
12" Max
Round Duct
Diameter
100 sq. In
12 inches
100 sq. In
12 inches
100 sq. In
12 inches
100 sq. In
12 inches
120 sq. In
13 inches
140 sq. In
14 inches
160 sq. In
15 inches
Vent or
Chimney
12" Max
- - -
- - -
- - -
See
Note
- - - -
- - -
- -
12" Max
Water Heater
Total Input Rating
Minimum Free Area
(Btuh)
(Each Opening)
40,000
60,000
80,000
100,000
120,000
140,000
160,000
Figure 4. Combustion Air Drawn from Outside
Through Horizontal Ducts
Air Directly Through An Exterior Wall
If combustion air is provided directly through an exterior
wall, the two openings must each have free area of at
least one square inch per 4,000 Btuh of total appliance
input (Figure 5).
Vent or
Chimney
12" Max
- - - -
- - -
- -
Furnace
See
Note
- - - -
- - -
- -
Water Heater
12" Max
Total Input Rating
Minimum Free Area
(Btuh)
(Each Opening)
40,000
60,000
80,000
100,000
120,000
140,000
160,000
Figure 5. Combustion Air Drawn from Outside
Through an Exterior Wall
- - - -
NOTE: Each
Air Duct
- - - -
-
opening to
Furnace
outside must
be at least 1
sq. in. per 2,000
Btuh of total
- - - -
input rating.
Air Duct
- - - -
-
Round Duct
Diameter
10 sq. In
5 inches
15 sq. In
6 inches
20 sq. In
7 inches
25 sq. In
8 inches
30 sq. In
9 inches
35 sq. In
10 inches
40 sq. In
10 inches
NOTE: Each opening
to outside must be at
least 1 sq. in. per
4,000 Btuh of total
input rating.
Round Duct
Diameter
10 sq. In
4 inches
15 sq. In
5 inches
20 sq. In
5 inches
25 sq. In
6 inches
30 sq. In
6 inches
35 sq. In
7 inches
40 sq. In
8 inches
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