9
Total Input Rating
(Btuh)
Minimum Free Area
(Each Opening)
Round Duct
Diameter
40,000
20 sq. In
5 inches
60,000
30 sq. In
6 inches
80,000
40 sq. In
7 inches
100,000
50 sq. In
8 inches
120,000
60 sq. In
9 inches
140,000
70 sq. In
10 inches
160,000
80 sq. In
10 inches
Figure 4. Combustion Air Drawn from Outside
Through Horizontal Ducts
Furnace
Water
Heater
Air Ducts must be
at least 1 sq. in.
per 2,000 Btuh of
total input rating.
Vent or
Chimney
Air Duct
Air Duct
Total Input Rating
(Btuh)
Minimum Free Area
(Each Opening)
Round Duct
Diameter
40,000
10 sq. In
4 inches
60,000
15 sq. In
5 inches
80,000
20 sq. In
5 inches
100,000
25 sq. In
6 inches
120,000
30 sq. In
6 inches
140,000
35 sq. In
7 inches
160,000
40 sq. In
8 inches
Figure 3. Combustion Air Drawn from Outside
Through Vertical Ducts
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
Furnace
Water
Heater
Vent or
Chimney
Outdoor Air from a Crawl Space or Vented Attic
When the openings can freely exchange air with the
outdoors, each opening shall have a minimum free area
of 1 square inch per 4,000 Btuh of total appliance input.
The openings shall exchange directly, or by ducts, with
the outdoor spaces (crawl or attic) that freely exchange
with the outdoors (Figure 2).
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 (Figure 3).
Figure 1. Combustion Air Drawn from Inside
Total Input Rating
(Btuh)
Minimum Free Area
(Each Opening)
Round Duct
Diameter
40,000
100 sq. In
12 inches
60,000
100 sq. In
12 inches
80,000
100 sq. In
12 inches
100,000
100 sq. In
12 inches
120,000
120 sq. In
13 inches
140,000
140 sq. In
14 inches
160,000
160 sq. In
15 inches
Vent or
Chimney
Furnace
Water
Heater
12” Max.
12” Max.
See
Notes
See
Notes
NOTES:
Each opening must be
at least 100 sq. in. or
1 sq. in. per 1,000 Btuh
of total input rating,
whichever is greater.
Openings must start at
no more than 12 inches
from the top and bottom
of the enclosure.
Figure 2. Combustion Air Drawn from a Crawl
Space or Vented Attic
Water
Heater
Vent or
Chimney
- - - - - - - - -
- - - - - - - - -
Furnace
Ventilation Louvers For
Unheated Crawl Space
- - - - - - - - -
Inlet Air
Ventilation Louvers
(each end of attic)
NOTE:
Air openings shall
each have a free area of
not less than one square
inch per 4,000 Btuh of the
total input rating of all
equipment in the enclosure.
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Alternate
Air Inlet
Outlet
Air
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). Ducts must have cross - sectional area at
least as large as the free area of their respective openings
to the furnace space.
