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by the total input rate of all appliances in the space. In
all cases, the minimum dimension of any combustion air
opening is 3 inches.
Air From Inside
If combustion air is taken from the heated space, the two
openings must each have a free area of at least one square
inch per 1,000 Btuh of total input of all appliances in the
confined space, but not less than 100 square inches of
free area. See example and
Example:
If the combined input rate of all appliances is less
than or equal to 100,000 Btuh, each opening must
have a free area of at least 100 square inches. If the
combined input rate of all appliances is 120,000 Btuh,
each opening must have a free area of at least 120
square inches.
Vent or
Chimney
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.
Total Input Rating
Minimum Free Area
(Btuh)
(Each Opening)
40,000
60,000
80,000
100,000
120,000
140,000
160,000
Figure 2. Combustion Air Drawn from Inside
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
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
4).
Figure
2.
12" Max.
Notes
Water
Heater
Furnace
See
Notes
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
3).
Vent or
Chimney
Furnace
Alternate
Air Inlet
Ventilation Louvers For
Unheated Crawl Space
Figure 3. Combustion Air Drawn from a Crawl
Space or Vented Attic
Ventilation Louvers
at each end of attic
Vent or
Chimney
See
Water
Heater
Total Input Rating
(Btuh)
40,000
60,000
80,000
100,000
120,000
140,000
160,000
Figure 4. 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. Ducts must have cross - sectional area at least as
large as the free area of their respective openings to the
furnace space. See
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. See
Figure 6 (page
Ventilation Louvers
(each end of attic)
Outlet
NOTE: Air openings shall
Air
each have a free area of
Water
not less than one square
Heater
inch per 4,000 Btuh of the
total input rating of all
Inlet Air
equipment in the enclosure.
Air Duct must be
at least 1 sq. in.
per 4,000 Btuh of
Attic
total input rating.
Insulation
Ducts must
extend above
attic insulation.
Furnace
Air Duct must be
at least 1 sq. in.
per 4,000 Btuh of
total input rating.
12" Max
Minimum Free Area
Round Duct
(Each Opening)
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
Figure 5 (page
10).
10),
9
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