9
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
confi ned space, but
not less than
100 square inches of
free area (Figure 5). For 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.
Figure 3. 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
Figure 5. 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 4. Combustion Air Drawn from Outside
Through an Exterior Wall
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
NOTE:
Each opening to outside
must be at least 1 sq. in. per
4,000 Btuh of total input rating.
12" Max.
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Furnace
Water
Heater
Vent or
Chimney
12"
Max
See
Note
See
Note
Alternate Method of Providing Air from Outside:
If acceptable under local Codes, it is permitted to
provide outside air using one opening (See NFGC).
Generally, confi ned spaces must have two openings
in the space for combustion air. One opening must
be within 12 inches of the ceiling, and the other
must be within 12 inches of the fl oor. However, an
alternative method recently adopted by the NFGC
uses one opening within 12 inches of the top of the
space. This method may be used if it is acceptable
to the local codes.
The following conditions must be met:
1. The opening must start within 12” of the top of the
structure and connect with the out of doors through
vertical or horizontal ducts or be ducted to a crawl
or attic space that connects with the out of doors.
2. The opening must have a minimum free area of 1
sq. in. per 3,000 Btu per hour of the total input rating
of all equipment located in the enclosure.
3. The free area must not be less than the sum of all
the areas of the vent connectors in the enclosure.
