8
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.
Installation In An Unconfi ned Space
An unconfi ned space is an area including all rooms not
separated by doors with a volume greater than 50 cubic
feet per 1,000 Btuh of the combined input rates of all
appliances which draw combustion air from that space.
In general, a furnace installed in an unconfi ned space will
not require outside air for combustion. However, in homes
built for energy effi ciency (low air change rates), it may
be necessary to provide outside air to ensure adequate
combustion and venting, even though the furnace is located
in an unconfi ned space. See example.
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 4).
Figure 4. Combustion Air Drawn from Outside
Through an Exterior Wall
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12" Max
12" Max
Furnace
NOTE:
Each opening
to outside must be at
least 1 sq. in. per
4,000 Btuh of total
input rating.
Vent or
Chimney
Water Heater
See
Note
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
Furnace
12" Max.
Water
Heater
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.
12” Max.
See
Notes
See Notes
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
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.
Example
:
A space with a water heater rated at 45,000 Btuh
input and a furnace rated at 75,000 Btuh requires a
volume of 6,000 cubic feet [50 x (45 + 75) = 6,000] to
be considered unconfi ned. If the space has an 8 foot
ceiling, the fl oor area of the space must be 750 square
feet (6,000 / 8 = 750).
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