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Vent (
Air Duct
Air Ducts must be
at least 1 sq. in.
per 2,000 Btuh of
total input rating.
40,000
60,000
80,000
100,000
120,000
140,000
160,000
20 sq. In
30 sq. In
40 sq. In
50 sq. In
60 sq. In
70 sq. In
80 sq. In
5 inches
6 inches
7 inches
8 inches
9 inches
10 inches
10 inches
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. See Figure 5.
Ventor__
Chimney
.//
ng to outside
must be at least 1 sq. in. per
4,000 Btuh of total input rating.
I
See--
Note
s Max
Water
Heater
Furnace
Seei
/
/
Note
i
12" Max=3_
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J
40,000
60,000
80,000
100,000
120,000
140,000
160,000
10 sq. In
15 sq. In
20 sq. In
25 sq. In
30 sq. In
35 sq. In
40 sq. In
4inches
5inches
5inches
6inches
6inches
7inches
8inches
Figure 5. Combustion
Air Drawn from Outside
Through
an Exterior Wall
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, confined 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 floor. 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.
installation in Unconfined
Spaces
An unconfined 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 unconfined space will
not require outside air for combustion. However, in homes
built for energy efficiency (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 unconfined space. See example.
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 unconfined. If the space has an 8 foot
ceiling, the floor area of the space must be 750 square
feet (6,000 / 8 = 750).
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