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Vent or
Chimney
Water
Heater
Total Input Rating
Minimum Free Area
(Btuh)
(Each Opening)
40,000
60,000
80,000
100,000
120,000
140,000
160,000
Figure 5. Combustion Air Drawn from Outside
Through Horizontal Ducts
Vent or
Chimney
- - -
See
- - -
- - -
12"
Note
Water
Max
Heater
See
Note
- - -
12" Max.
- - -
- - -
Total Input Rating
Minimum Free Area
(Btuh)
(Each Opening)
40,000
60,000
80,000
100,000
120,000
140,000
160,000
Figure 6. Combustion Air Drawn from Outside
Through an Exterior Wall
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 sq. ft.
(6,000 / 8 = 750).
10
Air Duct
Air Ducts must be
at least 1 sq. in.
Furnace
per 2,000 Btuh of
total input rating.
Air Duct
Round Duct
Diameter
20 sq. In
5 inches
30 sq. In
6 inches
40 sq. In
7 inches
50 sq. In
8 inches
60 sq. In
9 inches
70 sq. In
10 inches
80 sq. In
10 inches
Furnace
Round Duct
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
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.
Conventional Vent Systems - 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 below.
Category IV Venting
WARNING:
Upon completion of the furnace installation,
carefully inspect the entire flue system both
inside and outside the furnace to assure it is
properly sealed. Leaks in the flue system can
result in serious personal injury or death due
to exposure of flue products, including carbon
monoxide.
This furnace is classified as a "Category IV" appliance,
which requires special venting materials and installation
procedures. This section specifies installation requirements
for Conventional (1-pipe) and Direct Vent (2-pipe) piping.
For 1- pipe installations, install vent piping as described in
this section and provide air for combustion and ventilation
according to
pages
7-10. The length of vent and
combustion air piping for either type of installation are
llisted in
Table 1 (page
11).
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