9
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.
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
Figure 6. Combustion Air Drawn from Outside
Through an Exterior Wall
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,
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
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).
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
,
,
&
.
The length of vent and combustion air piping for either type
of installation are llisted in
.
Category IV appliances operate with positive vent pressure
and therefore require vent systems which are thoroughly
sealed. They also produce liquid condensate, which is slightly
acidic and can cause severe corrosion of ordinary venting
materials. Furnace operation can be adversely affected by
restrictive vent and combustion air piping.
The inducer assembly on this furnace can be rotated to
vent the flue products out of the top, left or right side. This
increases the flexibility of which direction the vent pipe can
exit the furnace.
