2. If required per Table 11, insert perforated disk assembly
(factory-supplied in loose parts bag) in intake housing where
combustion air pipe will be connected. If half disk set is
required, install only shouldered disk half.
Attachment of Combustion Air Pipe
1. Determine location of combustion air intake pipe connection
to combustion air intake housing as shown in Fig. 37 for
application.
2. Reposition combustion air intake housing plug fitting in
appropriate unused intake housing connection.
3. Install pipe grommet (factory-supplied in loose parts bag) into
selected furnace casing combustion-air pipe hole.
4. Insert assembled combustion air inlet pipe into intake housing
as shown in Fig. 37.
5. Drill a 1/8-in. hole in 2-in, combustion air pipe using the hole
in intake housing as a guide.
6. Install a field-supplied No. 6 or No. 8 sheet metal screw into
combustion air pipe.
7. Install casing hole filler cap (factory-supplied in loose parts
bag) in unused combustion air pipe casing hole.
NOTE:
Do not attach combustion air intake pipe permanently to
combustion air intake housing since it may be necessary to remove
pipe for service of igniter or flame sensor.
Attachment of Combustion Air Intake Housing Plug Fitting
The combustion-air intake plug fitting must be installed in unused
combustion air intake housing. This fitting must be attached by
using RTV sealant, or by drilling a 1/8-in. hole in fitting, using
hole in intake housing as a guide. Install a field-supplied No. 6 or
No. 8 sheet metal screw.
NOTE:
DO NOT OVERTIGHTEN SCREW. Breakage of intake
housing or fitting may cause air leakage to occur.
A plugged drain connection has been provided on this fitting for
use when moisture is found in combustion air intake pipe and
combustion box. If use of this drain connection is desired, drill out
fitting’s tap plug with 3/16-in. drill and connect a field-supplied
3/8-in. tube. This tube should be routed to open condensate drain
for furnace and A/C (if used), and should be trapped, as shown in
Fig. 40.
NOTE:
(Direct Vent/2-Pipe System ONLY) Moisture in combus-
tion air intake may be a result of improper termination. Ensure
combustion air pipe termination is similar to those as shown in Fig.
Table 10—Maximum Allowable Exposed Vent Pipe Length (ft) With and Without Insulation in Winter Design
Temperature Ambient*
FURNACE
SIZE
WINTER DESIGN
TEMPERATURE
(°F)
MAX PIPE
DIAMETER
(IN.)
WITHOUT
INSULATION
WITH 3/8–IN. OR
THICKER INSULATION†
040
20
1.5
51
70
0
1.5
28
70
-20
1.5
16
70
20
2
45
70
0
2
22
70
-20
2
10
58
060
20
2
65
70
0
2
35
70
-20
2
20
70
080
20
2
55
55
0
2
48
55
-20
2
30
55
20
2.5
70
70
0
2.5
47
70
-20
2.5
28
70
100
20
2.5
40
40
0
2.5
40
40
-20
2.5
38
40
20
3
70
70
0
3
50
70
-20
3
28
70
120
20
3
70
70
0
3
61
70
-20
3
37
70
20
4
70
70
0
4
48
70
-20
4
23
70
140
20
3
60
60
0
3
60
60
-20
3
44
60
20
4
70
70
0
4
57
70
-20
4
30
70
* Pipe length (ft) specified for maximum pipe lengths located in unconditioned spaces. Pipes located in unconditioned space cannot exceed total allowable pipe length
as specified in Table 11.
† Insulation thickness based on R value of 3.5 per in.
33