Lennox MERIT Series Installation Instructions Manual page 23

Maximum Allowable Exhaust Vent Pipe Length (in ft.) Without Insulation In Unconditioned Space For
Winter Design
Vent Pipe
Tempera-
Diameter
tures1 °F (°C)
2 in
32 to 21
(0 to -6)
2-1/2 in
3 in
2 in
20 to 1
2-1/2 in
(-7 to -17)
3 in
2 in
0 to -20
2-1/2 in
(-18 to -29)
3 in
1Refer to 99% Minimum Design Temperature table provided in the current edition of the ASHRAE Fundamentals Handbook.
2 Poly-Propylene vent pipe (PP) by Duravent and Centrotherm.
NOTE - Concentric terminations are the equivalent of 5' and should be considered when measuring pipe length.
NOTE - Maximum uninsulated vent lengths listed may include the termination(vent pipe exterior to the structure) and cannot exceed
5 linear feet or the maximum allowable intake or exhaust vent length listed in TABLE 7 or TABLE 8 which ever is less.
NOTE - If insulation is required in an unconditioned space, it must be located on the pipe closest to the furnace. See FIGURE 27.
Winter Design Temperatures Two - Stage High Efficiency Furnace
045
PVC
21
16
12
11
7
2
6
1
1
Conditioned
Space
TABLE 9
Unit Input Size
070
PP
PVC
2
18 33
n/a 26
12 21
9 19
n/a 14
2 9
4 12
n/a 7
1 2
Conditioned
Space
Pipe Insulation
Unconditioned
Space
FIGURE 27
Page 23
090
PP
PVC
2
30 46
n/a 37
21 30
17 28
n/a 21
9 16
10 19
n/a 13
2 8
Exhaust
Pipe
Intake
Pipe
PP
2
42
n/a
30
25
n/a
16
16
n/a
8