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NOTE
- In Direct Vent installations, combustion air is taken
from outdoors and flue gases are discharged outdoors.
Intake and exhaust pipe sizing --
Size pipe according to ta
bles 20 and 21. Count all elbows inside and outside the
home. Table 20 lists the
minimum
vent pipe lengths permit
ted. Table 21 lists the
maximum
pipe lengths permitted.
Regardless of the diameter of pipe used, the standard roof
and wall terminations described in section
Exhaust Piping
Terminations
should be used. Exhaust vent termination
pipe is sized to optimize the velocity of the exhaust gas as
it exits the termination. Refer to table 24.
In some applications which permit the use of several differ
ent sizes of vent pipe, a combination vent pipe may be
used. Contact Lennox' Application Department for assis
tance in sizing vent pipe in these applications.
NOTE
- The exhaust collar on all models is sized to ac
commodate 2” Schedule 40 vent pipe. In horizontal appli
cations, any transition to exhaust pipe larger than 2” must
be made in vertical runs of the pipe. Therefore a 2” elbow
must be added before the pipe is transitioned to any size
larger than 2”. This elbow must be added to the elbow
count used to determine acceptable vent lengths. Con
tact the Application Department for more information
concerning sizing of vent systems which include multiple
pipe sizes.
FIGURE 20
Horizontal Installation Offset Requirements
NOTE -
All horizontal runs of exhaust pipe must slope back to
ward unit. A minimum of 1/4” (6mm) drop for each 12” (305mm)
of horizontal run is mandatory for drainage.
NOTE
- Exhaust piping should be checked carefully to make
sure there are no sags or low spots.
Exhaust Pipe
Horizontal
Gas Furnace
12” Min.
12” Max.
TABLE 20
MINIMUM VENT PIPE LENGTHS
SL297UHNV
MODEL
MIN. VENT LENGTH*
040, 060, 080
15 ft. or
5 ft. plus 2 elbows or
10 ft. plus 1 elbow
*Any approved termination may be added to the minimum length listed.
Use the following steps to correctly size vent pipe diameter.
FIGURE 21
Piping Size Process
1
2
3
4
5
6
Which style termination
being used?
Standard or concentric?
Which needs
most elbows?
Intake or
exhaust?
How many elbows?
Count all elbows inside
and outside house.
Desired pipe size?
2”, 2-1/2”, 3”
Use table 21 or 22 to find
max intake or exhaust pipe
length. Includes all vent
pipe and elbows inside
and outside the house.
What is the altitude of
the furnace installation?
7
What is the
furnace capacity?
040, 060, 080?
IMPORTANT
Do not use screens or perforated metal in exhaust or
intake terminations. Doing so will cause freeze-ups
and may block the terminations.
