Page 23
The ML193UHE can be installed as either a Non-Direct
Vent or a Direct Vent gas central furnace.
NOTE - In Non‐Direct Vent installations, combustion air is
taken from indoors and flue gases are discharged outdoors.
In Direct Vent installations, combustion air is taken from out
doors and flue gases are discharged outdoors.
Intake and exhaust pipe sizing -- Size pipe according to ta
bles 11 and
12. Count all elbows inside and outside the
home. Table 11 lists the minimum vent pipe lengths permit
ted. Table 12 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 15.
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 22
Exhaust Pipe
NOTE - Exhaust pipe MUST be glued to furnace exhaust fittings.
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
12” Min.
Horizontal
Gas Furnace
12” max
Of straight pipe
TABLE 11
MINIMUM VENT PIPE LENGTHS
ML193UHE
MODEL
MIN. VENT LENGTH*
030, 045, 070, 090, 110
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 23
Piping Size Process
1
2
3
4
5
6
Which style termination
being used?
Standard or concentric?
See table 10.
Which needs
most elbows?
Intake or
exhaust?
How many elbows?
Count all elbows inside
and outside house.
Desired pipe size?
1-1/2'”, 2”, 2-1/2”,
3”
Use table 12 or 13 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?
030, 045, 070,
090, 110?
IMPORTANT
Do not use screens or perforated metal in exhaust or
intake terminations. Doing so will cause freeze-ups
and may block the terminations.
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