Figure; Vent Piping Guidelines - Lennox G71MPP Installation Instructions Manual

Vent Piping Guidelines

The G71MPP can be installed only as a Direct Vent gas
central furnace.
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
tables 6 and 7. Table 6 lists the minimum equivalent vent
pipe lengths permitted. Table 7 lists the maximum equiva-
lent pipe lengths permitted.
Maximum vent length is defined as:
Total length (linear feet) of pipe,
Plus Equivalent length (feet) of fittings,
Plus Equivalent length (feet) of termination.
NOTE − Include ALL pipe and ALL fittings, both indoors
and outdoors. Measure equivalent length of intake and
exhaust pipe separately. Use the greater of the two
lengths to determine vent pipe diameter to be used for
both intake and exhaust.
Regardless of the diameter of pipe used, the standard roof
and wall terminations described in section Exhaust Piping
Upflow and Downflow Application
Rooftop Termination
12" Min.
Exhaust Pipe Offset
12" Min.
Horizontal Application
Rooftop Termination
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 8.
NOTE − The exhaust pipe should be offset a minimum of 12
inches to avoid the possibility of water droplets being re-
leased from the exhaust termination. The minimum ex-
haust vent length is 15 ft. Shorter exhaust vent lengths may
result in the discharge of water droplets from the exhaust
termination, in spite of the 12−inch vertical offset. See figure
17.
Each 90° elbow (including those provided with the furnace)
of any diameter is equivalent to 5 feet (1.52m) of vent pipe
of the same diameter. Two 45° elbows are equivalent to
one 90° elbow of the same diameter. One 45° elbow is
equal to 2.5 feet (.76m) of vent pipe of the same diameter.
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.
Upflow and Downflow Application

FIGURE 17

Page 16
12" Min.
Side Wall Termination
12" Min.
Horizontal Application
Side Wall Termination