Vent Piping Guidelines - Lennox G51MP SERIES Installation Instructions Manual

Vent Piping Guidelines

The G51MP 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 in Direct Vent applications
and exhaust pipe sizing in Non-Direct Vent applications −−
Size pipe according to tables 6 and 7. Table 6 lists the mini-
mum equivalent vent pipe lengths permitted. Table 7 lists
the maximum equivalent 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 in
doors and outdoors.
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 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
21.
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.
Upflow Application
Rooftop Termination
12" Min.*
Exhaust Pipe Offset
12" Min.*
Horizontal Application
Rooftop Termination
*A minimum of 1/4" (6mm) drop for each 12" (305mm) of horizontal run is mandatory for drainage.
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 flue collar on all models is sized to accommo-
date 2" Schedule 40 flue pipe. When vent pipe which is
larger than 2" must be used in an upflow application, a 2"
elbow must be applied at the flue collar in order to proper-
ly transition to the larger diameter flue pipe. This elbow
must be added to the elbow count used to determine ac-
ceptable vent lengths. Assign an equivalent feet value to
this elbow according to the larger size pipe being used.
Contact the Application Department for more information
concerning sizing of vent systems which include multiple
pipe sizes.
Use the following steps to correctly size vent pipe diameter.
Refer to Vent Pipe Size Determination Worksheet on
Page 53.
1 − Determine the vent termination and its corresponding
equivalent feet value according to table 5.
2 − Determine the number of 90° elbows required for both
indoor and outdoor (e.g. snow riser) use. Calculate the
corresponding equivalent feet of vent pipe.
3 − Determine the number of 45° elbows required for both
indoor and outdoor use. Calculate the corresponding
equivalent feet of vent pipe.
4 − Determine the length of straight pipe required.
5 − Add the total equivalent feet calculated in steps 1
through 4 and compare that length to the maximum
values given in table 7 for the proposed vent pipe di-
ameter. If the total equivalent length required exceeds
the maximum equivalent length listed in the appropri-
ate table, evaluate the next larger size pipe.
FIGURE 21
Page 18
12" Min.
Upflow Application
Side Wall Termination
12" Min.*
Horizontal Application
Side Wall Termination