MG3P Installation Manual

10
Vent Pipe Length & Diameter
In order for the furnace to operate properly, the combustion air
and vent piping must not be excessively restrictive.
• The venting system should be designed to have the minimum
number of elbows or turns.
• Transition to the final vent diameter should be done as close
to the furnace outlet as practical.
• Always use the same size or a larger pipe for combustion air
that is used for the exhaust vent.
Table 1 specifies the maximum allowable pipe length for vent and
combustion air for a furnace of known input rate, when installed
with piping of selected diameter and number of elbows. Before
using the table, the furnace input rate, the centerline length and
the number of elbows on each pipe must be known.
When estimating the length of vent runs, consideration must
be made to the effect of elbows and other fittings. This is
conveniently handled using the idea of “equivalent length”. This
means the fittings are assigned a linear length that accounts for
the pressure drop they will cause. For example: a 2” diameter,
long radius elbow is worth the equivalent of 2.5 feet of linear
run. A 90 degree tee is worth 7 ft.
The equivalent lengths of tees and various elbows are listed in
Table 1. Measure the linear length of your vent run and then
add in the equivalent length of each fitting. The total length,
including the equivalent fitting lengths, must be less than the
maximum length in the table.
Vent Pipe Installation
CAUTION:
Combustion air must not be drawn from a
corrosive atmosphere.
This furnace has been certified for installation with zero clearance
between vent piping and combustible surfaces. However, it is
good practice to allow space for convenience in installation
and service.
• The quality of outdoor air must also be considered. Be sure
that the combustion air intake is not located near a source of
solvent fumes or other chemicals which can cause corrosion
Table 1. Vent Pipe Lengths
FURNACE
MODELS
(BTU)
FURNACE
INSTALLATION
DUAL VENT PIP
LENGTH (FT.)
WITH 1 LONG RADIUS
ELBOW ON EACH PIPE†
INLET / OUTLET
2” DIAMETER
INLET / OUTLET
3” DIAMETER
54,000 Upflow 70 90
72,000 Upflow 50 90
80,000 Downflow
30 90
90,000 Upflow
60 90
108,000 Upflow N/A 90
†
NOTES:
• The length of 2” pipe needed between the inducer and the exit hole (top of
cabinet) is 8 3/4” for upflow models and 16” for downflow models.
• Subtract 2.5 ft. for each additional 2 inch long radius elbow, 5 ft. for each
additional 2 inch short radius elbow, 3.5 ft. for each additional 3 inch long radius
elbow, and 7 ft. for each additional 3 inch short radius elbow. ubtract 5 ft for
each 2” tee and 8 ft for each 3” tee.
• Two 45 degree elbows are equivalent to one 90 degree elbow.
• This table applies for elevations from sea level to 2,000 ft. For higher elevations,
decrease pipe lengths by 8% per 1,000 ft of altitude.
Figure 8. Vent Locations
12 in.
12 in.
4 ft.
Note 3
Less
than
10 ft.
3 ft.
NOTES:
1. All dimensions shown are minimum requirements.
2. Exterior vent terminations must be located at
least 12” above the maximum expected snow level.
3. Mechanical draft vent terminal
4. Direct vent terminal - more than 50,000 Btuh.
5. Direct vent terminal - less than 50,000 Btuh.
9 in.
4ft.
Forced
Air Inlet
Notes
2 & 4
Note 3
Notes
2 & 3
Notes
2 & 5
Figure 9. Alternate Horizontal Vent Installation
Outside
Wall
Support
Vent Configuration to
Provide 12" Minimum
height above
Snow Level.
1/2"
Armaflex
Insulation or
Equivalent
(if required)
12" Above
Maximum
Expected
Snow Level
12" Min.
19" Max.
12” min. to maximum
expected snow level
(both pipes)
90° Elbow
Exhaust vent
option B
Exhaust vent
option A
Mounting kit faceplate
secured to wall with screws
(both pipes)
Combustion
air inlet
Exhaust vent
option C
18” Min.
36” Max.
8” Min.
36” Max.
(all positions)
Figure 7. Inlet & Exhaust Pipe Clearances