52
Venting System Length Calculations
The Total Equivalent Vent Length (TEVL) for
EACH
combustion air or vent pipe equals the length of the venting system, plus the equivalent
length of elbows used in the venting system from Table 17.
Standard vent terminations or factory accessory concentric vent terminations count for zero deduction.
See vent system manufacturer’s data for equivalent lengths of flexible vent pipe or other termination systems.
DO NOT ASSUME
that one
foot of flexible vent pipe equals one foot of straight PVC/ABS DWV vent pipe.
Compare the Total Equivalent Vent Length to the Maximum Equivalent Vent Lengths in Table 16.
Example 1
A direct--vent 60,000 Btuh furnace installed at 2100 ft. (640 M). Venting system includes,
FOR EACH PIPE
, 100 feet (30 M) of vent pipe,
95 feet (28 M) of combustion air inlet pipe, (3) 90
_
long radius elbows, (2) 45
_
long radius elbows and a factory accessory concentric vent
kit.
Can this application use 2--in. (50 mm ND) PVC/ABS DWV vent piping?
Measure the required linear length of air inlet and vent pipe; insert the
longest of the two here:
100 ft
Use length of the longer of the vent
or air inlet piping system
Add equiv length of (3) 90º long-radius elbows
(use the highest number of elbows for either the
vent or inlet pipe)
3
x
3 ft
=
9 ft.
From Table 17
Add equiv length of (2) 45º long-radius elbows
(use the highest number of elbows for either the
vent or inlet pipe)
2
x
1.5 ft
=
3 ft.
From Table 17
Add equiv length of vent termination
0 ft.
From Table 17
Add correction for flexible vent pipe, if any
0 ft.
From Vent Manufacturer’s instructions;
zero for PVC/ABS DWV
Total Equivalent Vent Length (TEVL)
112 ft.
Add all of the above lines
Maximum Equivalent Vent Length (MEVL)
127 ft.
For 2” pipe from Table 16
Is TEVL less than MEVL?
YES
Therefore, 2” pipe may be used
Example 2
A direct--vent 60,000 Btuh furnace installed at 2100 ft. (640 M) Venting system includes,
FOR EACH PIPE
, 100 feet (30 M) of vent pipe,
95 feet (28 M) of combustion air inlet pipe, (3) 90
_
long radius elbows, and a polypropylene concentric vent kit. Also includes 20 feet (6 M)
of flexible polypropylene vent pipe, included within the 100 feet (30 M) of vent pipe.
Assume that one meter of flexible 60 mm or 80 mm polypropylene pipe equals 2 Meters of PVC/ABS pipe.
VERIFY FROM VENT
MANUFACTURER’S INSTRUCTIONS.
Can this application use 60 mm (O.D.) polypropylene vent piping? If not what size piping can be used?
Measure the required linear length of air inlet and vent pipe; insert the
longest of the two here:
100 ft
Use length of the longer of the vent
or air inlet piping system
Add equiv length of (3) 90º long-radius elbows
(use the highest number of elbows for either the
vent or inlet pipe)
3
x
3 ft
=
9 ft.
From Vent Manufacturer’s instructions
Add equiv length of (2) 45º long-radius elbows
(use the highest number of elbows for either the
vent or inlet pipe)
0
x
=
0 ft.
From Vent Manufacturer’s instructions
Add equiv length of vent termination
9 M
x
3 ft/M
=
18 ft.
From Vent Manufacturer’s instructions
Add correction for flexible vent pipe, if any
1.8
x
20 ft
=
36 ft.
From Vent Manufacturer’s instructions
Total Equivalent Vent Length (TEVL)
163 ft.
Add all of the above lines
Maximum Equivalent Vent Length (MEVL)
127 ft.
For 2” pipe from Table 16
Is TEVL less than MEVL?
NO
Therefore, 60mm pipe may NOT be used;
try 80 mm
Maximum Equivalent Vent Length (MEVL)
250 ft.
For 3” pipe from Table 16
Is TEVL less than MEVL?
YES
Therefore, 80 mm pipe may be used
NOTES
:
1. Use only the smallest diameter pipe possible for venting. Over---sizing may cause flame disturbance or excessive vent terminal icing or freeze---up.
2. NA --- Not allowed. Pressure switch will not close, or flame disturbance may result.
3. Total equivalent vent lengths under 10’ for 40,000 BTUH furnaces from 0 to 2000 ft. (0 to 610 M) above sea level require use of an outlet choke plate .
Failure to use an outlet choke when required may result in flame disturbance or flame sense lockout.
4. Not all furnace families include 140,000 BTUH input models.
5. Vent sizing for Canadian installations over 4500 ft (1370 M) above sea level are subject to acceptance by local authorities having jurisdiction.
6. Size both the combustion air and vent pipe independently, then use the larger size for both pipes.
7. Assume the two 45
_
elbows equal one 90
_
elbow. Wide radius elbows are desirable and may be required in some cases.
8. Elbow and pipe sections within the furnace casing and at the vent termination should not be included in vent length or elbow count.
9. The minimum pipe length is 5 ft. (2 M) linear feet (meters) for all applications.
10. Use 3---in. (76 mm) diameter vent termination kit for installations requiring 4---in. (102 mm) diameter pipe.
59SC
2C
