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Vent Pipe Material
Vent and combustion air pipe and fittings must
be one of the following materials and must
conform to the indicated ANSI/ASTM stan-
dards:
Material
Schedule 40 PVC
PVC-DWV
SDR-21*
& SDR-26*
ABS-DWV
Schedule 40 ABS
Foam/Cellular Core PVC
*In Canada, check the local codes to ensure that
SDR is approved for use. SDR is not approved
for all Canadian installations.
Cement and primer must conform to ATSM
Standard D2564 for PVC and Standard D2235
for ABS. When joining PVC piping to ABS, use
PVC solvent cement. (See procedure specified
in ASTM Standard D3138.)
Vent Pipe Length and Diameter
In order for the furnace to operate properly, the
combustion air and vent piping must not be exces-
sively restrictive. To ensure this use Table 3,
which indicates the maximum allowable piping
length for a furnace of specified input rate, when
installed with piping of selected diameter and
number of elbows. This table applies to the length
and number of elbows for each pipe. To use the
table, the furnace input rate, the centerline length
and the number of elbows on each pipe must be
known. Choose the diameter for which the tabu-
lated length is equal to or greater than required.
Proper use of the table is illustrated by the
following example:
Example:
A 120,000 Btu/h furnace is to be installed in a
"one-pipe" system with 40 feet of vent piping.
There are a total of four long radius 90-degree
elbows used in the vent, including the one
exterior to the building.
Solution:
For this particular installation, the equivalent
vent length must be calculated. This equivalent
vent length will then be compared to the maxi-
mum allowable vent length given in Table 3.
Then, the diameter of the piping can be chosen
for which the equivalent vent length is less than
the maximum allowable vent length. Returning
to our example, we consult Table 3 and deter-
mine that for a 120,000 Btu/h furnace the maxi-
mum vent length for 3" diameter piping is 110
feet. Note that the maximum vent length given
Standard
in Table 3 includes one long radius elbow.
D1785
Therefore, for our example, we have three
D2665
additional long radius elbows for which we must
D2241
add to our piping. Each long radius elbow is
equivalent to 2.5 feet, so we must add 7.5 feet
D2661
to our vent length. Therefore, the equivalent
F628
vent length for our installation is 47.5 feet. We
F891
compare this with the maximum vent length for
3" diameter piping. Our equivalent vent length is
less than the maximum allowable vent length, so
for our "one-pipe" installation we can use 3"
diameter piping.
Condensing furnace combustion products have
very little buoyancy, so Table 3 is to be used
without consideration of any vertical rise in the
piping.
Vent Pipe Installation
Pipe Routing and Support
Route piping as directly as possible between the
furnace and the outdoors and remember that
routing affects pipe size requirements per the
preceding section. If a two pipe system is used,
locate the combustion air intake and the vent
exhaust in the same atmospheric pressure
zone - i.e. both must exit the building though the
same portion of exterior wall or roof. Vent piping
must be sloped upwards not less than 1/4" per
foot in the direction from the furnace to the
terminal. This is to ensure that any condensate
flows back to the furnace (where it can be
disposed of through the condensate disposal
system).
The quality of outdoor air must also be consid-
ered. Be sure that the combustion air intake is
not located near a source of solvent fumes or
other chemicals which can cause corrosion of
the furnace combustion system.
Combustion air must not be drawn from
a corrosive atmosphere.
CAUTION:
!
15
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