15
Note: In Canada, vent materials must be certi-
fi ed to the Standard for Type BH Gas Venting
System, ULC S636. Any certifi ed ULC S636
material must not be interchanged with unlisted or
non certifi ed pipes and/or fi ttings. The plastic
components, specifi ed primers and glues of
the certifi ed vent system must not be intermixed
with any other manufacturer’s vent systems or
parts.
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 specifi ed
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
excessively restrictive. To ensure this use Table
3, which indicates the maximum allowable pip-
ing length for a furnace of specifi ed 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 center-
line length and the number of elbows on each
pipe must be known. Choose the diameter for
which the tabulated length is equal to or greater
than required.
Proper use of the table is illustrated by the fol-
lowing 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 maximum
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 determine that
for a 120,000 Btu/h furnace the maximum vent
length for 3” diameter piping is 110 feet. Note
that the maximum vent length given in Table 3
includes one long radius elbow. Therefore, for our
example, we have three additional long radius
elbows for which we must add to our piping. Each
long radius elbow is equivalent to 2.5 feet, so we
must add 7.5 feet to our vent length. Therefore,
the equivalent vent length for our installation is
47.5 feet. We 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 fl ows 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.
CAUTION:
Combustion air must not be drawn
from a corrosive atmosphere.
Piping must be mechanically supported so that its
weight does not bear on the furnace. Supports
must be at intervals no greater than fi ve feet,
and at smaller intervals if necessary to ensure
that there are no sagging sections to trap water.
(See Figures 10 and 11.) It is recommended to
install couplings along the vent pipe, on either
side of the exterior wall. These couplings may
be required by local code.
Figure 12 illustrates vent and combustion air pipe
sizes exiting the furnace. Transition to the correct
pipe size (i.e. from 2” to 3” diameter) must be
done close to the furnace so that the full length
of pipe is of proper size.
