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Proper use of the table is illustrated by the
following example:
Example:
A 110,000 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 4.
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 4 and deter-
mine that for a 110,000 Btu/h furnace the maxi-
mum vent length for 3" diameter piping is 100
feet. Note that the maximum vent length given
in Table 4 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 4 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.
!
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. Sup-
ports must be at intervals no greater than five
feet, and at smaller intervals if necessary to
ensure that there are no sagging sections to trap
water. (See Figures 16 and 17.) It is recom-
mended to install couplings along the vent pipe,
on either side of the exterior wall. These cou-
plings may be required by local code.
Figure 18 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.
Straight neoprene couplings are supplied with
the downflow furnaces only. These couplings
are to be installed in the combustion air inlet (if
present) and exhaust vent piping at the furnace
as shown in Figure 16. For an upflow furnace
installation, if breakable connections are re-
quired in the combustion air inlet (if present) and
exhaust vent piping, then straight neoprene
couplings for 3" piping with hose clamps can be
used. These couplings can be ordered through
your local furnace distributor.
To install a coupling, slide the rubber coupling
over the end of the pipe that is attached to the
furnace and secure it with one of the hose
clamps. Then slide the other end of the rubber
coupling onto the other pipe from the vent and
secure the coupling with the second hose clamp.
For the upflow models, the combustion air pipe
can be attached to the furnace. Ensure that the
connection is tight and leak free.
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