equivalent vent length is less than the maximum allowable vent length. Returning to our example, we consult Table 5 and determine that for an 80,000 Btu/h furnace the maximum vent length for 2” diameter piping is 60 feet or for 3” diameter piping is 150 feet. Note that the maximum vent length given in Table 5 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 2” and 3” diameter piping. For both cases, our equivalent vent length is less than the maximum allowable vent length, so for our
Condensing furnace combustion products have very little buoyancy, so Table 5 is to be used without consideration of any vertical rise in the piping.
NOTE: Always use the same or larger size piping for combustion air as is used for the exhaust vent.
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 (See Figure 22). 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 Figure 16.) 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 18 illustrates vent and combustion air pipe sizes exiting the furnace. Transition to the correct pipe size 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 Figures 16. For an upflow furnace installation, if breakable connections are required in the combustion air inlet (if present) and exhaust vent piping, then straight neoprene couplings for 2” or 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.Ensure that the connection is tight and leak free.
NORDYNE condensing furnaces have been certi- fied 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.
Location of Outdoor Terminations
Horizontal Venting
Vent and combustion air intake terminations must be as shown in Figure 19 and 20. Vent termination clearances shall be consistent with the National Fuel Gas Code, ANSI 2223.1/NFPA 54 and/or the CSA B149.1, Natural Gas and Propane Installation Code.
All minimum clearances specified must be maintained to protect building materials from degradation by flue gases.
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