high efficent furnaces

[Zzz]

Here is an interesting idea. Most companys are using 636 for exhaust, as specified in the code, but using pvc for the intake. If you look at a 636 90* elbow and compare it to a pvc 90*, 636 has a long radius and pvc has a short radius. In my expirience this is taking out pressure switches because, the equivelent length is different by double for each 90*, we have been having trouble with this especially on the new g71 since they claim you can go forever on 2" pipe. Weak pressure switches are very hard to diagnose.

[Admin]

The G71P is a direct vent appliance, so you have to use an intake and exhaust vent.  You're right you can still use ABS as the intake.  You can still buy long and short radius elbows for both 636 and PVC or ABS.  It's recommended to only use the long radius elbows. 

Be careful with Lennox installation manuals.  There have been many revisions regarding the vent length.  Make sure you're refering to the most up to date chart.  The G71P has a variable speed inducer motor, so the vent length can go further then the non variable speed inducer, as found on the G61V.

There are also special termination requirements when installing a direct vent Lennox furnace.  The exhaust vent should be reduced down in size at the termination.  Refer to the installation manual for proper sizing.  This does not apply to non direct vent installations.

For the record I rarely install an intake pipe to a non direct vent appliance, like the G61V.  I prefer to use the existing combustion air pipe, if there is one.  You will never get a no heat call for snow blockage by doing this.  If there is no existing combustion air pipe in the furnace room, then I will install an intake vent.