I had to replace an AC unit and A-coil and decided to upgrade the furnace to a 95% efficiency unit. The intake and exhaust are ran in PVC pipe. Coming out of the furnace is 2" pvc for both. The size of my furnace requires it to be ran with 3" so I used a 2-3" fitting and ran both with 3". The installation manual says to put a short piece of 2" pvc at the outlet as an accelerator for the exhaust. It also says to reduce the intake to 2" to put on a supplied debris screen. Seems confusing to have 2" in and out at the furnace, increase to 3" then reduce back to 2".