High efficiency furnace intake and exhaust

[Bob Vavricka]

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".

[Paul F Franklin]

Newer furnaces have sensors to monitor the combustion air input and exhaust outlet; they can sense an obstruction in either, but this means the pressure or vacuum in the lines needs to be kept within a certain range. One way to do this is adjust the pipe diameter (and thus resistance to air flow) based on the length of the runs and how many fittings are in the path. Another way is to add restriction at the end of the exhaust line to ensure the right amount of back pressure for the sensor to operate properly. With the power vent water heater I have, the manufacturer supplied several screens with different size and quantity of holes for the end of the exhaust. They provided instructions for selecting the proper screen based on the factors I mentioned earlier. Pipe size also depended on those factors.