Why does water vapor drip back down a vent pipe.

Originally Posted by munshin

Out here in De. we are allowed to use Pvc for the High 90% over above. I am gathering that is the type of heater in use. If that is the case, then, there sounds like you have a pitch pblm.

Originally Posted by pcplumber

When we replace an existing FAU, and connect with single wall pipe, and connect to a transite pipe, water will pour backwards into the furnace and flood the controls. Often, there is an oily substance that comes down the pipe. At one time I thought the customer stored a can of motor oil on the furnace and the can leaked. It looked exactly like 3-in-1 oil. Most people think their roof jack is leaking, but these leaks occur when we haven't had rain in over a leak.


The furnace instructions require us to install double wall metal pipe from the outlet of the furnace through the roof.