Discovered water pooling out on to floor and dripping from within furnace--seemingly coming from furnace vent... there is quite a bit of rust that has formed which leads me to think its been going on for a while within the furnace but was not noticed till now after heavy rains, and the pooling outside the furnace... after spotting a tiny tear in sealant around furnace vent on roof and had that repaired.. thought that did the trick (was repaired on a rare rain free day here in PDX, OR) Seems that once the rain began again, the puddling and water inside furnace began again. This is a forced air gas furnace "weathermaker8000" I do not have air conditioning or a water heater that is in proximity to furnace. How would one determine if there was a furnace vent obstruction and could that be the cause of leakage.. I'm completely "mistified" due to the rain factor.

If the furnace ran quite a bit on days when it wasn't raining and you didn't have water, you should be able to rule out a blocked vent and condensation. You are satisfied with the repair to the roof around the vent? Do you have an attic? Go up in it when is is raining. Rain could be coming in somewhere else and running down the vent.

Did the vent have a cap that disappeared? There is an alternative system less restrictive than a cap, a slightly larger pipe extending up from the vent, but not sealed to it. You need one or the other to keep rain from falling into the vent.

One way to inspect a vent is to shine a light down it. You should be able to see to the first elbow. Look for both obstructions and for holes. Holes are a serious problem. If the vent has holes rusted in it, you must replace it.

There was no actual rain today, thought threatening to and yes there still was leaking... didn't seem as much though. The vent cap is in place and "looked fine" according to the person who did the sealing. It seems it a long way down to the eblow to inspect for holes... how would one see all the way down the vent? And would it be done from above or below or both? Checked attic, no leakage there. All other seals around chimney and flashings are good.. sure appreciate your response!

You would be surprised how far down you can see with a light. Usually vents are easier to get at from the top. You can also try lowering a light into it.