Dripping in Furnace vent

[Gatekeeper]

I have a question concerning my furnace which I hope someone here can help me with...

Our Furnace, located in our basement and fueld by Natural gas has a vent that goes right through the house out of the roof on the second floor.

Now there are many times we here a "drip.. drip.. drip" sound coming from there, and so one day we called a roofing guy to come have a look at our roof.

He told us that the seal around the vent had gone, and so he used this liquid tar sealant stuff and sealed around it. Wonderfull. I thought it was fixed, it seemed like it to me..

But a few months later we noticed it once more.. drip... drip.. drip..

So my Dad and I went up there, and sure enough, the guy did a pretty good job of sealing it. But to be sure, we used our own stuff and sealed everywhere possible.

But still, we hear a drip.. drip.. every now and then. WTF? Where can the water be coming from?

My Dad has a theory though, and I don't know if this holds up.

The vent on the roof has a metal cone type hood on it. He things that when the temperature drops, condensation forms under this hood, and drips back down the pipe...

Is this what is happening? or is there something else I need to look into?
And what can I do to fix it (if anything?)

[HayZee518]

you say the vent goes straight up through the roof. my thoughts are if this vent is galvanized metal, up through the attic, through the roof, through a rubber stack seal and forms a goosebeck at the roofline. Warm air will go through this vent. if the surrounding air is colder than the air going up, some form of condensation will form on the outside of the pipe and where it makes a transition from vertical to horizontal, there will be a dripping - off a screw or rivet or even a joint. If you have access to this "tube" insulate it with a wrap around fiberglass seal. If you do than your problems should be solved.

[Gatekeeper]

Originally Posted by HayZee518

you say the vent goes straight up through the roof. my thoughts are if this vent is galvanized metal, up through the attic, through the roof, through a rubber stack seal and forms a goosebeck at the roofline. Warm air will go through this vent. if the surrounding air is colder than the air going up, some form of condensation will form on the outside of the pipe and where it makes a transition from vertical to horizontal, there will be a dripping - off a screw or rivet or even a joint. If you have access to this "tube" insulate it with a wrap around fiberglass seal. If you do than your problems should be solved.

So (just so i'm getting this right) you suggest that I wrap a fiberglass insulation seal around the metal vent pipe in the attic?

That is the only place I have access to it. The attic, on top of the roof, and a small portion in the furnace room in the basement. The pipe is behind walls in the main floor and the second floor and I have no access at those spots.

[HayZee518]

good a place as any to start. to explain this, in the summer, pour an icy glass of iced tea in a glass - glass, wait a few minutes. notice the water droplets on the outside of the glass? The surrounding air is warmer than the iced drink. Now transfer this warm air into a vent pipe surrounded by cold air. the outside of the pipe will sweat. and the humidity in an attic is a bit higher than the house and is not moving even with gable vents. insulate what you can see. maybe the pipes inside the walls are warm enough that the outside air (within the walls) the pipe won't sweat.