Furnace Vent Issue

In the last couple months I have been called out to look at a leak around a furnace vent at two seperate propertys. That in itself sounds routine but these two were not leaking from the roof itself. Inside the attic the furnace vent pipes connected to another piece of pipe just a few feet below the roof line. The water stains started at that connection. The water was coming from "inside" the pipe itself and getting out at the connection and running down from there. One of them 2 stories running down all the way to the basement before revealing a leak on the garage floor. lol The furnace vent pipes were both double pipes seperated by only about 1/16th of an inch. I took the top off the last one from the roof and it was all rusty on the inside. Not the outside. I bent back the inside pipe and stuck my finger down about two inches between the two pipes and when i pulled it back out there was moist rust on my finger. Im still not 100% sure how the water is getting in there but i put a new top on and hopefully that will stop the problem. Anyone else ran into this before?

I don't have any and if i did i wouldn't know how to post them. lol I went out and bought 3 digital cameras, printers, adapters, etc. and now they are all gone. Never needed any pictures anyways so i just gave up.

Its been a long time, but I seem to remember that there is 3 or 3/10 gallons of water per 1000 cubic ft. of gas. I have a hi-efficiency heater and hot water boiler, and moisture pours out of the PVC flue w/ a positive exhaust that is horizontal(winter and summer.) On gravity flues, this excessive moisture should condense on the first cold surface it finds.

Seenitall you lost me. Its not a chimney just a heater vent exhaust. Copperman you might have something there i dont know becuase he says it only leaks when its raining. Does it only condensate during rain? When the vent is heated up and the rain is cold? Is the customer mistaken about only during a rain? lol I think i will call him and ask him if he put in a new furnace recently! lol

Was there a new furnace installed that is causing the condensation. Some of the high efficiency furnaces require PVC piping because of the condensation they create

Does most of your rain come from one direction? Sometimes if a chimney cap is sloped for the rain to drip off on the side of a prevailing wind, the wind will blow the drips down the pipe and cause it to leak. Have had lots of these over the years. Try sloping the cap to drip away from any prevailing wind direction or make a shield out of some metal flashing and screw onto the cap as a rain deflector.

Hi Guys, These two are strange ones. Woody, the stains on the pipes begin inside the attic at the connection point of the pipes shown in the lower right corner of your picture. There are no stains whatsoever coming from that point up and absolutely nothing at all wrong with the roof. That includes height of the vent pipe and everything else. The base of the flange is in excellent condition as well as the outside of the vent pipe. No rust whatsoever on the outside. Only on the inside. lol And there is no rust on the inside of the inner pipe. The rust is between the two pipes and nowhere else. Also both customers say it only leaks when its raining ruling out condensation. ( for the most part ) Its really strange because in all my years of doing repairs i had never seen this situation before until this year and now i've seen it twice. Im not 100% sure what the flick is going on but im leaning toward the caps being the problem. lol

IMHO nothing more than condensation contingent on tightness of flange to roof. I always thought the Amervents had a double liner w/ at least 1" separation. Could possibly be caused by insufficient flue height, which should normally be above highest point of the roof, in order to exhaust gas properly. If not, backdrafts can occur, and flue gas has considerable amount of moisture within it regardless of the temperature. With a short stack height, backdraft diverters are commercially available.

Could they be sweating?and not leaking.