I see a lot of damaged B-vent pipe while inspecting and I have blogged about this issue before. (B-vent is the typical type of vent pipe used to vent mid-efficiency gas furnaces.) Usually the worst of this damage is limited to the portions of the pipe exposed above the roof. On a recent inspection I found some of the worst deterioration of b-vent that I have ever found. All of this deterioration was in the attic space of the home.
It is also pretty rare for the interior liner of this double-wall type pipe to actually corrode all the way through. But this pipe had at least one area where venting into the attic space was obvious as it was starting to corrode another b-vent pipe that was running above it.
Nearly the entire run of this vent was close to horizontal. I am guessing that there was about 16 feet of horizontal run to its approximately 8 feet of vertical pipe----a combination not suitable for proper drafting of the exhaust gases.
You might ask how this could have ever been installed this way. Simple----form not following function. In this case the "look" of the front side of the home was more important than the safe installation of the b-vent pipe (and thus more important than the safety of the home's occupants). The vent could have gone straight up through the roof above the furnace without any horizontal run of pipe---but it would have been visible from the Front side of the home. When "aesthetics" rules over safety, there is something wrong with the aesthetic in my opinion.
Charles Buell
PS, for those of you that are new to my blog (or for some other "unexplained" reason have never noticed)
all pictures and smiley-face inserts (emoticons) (when I use them) have messages that show up when you point at them with your cursor.
* * * * * * * * * * * * * * * * * * * * * * * * *
Click on the Rose 
to check out: AHA!---A Forum of Landmark Proportions---your Group
PS, for those of you that are new to my blog (or for some other "unexplained" reason have never noticed)
all pictures and smiley-face inserts (emoticons) (when I use them) have messages that show up when you point at them with your cursor.
My WORDLESS WEDNESDAY pictures and some selected POEMS & STORIES.
that is some serious corrosion there, Charles. why would building codes allow this if it is not safe?
WOW! Didn't anyone consider the cost to repair damge like this? Common Sense 101> Veronica
Kara, because building codes are only "minimum" standards all sorts of conditions can happen out there. It is also highly unlikely that this installation meets current standards as it was installed. Whether it was actually inspected 21 years ago is anybody's guess.
Veronica, B-vent is actually fairly cheap---certainly "cost" is no excuse for not maintaining this type of vent. Typical materials cost for the whole configuration would be under $250.00. If the vent went straight up----perhaps as little as $80.00.
Great, clear, pictures. What's causing the corrosion in these particular spots?
Wow .... That is alot of corrosion. Hopefully the new owners decide on the proper installation for replacing this pipe.
Sean Allen
Brian, improper draft leads to exhaust gases condensing in the pipe which corrodes the pipe. As the outer wall fails you then get "hot spots" that assists in the corrosion.
Sean, I hope so as well----I am not holding my breath though----people hate to see these vents terminate where they can be seen from the front. The furnace was shot as well though, so I recommended perhaps a different type of furnace that wouldn't require a b-vent.
Charlie,
I think you can pretty safely call that out without being a deal killer
Steve, you mean not a time for duct tape?
Charlie,
This is one of the few times I have not heard you say: Cleaning, rust inhibitor and paint.
You are right Charles, I do not think that I have ever seen B-Vent damaged that badly on an interior installation. The horizontal runs had to have allowed water to build up in them...
Michael, there could also have been some sort of blockage----like a bee's nest or something.