Gas Furnace Pipe Coroded?

Sounds to me that the pipe is getting wet a lot. I installed the pipe to my furnance fifteen years ago and made sure that i put a flange with a rubber heat proof boot on the roof which keeps water from running down the pipe. The pipe is fifteen years old but looks new. There is the chance that you got hold of some inferior pipe which rotted. Usually the pipe for furnances is doubled walled which reduces the heat and keeps your roof from catching on fire. It doesn't take alot,so get the best. that way you don't have to worry about it. Hope this info helps.

Check to make sure there is a cap on top of the chimney to prevent water from getting down the chimney pipe. Condensation causes the pipes to rust. Make sure the pipes are clean. Any kind of debri holds the moisture in the pipes

If you replaced the pipe yourself, and didn't include a sediment tube, which collects the sediment and condensation, I would suspect that first. Use black pipe, not galvanized, which is for older water systems.