Question:

Black sticky residue in gas furnace vent pipe.

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I have a gas furnace on my upstairs floor that vents up through the roof. Last winter the sound of the furnace igniting started getting louder and louder (scary booming sound). It would try multiple times to ignite and sometimes shutoff and try again a few minutes later. Noticed that the vent pipe had sticky blackish/brown residue leaking down inside the pipe and would then leak to the outside of the pipe. Heating guy came out, cleaned pipes, furnace, internal fan, and replaced the igniter. We were okay for about a month and then the same problem started again. Heating guy came out AGAIN and said he was stumped -- he'd never seen anything like it. It was the middle of winter and the roof was covered with snow (unsafe to get on the roof for further exploration), so he cleaned it and said to call him in the spring or summer. Again, the cleaning lasted for about a month and we were back to square one again. We had to shut off the upstairs furnace for the rest of the winter. Considering we paid the heating guy $300 for no solution, we decided to try and fix it ourselves in the summer.During the summer we discovered rust in the vent pipe -- it was galvanized metal. Thinking the metal was defective or something (and the black sticky stuff was melted rust), we replaced the entire vent pipe with new galvanized metal and cleaned the furnace again. This winter the furnace has run for 3 months and the booming and ignition problem have started again. We are going to have the shut off the furnace again soon if we don't find a solution. PLEASE, does anyone know what this black sticky residue problem could be? and what we can do to fix it?

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