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Black sticky residue in gas furnace vent pipe.

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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  • Danny Clark
    Danny Clark May 11, 2010

    Did you also replace the pipe on the outside, on the roof including the vent cap?


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Unless this is some weird act of G-d, like sap from a nearby tree or something I will stick to furnace facts. LOL. Anyway, this sounds like an incomplete combustion situation. Let me explain. During the combustion process many elements are necessary to facilitate "complete combustion". If the correct combination is not met, the result will be a blackish residue. If you combine this residue with the condensation that normally occurs in the flu system, it could turn muddy or sludgy. This is can be a dangerous situation due to the amount of carbon monoxide (poison gas) nessessary to create this residue (similar to the film in the tailpipe of your car).

I would suggest calling another reputable company that offers true "combustion analysis" reports. The test takes around 30 minutes and they should be able to confirm my diagnosis. This test should cost anywhere between 100 and 200.00(US). Many times a simple adjustment to the gas valve or burner assembly will due the trick. Best of luck!

Posted on Jan 28, 2009

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