I have a hanging furnace in my garage (natural gas) that is run with single wall vent tubing to the ceiling where it connects to the double wall pipe, runs through the attic and out the roof. I installed the furnace last year and ran it all winter. This winter I have noticed a brown streak running down the single wall pipe, it looks like dirty water runs down and gets dried up from the heat leaving a stain, there is also a small (dried) puddle of this brown substance on top of the cabinet that sits under the heater. I am amusing this is condensation that is forming in the vent line and dripping down? Is this something I need to be concerned about?
