Originally Posted by
Scott Patterson
I would vote on condensation down the flue pipe. But this is only a guess. This is one of the camels that folks strain on but watch out for the gnats they will choke you!
This would not even make my report.
Scott if you run up on this it needs to make a report if it's from the flue gases as it is a defect, flues are not meant to condense on the inside unless it's a condensing furnace.
There are ways to fix these condensation issues that require the addition of a post purge circuit.