Can you post a pic of the furnace, and the venting?
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Can you post a pic of the furnace, and the venting?
I think beenthere's on the right track. I can tell you this from experience with these Ducane furnaces: Use 3" pipe. I know the manual says 2" is acceptable but EVERY one I've seen with the problem you describe has had 2" & NONE of the ones I've encountered with 3" had this problem.
if youve got a vertical run on your exhaust shouldnt you have apower ventor that would take the guess work out of it anyways