Ducane furnace - LED 03 - contractor stumped - Please help

[AAone]

Can you post a pic of the furnace, and the venting?

[GradyWhite244]

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.

[acman727]

if youve got a vertical run on your exhaust shouldnt you have apower ventor that would take the guess work out of it anyways