I Inspected a house today where water was dripping out of fireplace mantel. I has not rained here in over a week. So I go to the attic to see what it is and find that there is water dripping from the furnace flue. Now it is very cold here, like the rest of the county and this flue runs a considerable distance up 3 floors with a few 45’s in it. 2 stories of the flue are in the attic space. How can you control the excessive condensation? Will putting a boster fan in the flue work? Or a bigger induction fan and motror?
This happens a lot when an older inefficient furnace is replaced with a more efficient one. The more efficient one has cooler flue gases. You do have a very long run. Is the flue pipe running up through the chimney? Is it class B gas pipe? If there is no pipe and the furnace is venting into a masonry unlined chimney this is improper. Get the install sheet if you can and see what the max run is.
Any condensation on/in the chimney in the attic area means that the furnace run is too long and is not exhausting properly.
I would recommend a chimney professional evaluate and estimate costs of upgrading furnace flue. Sounds like the flue is not properly sized. Too many elbows.