I had a 35-yr old furnace which I replaced with a new heater/AC combo. The installer (with whom I've worked before) tied it into the existing furnace flue in the Laundry Room. Now that I'm running the heat, I'm noticing a yellow, oily liquid dripping from the flue seam. The flue exits the ceiling, then runs up a chase along the exterior of the 2nd floor, exiting out the roof. I went onto the roof, to see if there were any obstructions: there weren't. I did notice, however, that the flue lining appeared to be a fibrous/cementous gray material. I read in an earlier posting that replacing this might fix the problem. My question is: Should I expect that the contractor should have know this and taken care of replacing this in his replacement bid? Or, should I be expected to pay for switching out this liner.

Your new furnace vents with metal or plastic pipe?

New flue is metal, connecting to the old flue at the ceiling, which is the fibrous/cementous something-or-other!

I am having a similar issue. I just had a new hanging furnace installed in my garage 75k BTU with a 4 foot vent exiting the side wall hanginf under my eve. Furnace works great except that it is leaking yellow gue all over my sidewalk and also turning to large yellow chunks of ice and condensating yellow driplets all over the eve. It appears to be acidic in someway as it is eating the paint off my cedar siding. Im thinking about routing the vent furthur out and up with an elbow although it will not resolve the yellow gue that is the issue. My biggest concern beside eating away my house is damage to the new furnace. I checked the vent angle and it drops down as it exists the garage at about 6 or 7 degrees. I think 5 degrees is required. What should I do? Does it need a drop down canister inside or should it vent directly outside with the 4 inch tube provided?