Hello,

Have a Furnace in the basement and see that water is dripping down from the furnace aluminum tube that goes to the second floor, don't see any water leaking from the tube that goes to the first floor.

What could be the cause?

Thanks,

Any condensation should be at the evaporator coils at the furnace in the basement and it should be drained to the exterior or other point of discharge. If you are getting water coming from the second floor then there is some other issue, such as un-insulated ductwork, but then the water would be on the outside of the duct.