My father-in-law is complaining about a lot of condensation coming thru one of his a/c vents in the bedroom. Above the room (in the attic) is the furnace, so the duct is a short run. The furnace is about 18yrs. old but has been maintained & runs well. All of the vents are open in the house. From inside the attic, there appear to be no leaks around the duct or around the vent itself...why all the condensation suddenly? Do you think there is some kind of obstruction somewhere? Drain line problem?