When our central air is on, the underside of our vents, in the basement condensate. Eventually the water gets into the ductwork in the basement. The ductwork is double layered so the first time it happened I made a 3 to 4 inch slit in the outside layer and eventualy it dried out. I taped it back shut for winter time heating. I use a dehumidifer and this cuts down the condensation alot, but not completely. The basement is actually a cellar under the diningroom and a 4 foot crawlspace under the rest of the house. It is wet in the cellar, but we have a sump pump. If you have any ideas I would really appreciate them!