With this heat wave, we have received numerous calls regarding condensation on the ducts. The location of the ducts in many cases will dictate the solution. As an example, if your ducts are in an attic, you must have them properly insulated. But the vast majority of homes will have the ducts in the basement or crawl space.
The main culprit for duct condensation is humidity. When the temps are in the 90° F range, that also means the humidity level is very high. When the warmer humid basement air comes in contact with the much colder plenum on the furnace, the net result is sweating and dripping from the sheet metal.
The easiest resolution is to put a commercial grade dehumidifier in the basement and set it to 50% humidity. This product will solve the condensation problem and also improve the comfort level in the entire basement. A 50% humidity level will also prevent mold and mildew growth in your basement.