On hot and humid days, water is often found in and around air conditioner duct and vent systems, especially if a home is kept very cold. Ducts sweat because the outside metal surfaces are below the dew point of the air in their immediate surroundings. Ducts may leak from poor seals, or because algae has built up and clogged the drain line.
The primary methods to stop duct systems from sweating include raising the outside surface temperature of the duct, lowering the dew point of the air surrounding the duct with a dehumidifier, and clearing any algae from drain lines with bleach or by using a CO2 gun to blow the line. If the supply duct is sweating, increased air flow will moderate the cooling coil's temperature and raise the duct surface temperature.
To avoid serious injury or death, disconnect the unit from its power source before beginning repair work.