Lack of Insulation
Not having enough insulation between an indoor air handler and the warm outside air is a common cause of sweating. When the cool air meets the warm air, condensation often occurs on the metal air handler. This is why it is important to properly insulate the space between the inside of the building and the outside where the air conditioner is placed.
Humidity
Humidity is another factor that can cause an air handler to sweat. If the air conditioner is located in a room with significant humidity, it often creates condensation. This is a problem in hot, moist environments during the summer months. Placing a humidifier near the air conditioner usually helps reduce air handler sweating.
Lack of Air Flow
When air handler ducts or filters clog up, they sometimes limits airflow. In turn, this restriction of air can sometimes cause sweating within an air handler. Cleaning the ducts or switching to a new filter will usually remedy the problem.
Indoor Coil Losing Cold Air
Another reason why an air handler sometimes sweats is because the evaporator coil keeps losing cold air. This often happens because a small layer of ice develops over the evaporator. As a result, the temperature within the air handler increases, which causes sweating.