What Is A Dehumidifier & How Does It Work? A dehumidifier removes moisture from the air. The usual technique used to remove the moisture is to condense the moisture onto a cold surface. A dehumidifier has both its hot and cold coils in the same box. A fan draws the room's air over the cold coil of the air conditioner to condense the moisture (which normally drips into a bucket). The dry air then passes through the hot coil to heat it back up to its original temperature.