When your air conditioner runs during the summer, air blows over the extremely cold cooling coil, through the filters and into your house. When the air moves over the coil, it
… deposits most of its heat into the coil, meaning that it's cooled down when it moves into your house. If you have a problem with your air conditioner, either as a result of poor airflow (usually caused by dirty filters), improperly charged refrigerant or even the outside temperature, the hot air crossing the coil will not be able to deposit enough heat. As a result, the coil stays really cold and the condensation on it freezes. (MORE) 
