Evaporator coil
The indoor coil that absorbs heat from your air to cool the house.
The evaporator coil sits inside, usually above the furnace or in the air handler. Cold refrigerant runs through it and absorbs heat from the air blowing across it — that's how your home gets cooler. If airflow is restricted (a dirty filter) or refrigerant is low, the coil gets too cold and freezes into a block of ice, and cooling stops. A frozen coil is the classic sign of a clogged filter or a refrigerant leak.