The evaporator coil is the indoor coil that makes the air in your home cold. It is the partner to the outdoor condenser, and it usually sits right on top of or next to the furnace or air handler. Warm household air blows across it, the coil absorbs the heat, and cooler, drier air returns to your rooms.
It works by evaporation, which is where the name comes from. Cold liquid refrigerant (the fluid that carries heat) flows through the coil's tubing. As warm air passes over it, the refrigerant absorbs that heat and boils into a gas — much like sweat cooling your skin pulls heat away. The coil also wrings water vapor out of the air, which is why a working AC dehumidifies as it cools.
Evaporator coils are a common failure point. They are made of thin copper and aluminum, and a tiny pinhole leak lets refrigerant escape, leaving the system blowing warm air. A leaking coil usually has to be replaced, not patched. Replacement runs about $1,000 to $2,500 while the part is under warranty and roughly $2,500 to $4,500 or more once the warranty has lapsed.
For a buyer, two things matter. When matching a new outdoor unit, the indoor coil should be replaced at the same time so the two are designed to work together — a mismatched coil drags down efficiency. And on an older system, ask whether a refrigerant leak is at the coil, because that repair can cost enough to tip the decision toward replacing the whole system.