Evaporator Coil

How the evaporator coil cools and dehumidifies

Cold, low-pressure refrigerant enters the evaporator coil as a liquid and boils into vapor at a temperature well below room air. As the blower pushes warm household air across the coil's fins, heat moves into the refrigerant and the air leaves cooler. Because the fin surfaces sit below the air's dew point, water vapor also condenses on the coil and drains away through the condensate line — which is why air conditioning dries the air as it cools. In a heat pump running in heating mode, this same coil reverses roles and releases heat instead.

The two classic evaporator coil problems are freezing and fouling. Restricted airflow (dirty filter, closed vents, weak blower) or low refrigerant lets the coil drop below freezing, building ice that blocks airflow entirely. Dust and biological growth on the wet fins reduce heat transfer and can add musty odors to the airstream.

Signs your indoor coil needs attention

• ✓Weak or warm airflow from supply registers while the system runs
• ✓Ice visible on the refrigerant lines or coil cabinet
• ✓Water around the furnace or air handler from an overflowing drain pan
• ✓Musty smell whenever the cooling kicks on
• ✓Cooling bills creeping up without a change in usage

Because the coil sits inside sealed ductwork, cleaning and leak-testing it is a technician's job. Regular filter changes are the single best thing a homeowner in the humid Philadelphia summer climate can do to protect it.