The evaporator coil is located inside or near the air handler where the blower fan is. Evaporator coils are made from copper, steel, or aluminum because these metals conduct heat easily. Most residential AC evaporators consist of tubes bent into U-shapes and set into panels.
The panels are typically positioned in the form of an “A.” These panels are lined with thin pieces of metal known as “fins,” which bring the passing air to be cooled closer to the coils in order to maximize the effect of the refrigerant.
As the air conditioner runs, the compressor pulls cold, low-pressure liquid refrigerant through the tubing in the evaporator coil. Before entering the evaporator coil, the refrigerant passes through the expansion valve. This valve relieves pressure from the liquid refrigerant, which rapidly cools it. The liquid refrigerant leaving the expansion valve is quite cold, which is what allows it to absorb heat from the air.
Because of the way they operate, evaporator and condenser coils both need to be kept clean to perform as intended and reach optimal energy efficiency. A dirty evaporator coil can experience a number of problems, including:
• Impaired heat absorption and cooling capacity
• Higher energy use
• Higher pressures and temperatures
• Frost and ice buildup
Even a fine layer of dust on the evaporator coil reduces its efficiency. The dust acts as an insulator, keeping the heat in and the air away from the cold coils. That means the coil can’t absorb as much heat as it can when clean. Your system will then have to run longer to provide the indoor temperature you want, which means it will use more energy.
Because it isn’t absorbing enough heat, the refrigerant running through a dirty evaporator coil doesn’t warm up as much as it should. This very cold refrigerant causes water vapor in your air to freeze rather than condense into a liquid. Eventually, the whole evaporator coil can frost over.
A layer of frost on your evaporator is never normal. Letting your system run with a frozen evaporator raises the temperature in the compressor and can eventually cause this component to fail. Dust on the evaporator coil, debris on the outdoor condenser unit, a dirty air filter, and a refrigerant leak can all cause the evaporator to freeze. If you can’t pinpoint the problem, contact a heating and cooling technician.
Evaporator coils can also develop tiny pinhole leaks due to corrosion caused by the mixing of moisture from condensation with chemicals commonly found in household air. Oily residue around the evaporator or in the drain pan is a sign your coil is leaky and requires replacement.
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