Air conditioning units use a coolant fluid that can be changed back and forth from a liquid to a gas to transfer heat from the air to the coolant where it is carried away. The evaporator coil is one of the most important parts of the AC system: it is cooled by the gas, and in turn cools the air as it passes by the evaporator fins. The heat passes through the evaporator coil to the coolant itself, and the cycle continuously repeats as long as the air conditioner is on.
Sometimes air conditioners can produce unpleasant smells and odors as they circulate air. This is caused by a build-up of mold and bacteria on the evaporator fins. These fins, in addition to cooling the air, also attract water vapor from the air, which condenses and drips down the evaporator coil and to a drain. In normal conditions, the leftover moisture evaporates before it can do any harm, but if the moisture remains too long, it becomes a breeding ground molds and foul-smelling bacteria. Air conditioning systems are flushed to remove these odors, and sometimes the fins are coated with a moldicide protectant.
The evaporator coils tend to be naturally cold, and generally hover right around the freezing point of water. If moisture stays on the fins too long and there is something else wrong with the AC system, the water can freeze and begin to coat the blades. This is typically caused by blocked coils or failed expansion valves, and the frost layers can seriously inhibit the performance of the air conditioning system.
In the normal process of operation, the evaporator coil and fins can become coating with normal dust, dirt and lint, which is often attracted by water. These layers of dirt are not as dangerous as frost, but they can still stop the evaporator from performing correctly and inhibit the efficiency of the entire system.
Evaporator coils can be cleaned with a soft cloth. A light-cleaning solution, such as slightly soapy water, can also be useful, especially if there is a light layer of oil or grease on the fins. There are some coats available for automotive evaporators that, in addition to taking care of bacteria problems, are also designed to make the coils dust free, but these may also inhibit performance.
If the evaporator seems to be in good condition, but is not working properly, there is probably a coolant issue. If the coolant lines have a leak, low coolant can lead to many different evaporator problems. Too much coolant can also create cooling problems, since it cannot pass through the coil fast enough.
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