Did your window air conditioner recently stop working efficiently and you opened it up to find signs of corrosion and rust on the coils? Coils are a vital part of the cooling process as the coils taken in refrigerant, chemically convert the refrigerant, and then become cold in order to cool passing air and your room. Problems with the coils can make the unit inefficient or completely nonfunctional.

What are the potential causes of corrosion and rust on the coils of a window air conditioner – and how can the problem be fixed?

Rusty Coils: Overfull Drain Tray

The cooling coils form a condensate that drips down into a tray at the bottom of the window air conditioner. The unit's installation is done at a slight tilt so that gravity can carry the condensate down the tray and out a drain hole. But an excess amount of condensate can overwhelm the tray and leave the bottoms of the coils soaking in water for extended period of times. And these water soaking sessions can cause rust to form.

Excess water can be due to a clogged drain, which is easy enough to check for with a cup of water poured directly over the drain. The water can also happen due to the coils freezing too much, which is typically due to a refrigerant issue. Refrigerant issues most be corrected by an air conditioning repair technician as the system fuel is a potentially dangerous chemical.

The excess water can also be due to a simple installation error. Call in an air conditioning service company to check the installation and to fix the unit's position, if necessary.

Corroding Coils: Salty or Acidic Air

While rust can cause corrosion, straight corrosion happens when the surface metal of the coils simply starts to flake away without any other obvious cosmetic flaws. Corrosion is more concerning than rust since the corrosion means the coils are actually starting to deteriorate.

The most likely cause of corrosion is that there is something in the air outside your home that is too much for your air filter to handle. The common culprits are overly acidic air, perhaps due to some chemical spraying or a fire, or excessive salt, such as living near the beach.

Contact an air conditioning service as soon as you spot corrosion to determine the problem. The tech might suggest a more heavy-duty filter that will adequately filter out the acid or salt. Or your area might simply be hard on window air conditioners and you will either need to do without or commit to replacing the entire unit periodically.

Note that you should buy a new unit if the rust or corrosion is severe. You don't want to risk the coils failing and spilling refrigerant all over the inside of your unit and potentially all over the floor and outside your house.