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If the air blowing from your air conditioning unit starts to smell bad, you may have mold growing in your AC Unit. This should be taken very seriously as the stagnant water can lead to fungi, molds, mildews, and bacteria being distributed through the air and water particles being blown out through the unit. Such growths can lead to illnesses like Legionnaires' disease, named after the previously unknown bacteria that caused illness during the 1976 convention of the American Legion in Philadelphia.
What you will need to clean out your AC Unit:
Remove the filter from the front grill and replace it with a new filter. If the filter is the non-disposable type, soak it for 15 minutes in a sink with a combination of dish detergent and hot water. Rinse the filter well and allow it to dry completely.
Remove the front grill cover from your unit. Check for springs or screws below the front of the unit and gently pull the cover forward while pressing down on the cover.
Remove all the screws attached to the metal cover of air conditioner and lift the cover straight up. Be careful to keep track of all the screws you are removing. An old ice cube tray or muffin pan works well for keeping track of the screws.
Check the fan motor for any oil holes or oil plugs. If the fan has any oil plugs, take caution when removing them as they might have become brittle over time and will break off, blocking the plug. If this happens, remove the broken plug with the tip of your screwdriver. Add a few drops of oil to each fan motor oil hole (do not over-oil as it could cause damage to your AC Unit.)
Clean the front fins using the AC aerosol spray cleaner to remove, dust, dirt, mildew, and stains. The aerosol spray will be able to get in-between the fins to properly remove any mold and dust. Be careful not to bend the fins as they are often made from aluminum or other easily-bendable metal. Because the fins are easily bendable, be sure to use an AC Condenser brush for cleaning the fins. Reshape any flattened or punched in fins, as these encourage mold growth and prevent proper airflow and cooling. The best tool for this task is a Fin Comb which will gently straighten multiple fins in minutes saving you hours of work. Flush fins with warm water (this is where working outside helps!) using the pump action spray bottle. Allow to dry for 15 minutes. Be careful not to spray or spill water onto any electrical parts, as a precaution; cover these parts with a clean rag .If your fins are stained you might need to do clean and flush them again, over a period of several months. Just be sure to unplug the unit every time.
Gently clean the condenser coils in the back of the unit with a rag or brush.
If there is any standing water from the drip pan or base, be sure to drain it.
Gently wipe down the blower fan blades.
Vacuum all surfaces of the AC cover inside and the front and back grill assembly. Be sure to vacuum the air vents or wipe off any dust and debris build up with a rag.
Reassemble any parts you removed from the unit, plug it in and test to make sure it works. You can place the cover over the unit, but do not put the screws back in, you want to make sure everything was properly assembled and aligned or else there might be noise from misalignment.
After you are satisfied that everything is running correct, either reinstall your unit or store it in a cool, dry place. You can set the unit on two pieces of woods to allow for air circulation under the unit and to discourage mold growth. To further protect your unit, use an air conditioner cover.