I have found that AC systems are really dirty and gross. First an air conditioner sucks up dirt, dust and hair to store it in the ductwork. You need to get ductwork cleaning every 5-7 years. Look up pictures sometimes of old ductwork. It will make your skin crawl how gross it looks. The air filter also catches debris from getting into your system. It can only do so much. The inner workings still need a good scrubbing from time to time. There is more than just dust that is a problem as well. The cooling coil in the system also is a wet place. Where there is wet, mold forms. A lot of people don’t even realize they are blowing air over the mold spores inside their system. It is smart to at least once a year have the machine learn to avoid mold and mildew growth inside of it. Another part is the condensate drain. So much water is flowing here constantly. It is easy for algae to back up and clog the drain. Have you ever seen an air conditioner leaking down the sides, spit out water or be in a puddle of water? This is all due to a condensate drain that is clogged with algae. It is an easy fix, just muck it out. It is really gross but it makes a world of difference. None of the AC cleaning jobs are hard, just icky. If you are willing to get your hands dirty you only need to call an air conditioning business for service once a year.