Upon my arrival I performed a 6 month checkup on the zoned mini split system. Flushed and treated water removal system, washed condenser coil using water, cleaned and treated the electrical cabinet then verified system safety and performance. I found that when I was changing the air filters the one on the second floor did not have a filter in it. Attempting to install a filter I found that then the home was built the builders put the filter grill on the supply air duct and the return side had the supply grill. I made corrections on the system today and advise to have the system chemically cleaned due to the amount of time it was getting unfiltered air on the return side.