Hey guy’s, I believe I know the answer to this this but I want to run it past you to verify. The air handler is in the attic, the return is located in the ceiling on the 2nd floor. The ducts from the air handler is fed into the 2nd and 1st floor apartment. The 1st floor apartment is sealed off from the rest of the home.
I believe the basement apartment should have a filter with a return duct. with out that, you are building positive pressure in the apartment and not getting proper air circulation. Is that correct?