AHU Filter on return air

[schrodingerscat]

I'm trying to set up a basic air system for a test building and I've put in a fan module, and on return air I've put an AHU filter cartridge. I've got nothing to do with HVAC so I don't understand the systems but I'm told that the filters are meant to be on return air. When I put a duct on the filter however, it automatically sets itself as a supply air duct.

Any ideas what is going on?

EDIT: Sorry if this is in the wrong section, but the only solution I can think of right now is editing the family and changing the duct connector to return air.