I recently discovered that my furnaces (Rheems) have reusable "permanent" filters, but they also have a disposable filter tray. Are they intended to both be used simultaneously, or are homeowners expected to choose only one?
The disposable filters are in a slide-out tray that's easy to access and are first to take in air. The washable filter (see photo) comes next after the disposable and requires unscrewing and taking the cover off the furnace. It's not an electronic air cleaner unit, just part of the furnace. I didn't actually discover/notice that there was a reusable filter until a year or two after having bought our house. Incidentally, there was a crazy amount of lint caked onto the reusable filter.
So, does using both filters cause unnecessary strain on the motors, or is this an intended design?