If I have a Zoned HVAC system, should there be more than one filter to change?
If the filter you know about is on a duct leading to &quot;the furnace&quot; in the attic, then there must be a filter associated with &quot;the other furnace&quot; in the basement. I've seen them in various locations on/behind/under the furnace itself, and I haven't even dealt with that many hot air furnaces (my area was boiler-centric for heating, and A/C was practically unknown.)
Figure out where the air goes towards the furnace and follow - gotta be there somewhere.
Likely a very understated edge-on view when you are looking for it.