Thanks Dan! But then why is the filter right along the front (inside the
house) grill of the air conditioner? And why is dust forming on this same
grill? It seems that the AC unit is sucking in air from the inside via the
grill then filter?



IOW, the filter rests parallel to the inside face of the A/C unit where the
controls are. It would seem the air would have to be coming from the inside.



Yes you are correct that the air is coming from inside. If there is a
fresh air selector on the a/c it opens a little door in the air path
so fresh air from outside is drawn in to the air stream (about 10%)

Steve B.