Can I Put a Filter on a Floor-Mounted Return Register?

Our Answer

I've never seen individual filters used to keep debris from falling into floor-mounted registers. They're typically easy to remove, and the occasional reach down with a vacuum or cloth – or just your hand – should be enough to keep floor registers clean. 
Filters are used exclusively on the main return duct, and that’s usually either at the main return (typically centrally-located in the house) or inside the furnace, on the return side of the blower fan.  Filters remove allergens and germs from the air, perhaps even limiting the amount of dust that makes its way into those floor-mounted registers.