I have a Aprilaire 2210 under my upflow furnace. Airflow is 1000 CFM in cooling and 1100/1300 in heating. I have two returns 20x10 and 20x14, feeding into a large plenum under the furnace. The return inlets currently have louvered grilles on them. I just cleaned out the plenum and found a lot of stuff in there - dirt, cobwebs, etc. I'm wondering if I could put filters on the return grilles to reduce the junk that ends up in the plenum, if I used very non-restrictive 1" filters in both of the return grilles. This is sort of a "rock catcher" idea. The Aprilaire seems to do a great job so I'd want to keep that in place.