I have a 24 by 24 inch return air filter in a transition that goes to about 16 inch diameter coiled type duct. The air handling system pulls a lot of air and there is only one intake. The problem I am having is that the suction will pull a new filter towards the round opening and after about a week, there is enough dirt on it to pull the filter firmly against the round opening. When I need to change the filter, I have a perfectly round dirt mark on the filter and the rest of the 24 by 24 inch filter is clean.
There is only a 2 inch clearance between square to round. Should this transition have a longer gradual change in dimensions from square to round? My nephew thinks I need to find a filter that will not collapse so easily. The filters I am using are the economical ones.
My thoughts are your unit needs a LOT more air than the 16" is giving it.I would suggest that you install a second return air filter grille with 16" or remove the 16" and conveert it to 20",or better yet put 20" on both grilles.
I'll take a closer look at the air handler which is in a open space above my ceiling to see what options I'll have.
In my last residence, my furnace filter had two springs diagonally across the filter to keep it in place. I'll look into this a short term fix as I am now busy with other jobs I started. Thanks.