I had an very old home that I upgraded in the Dallas area. It used to have a common return in the Den. When we upgraded, we ran flex duct under the house.
However, critters like the flex duct. Turns out, most of the underside is now paved. I could heavy vinyl over what little dirt is still exposed. Just to keep the mosture smell down. Then seal off the house crawl space like any other room and remove the old duct. This in essense would make the entire crawl space a return.
I could even put in a prefilter under the house as getting in the crawl space is easy and the access at that point is about 3 Ft.
Would that work? Could I have a common return under the house that pulls from the floor grates and just eliminate duct?
I am thinking that if I don't seal to tightly then a little bit of fresh air might be a good thing and help keep down dust.