My system has two return ducts, and each has a standard 1" filter behind the intake grille. The unit itself (XV-90) is in a crawlspace and has no filter rack.
Nobody in the house has a dust allergy and we vaccum regularly, so dust getting in the house really isn't an issue. I simply want to ensure that I don't have to purchase another furnace for a VERY long time.
I am getting the impression here that the Filtrete restrict too much (and they are fairly expensive to boot), so that rules those out.
For now, we have one return (upstairs) using PrecisionAire P.S.F. filters. (Fairly dense white spun nylon/polyester/fiberglass, I can't tell exactly what, with a similar texture to a ScotchBrite pad, only looser.) The other (main) return has the cheapo EZ Flow II loose blue fiberglass.
I can't find any guidance other than consumer reports, but it doesn't offer any help for people that just want their unit to run well, and don't care much about dust in the house.
[Edited by sirwired on 02-01-2005 at 12:06 PM]