The house I lived in for 13 yrs and that I just sold had a conditioned crawl space. That means it has zero foundation vents and instead the supply plenum for the furnace and air conditioning that was in the crawl space had two registers on it which blew heated or cooled air into the crawl space. It did not have a return air duct anywhere that I ever found on the various safaris I made into the crawl space during my 13 year tenure in the home. The house we recently bought and moved into also has a conditioned crawl space with no foundation vents. There is one register coming off the main supply plenum and about, I don't know, maybe ten feet from where that supply register is there's a rectangular hole cut into the OSB subfloor which shining a flashlight into it reveals it is the opening for a wall cavity return air passage going all the way up to the attic where it feeds into an insulated flex duct. So this home has an hvac return in the crawl space. The furnace is equipped with a 20x25x5" MERV 11 media filter (snug fit in air box, no bypass leakage) So which one was done "right"? I know that when laying out an HVAC you need to have returns in the system, and so since the crawl space is conditioned, then you'd want a return down there to balance the air flow. But given that it's "crawl space air" (this crawl space = pea gravel w/ plastic sheeting for vapor barrier) do we really want that air *actively* intermixed with the living space's air? I'm looking at you, termite and pest control crawl space treatments. Spray my crawl space and my hvac will pull the fumes right into the home via the return air vent in the crawl space. I suppose that without a return in the crawl space and only a supply register, technically the crawl space air would still go into the home by pressurization, very minute pressurization to be sure but crawl space air *would* draft in through plumbing penetrations in the subfloor, for instance. Which is probably what it did in my other house. So I guess it doesn't matter terrible much.