Return Air in [HVAC] Conditioned Crawl Space?

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Jul 2, 2007
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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. smile
 
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I've been listening to a lot of home improvement podcasts and the thing now is to completely encapsulate the crawl space. Plastic on the ground, closed cell foam on the foundation walls etc. Do you have good drainage with a sump system?
 
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I was in the heating and air business for 10 years and installed duct systems in Tennessee and Florida. All ductwork and return ducts were either flex lines or duct board and or metal duct..we never ever ran supply air to a crawl space to promote fresh air--Never...we would have been laughed out of town, Neither TVA or Florida power would have approved that method...this must have been some homeowners idea or something weird.
 

LoneRanger

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CourierDriver: No, it's actually built to code. The setup as it stands was designed and installed that way by the HVAC contractor for the initial build of the house in 2003. All ductwork is either FAT or traditional metal (main supply plenum traditional sheet metal type). Only the mentioned rough return intended to pickup crawl space air, is a wall cavity type. When you look up into it from crawl space, it's direct vertical run straight up to a FAT return duct inlet up in attic, the passage way is the interior of a sheet rock (drywall) wall between two 16" OC studs. dlundblad: Yes, sort of. There is a sump pit with HDPE sump liner (similar to a trash can) and a sump pump mounted in it, and it pumps to what looks like a 2" pvc pipe which exits through foundation block wall to wherever (likely a pop-up) in the yard or possibly to street. As for drainage, while grading on the exterior looks okay and roof gutters are clear, there is some minor water incursion into the crawl space with minor puddling on top of vapor barrier plastic in one area at foundation wall. Believe has to be due to negative grade on exterior side even though it "looks" okay by eyeball. Will pull off landscape rocks and add some fill and compact it to restore positive grade if needs it. Linctex: Loam and clay mix, think it's called Miami soil.
 
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Only thing I can offer is there is a "plenum rated" network cable, costlier than regular network cable. When a fire happens you don't want to be overcome with toxic smoke. In reality, if the fire starts down there you're done by the fire. I think the network cable has been traditionally run in office buildings, above-floor or in vertical spaces. Smoke could get you where fire might not.
 
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