underground air returns in phx. AZ
Has there been a study done which compares the costs and cooling capabilities of underground air returns as opposed to attic air returns. The company I work for builds very high end homes and we want to install the best possible system for our homeowners. We just have not been able to find a definitive study comparing the two methods. We have talked to several HVAC companies and they all say their system is the way to go but they cannot back up their statement with data; it is what they have always done and they are comfortable with their method. If it ain’t broke, don’t fix it. We would like to base our construction methods on solid, proven, verifiable information.
Remember, this is Phoenix, Arizona, and it is currently 112 degrees outside. Air conditioning is very important
Thanks for your help.
Allen
Replies
There is no definitive study of which I am aware. May want to chek out some of the "building science" site. Both methods, if done correctly, will work well. I have designed systems in which the returns were underground, and though uncommon, worked well, and more commonly, supply ducting run under slab, which also works well.
In any environment, it is best to keep duct and equipment out of unconditioned spaces, like attics. The approach of supplying the conditioned spaces from the floor at the perimeter and returning high from interior walls is a tried and true method ffor both cooling and heating, but is particularly well suited to cooling. Underfloor ducts require some attention to detail that ducts in other locations do not. They have to be sealed well, pitched properly, and provided with access to a low point in case water should be allowed to enter the system.
Thanks for your input. The material used for the underground return is a very heavy pvc, such as used in yard drainage systems currently. Two of the contractors have been doing underground for over 25 years and say they have yet to have a leak or a mold problem. With the rain we have had this past monsoon the media would have picked up on mold in the a/c systems of multi mllion dollar homes. 90 % of our competators are installing underground return air systems and we want to know why. We do not want to follow the crowd just because that's what everybody else is doing. We build slab on grade in the desert and insulate just under the roof sheathing, not at the ceiling. The attic temp. is therefore somewhat more tolerable. The idea is to deliver the conditioned air from about 8 to 10 foot AFF and have it be drawn across the room to the air return in the floor. The air then returns to the system in the coolness of the underground and not the heat of the attic. In the case of 15 tp 20 foot ceilings we have been told to not try and condition this space, but that it actually helps to insulate the room. Their position is why condition this air, no one lives up there. It also appears the contractors that install underground a/r also install rigid ductwork. Do not know if this is coincidental or necessary.In our area, underground is considered high end and above ground tract.
Notwithstanding Tim's advice (and he knows way more than I) I'd caution against underground returns.
First, what material will hold up sufficiently longterm?
Second, how will you guarantee no moisture/water in the system, with the potential of turning the system into a mold factory?
I will note that in my area slab construction is usually low end and underground ducts haven't been done for a few decades, and were in low end houses to start with.
Climate and soils types and conditions will be a big factor, of course.
"First, what material will hold up sufficiently longterm?"
All the old (60's installation) systems around here (northern IL) that I have surveyed used a product called "transite", an asbestos laden fiber cement product, that has held up very well.
In the systems for which I specified the material in the last decade, typically a 4X4 (4 mils inside, 4 mils outside) polyvinyl coated duct, w/ ss screws, painted/sealed and taped joints, installed above the underslab vapor barrier, imbedded in clean sand. Returns have to be insulated. The design conditions that I intend to acheive indoor during the cooling sealing is 75 degF dry bulb and 50% relative humidity. At these conditions, the air has a dew point of 62.5 degF. Underfloor, uninsulated duct at less that 65 deg will condense moisture out of the return air stream. The recommendation is that underfloor ducts be used primarily for supply except in very dry environments (like AZ).
Most of the contractors that I presently work with that install under-slab ductwork, do so with a pvc product, but it is not sch40 or dwv, but a specialized pvc air duct product.
"Second, how will you guarantee no moisture/water in the system, with the potential of turning the system into a mold factory?"
The International Mechanical Code has very specific requirements for material, construction and installation as pertains to underfloor duct systems. All ducts must pitch to a low point with access (to drain as necessary). This intended for the possiblility of spillage and or fire fighting/sprinkler use, however inleakage should be accomodated as well, though very undesirable.
In a smooth, waterproof and pitched duct system, there will (should?) be no mold growth unless it is also growing on the inside of the walls of the house and someone has introduced a good supply of food (dirt, dust, debris) into the ducting. I have never seen mold grow on PVC or steel, unless that material was in direct contact with something growing mold already. Air returning from a resonably conditioned space, should not wet the ducts but it is possible in humid environments. However, elimination of this concern is the reason that underfloor ducts are usually used in supply systems instead of return systems. Should ground water level be of real concern, outside of the 500 year flood, then the underfloor duct system should be avoided.
In the sprawl eminating from Chicago, underfloor supply systems have become very popular because of the fact that the new, cheap, every-corner-is-cut-twice construction does not include basements, crawl spaces or attics of any consequence.
Edited 9/1/2005 2:45 pm ET by Tim
Great info, thanks!
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