Under Slab PVC Ductwork
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  1. #1
    Join Date
    Oct 2012
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    3

    Under Slab PVC Ductwork

    Any reason why I can't use schedule 40 or schedule 80 PVC pipe for underslab HVAC ductwork? I have found thin wall PVC pipe (Black Max) supposedly manufactured for air duct. It is 1/3 the thickness and weight of schedule 40, and much more costly. Why shouldn't I just use schedule 40? It is thicker, stronger and less expensive. I may ask building inspector if schedule 40 is acceptable to him, but I would first like to know if there are issues about using the schedule 40 that I am unaware of. By the way, slab is high above grade with 2 feet gravel and drainage below so water/moisture in the pipe's environment is not an issue. This duct will be for cooling only. Heating will be by hydronic in slab radiant.

  2. #2
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,279
    How high above soil will the ducts be? How warm does the soil temp get to in summer when you want a/c? Hopefully above 50-55 degrees F.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  3. #3
    Join Date
    Oct 2012
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    3
    18" to 36" above the soil. This is a rather unusual high slab built up on deep gravel base. I think soil/gravel temp will be about 50 degrees F, but not much higher than that.

  4. #4
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,279
    Quote Originally Posted by Rye Builder Joe View Post
    18" to 36" above the soil. This is a rather unusual high slab built up on deep gravel base. I think soil/gravel temp will be about 50 degrees F, but not much higher than that.
    Well, I'm wondering how warm the slab will be where the ducts will reside. If the ground temp never rises above 50 or so, I would want to know if the concrete surrounding the ducts would ever dip below 55 degrees or so. Reason I ask? Possible condensation in ducts. If you put some form of insulation below the slab, then no worries, of course. Otherwise, hmm...
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  5. #5
    Join Date
    Oct 2012
    Posts
    3
    Quote Originally Posted by Shophound View Post
    Well, I'm wondering how warm the slab will be where the ducts will reside. If the ground temp never rises above 50 or so, I would want to know if the concrete surrounding the ducts would ever dip below 55 degrees or so. Reason I ask? Possible condensation in ducts. If you put some form of insulation below the slab, then no worries, of course. Otherwise, hmm...
    It will be insulated; no worry about condensation. My question centers on the type of duct being used. I want to use schedule 40 or 80 PVC. Do you know of any reason why I can't use that material for the ductwork? The reason I ask is that I've found non schedule 40/80 ductwork - brand name Black Max - supposedly specifically made for HVAC air duct, But it is very thin walled, lightweight and expensive. Schedule 40 or 80 seems tougher, better and less expensive, so I am wondering if there is a reason for the thin walled duct. Why else would anyone pay more for a thinner pipe?

  6. #6
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,279
    It may come down to what your AHJ (authority having jurisdiction) says. If he's okay with Schedule 40, then you can go with it. Whether that is the correct choice for moving air for HVAC, I don't know. I would want to know something about its pressure drop properties before burying it beneath concrete forever. Nothing would suck more than to mud all that pipe in and then discover it's horribly restrictive to airflow and your HVAC grossly underperforms.

    May I ask why you want slab ducts vs. ducts in furdowns inside conditioned space?
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  7. #7
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,058
    IMC 603.8 Underground Ducts.
    Ducts shall be approved for underground installations. Metallic ducts not having an approved protective coating shall be completely encased in a minimum of 2 inches of concrete.

    603.8.3 Plastic Ducts and Fittings

    Plastic ducts shall be constructed of PVC having a minimum pipe stiffness of 8 psi at 5 percent deflection when tested in accordance with ASTM D 2412. Plastic duct fittings shall be constructed of either PVC or high density polyethylene. Plastic ducts and fittings shall be utilized in underground installations only. The maximum design temperature for systems utilizing plastic duct and fittings shall be 150 degrees F.

    Yes you can use PVC underground only

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