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HVAC duct design

ARMANDO COBO | Posted in General Questions on

I just finished designing a 3700 sf / 2 story high-performing house in Dallas, TX. My Manual J calcs tell me I have 28 Kbtuh heat loss and 20 Kbtuh heat gain.

I will like to use a 1- 2ton – 16 SEER variable speed condenser and 1- 95 AFUE furnace, all ducts in the conditioned space running though dedicated chases in 18″ deep floor trusses. Since I want to install minimal duct runs to the supply registers on the first and second floors and blowing air across the room from high in the walls, can I use the wall cavities to install metal runs in them to the supply registers? Is there a better way?

FWIW, if I use the “typical” building enclosure specs most houses are built with, my loads double.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Armando,
    The type of duct you want is called wall stack -- it typically measures 3 1/4 inch by 14 inches. The drawback to using wall stack duct is that it can't handle much airflow (low cfm).

    If you need bigger ducts, you can frame your partitions with 2x6s, or build vertical chases.

  2. Expert Member
    ARMANDO COBO | | #2

    Thank you, Dan. I actually have 2x6 @ 24" oc, so probably we can use a 5 1/4 x 22? I could not find them on Google, so I may need to have them custom made, but costing less than chases and drop down ceilings since I have a ventilated attic.

  3. GBA Editor
    Martin Holladay | | #3

    Armando,
    For cavities in a 2x6 partition, use oval duct. You can get 5x12 oval duct if you want, or larger - up to 5x20. Here is a catalog:
    http://www.mcgillairflow.com/textDocs/swDuct/swDuct_FOduct.htm

    Here is a calculator that helps you determine air flow for oval duct:
    http://www.engineeringtoolbox.com/equivalent-diameter-d_205.html

  4. Expert Member
    ARMANDO COBO | | #4

    This is good information, Thank you. I'm finding the more efficient and high performing houses we design, the harder it gets to do HVAC designs with the current system. I did read a while back that ACCA was developing the guidelines to design high performing systems… I guess they are finally getting it. Now we just need the manufacturers to get the hint!!! ;-))

  5. GBA Editor
    Martin Holladay | | #5

    Armando,
    I know that you are a smart and experienced builder. I'm guessing that there are differences between how people in the Northeast design duct systems and how it's done out West.

    I designed duct systems for a heating and plumbing wholesaler in the 1970s. All of our duct systems were 100% galvanized, and we had a big warehouse full of galvanized ductwork -- rectangular ductwork, round ductwork, wallstack, and oval ductwork.

    My guess is that everyone out West uses flex duct -- which is why you're asking questions about duct fittings that are fairly standard here in Vermont.

  6. Expert Member
    ARMANDO COBO | | #6

    You are so right. The typical system in TX is equipment in the vented attic with flex ducts (ductupus, as Allison calls them). When I was building in the Midwest, we only used metal ducts. NM, AZ, NV & CA are about half & half, but TX is a whole country to itself about everything.... "This is the way we do it in TX". I had the second largest HVAC contractor in DFW tell me that even thought he knew I was doing the right thing; his company was NOT willing to go there. How you like that!!! Kind off like, “do it our way or find someone else”… good business practice.
    I contacted an HVAC manufacturer and the North TX chapter for ACCA about training for our HBA members, and never got a return call from either of them. Funny thing, the HVAC manufacturer in Dallas talked to the HVAC contractor about my call, and they probably decided not to fix the issues since I was probably an HVAC extremist nut job (come to think of it, maybe they are right, eh?).

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