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Community and Q&A

Horizontal ducted minisplit ducting

Jim Sweazey | Posted in Mechanicals on

I plan on using a Mitsubishi 12K BTU horizontal ducted mini split, Model: SEZ-KD12NA4R1.TH to provide both heat and AC to two second floor bedrooms. The unit would be inside the building envelope in a dropped ceiling in a hallway between the bedrooms. I’m having trouble determining what adequate ducting would be.

The Mitsubishi submittal document has graphs for different static pressure that relate to CFM but I struggle to relate this to a duct design.

The duct would need to be 8′ to one bedroom and 6′ in length to the other. I have a lot of horizontal wall space and the unit is just less than 8″ high. I would like to use that height for the duct. The blower maximum dry CFM is 388.

Any advice would be greatly appreciated. I’m in zone 6A.

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Jim,
    Here are some links to provide you information on static pressure calculations:

    Using Total Effective Length in Duct Design

    Help me understand static pressure in ducted minisplit systems

    HVAC pressure design

    I'd like to highlight two points from these links.

    The first is a previous comment that I posted:
    "At a recent conference, I heard Dean Gamble give a presentation on static pressure. Good stuff. Here is a link to the presentation -- there might be some info in there that helps you:
    http://conference2016.resnet.us/data/energymeetings/presentations/RESNET-Understanding-Static-Pressure-2016-03-02.pdf "

    The second is a quote from Isaac Savage in one of the threads:

    "To size your ducts, you'll need to determine the friction rate (FR). To get to the friction rate, you need to know the TEL of your ducts (equivalent length of fittings+linear distance), the ASP (available static pressure) of your blower, and the static drop of the other components in the airflow stream (grilles & filters in this case).

    "So, in this case, you have 0.36 ASP & 115 ft for the TEL. For the components, you can use 0.1 to account for grilles and balancing dampers (assumes ~0.03 for each). For the filter, it'll depend on what type you're using. You'll want to consult the product specific info for it. For the sake of this example, let's say it's going to produce a drop of 0.05" at the max airflow for your unit.

    "You'll first subtract the component drops from your 0.36 (0.36-0.05-0.10 = 0.21), giving you an available static pressure of 0.21". This is the blower power you'll actually have to move air through the ductwork.

    "Next you use this equation: ASP/TEL*100 (0.21 / 115 * 100 = 0.18).
    This is your friction rate - 0.18

    "Now you can use a duct wheel to match up your desired CFM with the FR to determine the required duct size. Be sure your duct wheel is providing you with sizes based on the duct material you plan to use. It'll be different for sheet metal vs flex duct etc."

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