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Duct and Plenum – Mini Split

ATXMatt | Posted in General Questions on

6 months since my last question, we are now finally to HVAC. I had a company run a manual J for us and we are going with an 18k BTU ducted Mini – The Fujitsu 18RLFCD. Yes I know the total CFM here is a little higher than the unit runs at.

We are planning rigid round ducts. Am I ok to build the plenums out of Ductboard? the learning curve and availability of sheetmetal seems a little high. 

Also, this is what I was thinking for the plenum and Duct sizing. 

While the closet does share two exterior walls, I am not convinced we need to pump that much air into it, but will for now. Also, not sure we need the second return in the master, but might be nice so we can keep the door closed at night.

Texas – metal building, extreme insulation and very tight, with an ERV.

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  1. Expert Member
    AKOS TOTH | | #1

    The simple way I've made plenums is by taking a ducts of appropriate size (say 8x16) and cap the two ends. You then cut an opening 1.5" smaller than the one on the mini split which leaves you 3/4" of material to bend as a lip. You can use S cleats on the top and bottom to connect to the mini split and screw the sides together. A bit of duct seal mastic and you have a quick, cheap, no leak plenum.

    For return, I've found the best is to convert the unit to bottom intake (some need an adapter plate for this) and mount a filter grill about 1" bellow the bottom of the unit and seal it against it with foam essentially using the bottom of the unit as a manifold. The also gives you a conveniently large service opening to access the blowers if needed.

    It is good to oversize ducts, but with a simple layout like yours you can get closer to 600FPM without causing pressure issues. 6" is more than enough for a 125CFM feed.

    Not sure if a second return in the bedrooms is worth it, more important item is getting a good intake filter as this will keep your ducts and coil clean for many years.

  2. walta100 | | #2

    I have never been convinced that duct board with taped joints will stand the test of time. I say spend the money and have a steel plenum fabricated and install it yourself.

    The only downside of having oversized supply ductwork is the cost when in doubt go bigger.

    I am not convinced return do anything but I tend to leave my doors open.

    Your “bedroom #2” is a death trap if it only has one exit. Consider adding a window or door to the garage.


  3. ATXMatt | | #3

    We've got a tiny kids door from the back corner of the room into the garage, so no stress on bedroom 2. A window to the garage would be strange.

    So I see a lot of rectangular plenums, but about about a wedge shaped plenum? Or a "pentagon"?

    I understand some of duct sizing, but how about plenum sizing and duct flange distance/angle. Is it as critical with a low pressure system, where you don't have as much high speed turbulent air to tame in the plenum.

    On the attached drawings, the larger circle is the stick on flange for the duct flange. I was planning on doing dampers at the plenum.

  4. Expert Member
    AKOS TOTH | | #4

    If the velocity is low, the exact shape of the plenum doesn't matter much. When in doubt, make it big. I've had no issues with ducting even the 0.2" units.

    If your takeoffs go mostly at 90 deg to the flow direction, a V shaped flow splitter in the middle does make a noticeable difference.

    Man D has the coefficients for different takeoffs when they are at 90deg to flow. Just add on the EL to your ducts and see if you actually need to make your plenum better.

    Because it is custom it will cost a bit more, but the pentagon shaped plenum would be ideal in your case, especially if you are trying to keep it compact.

    1. ATXMatt | | #8

      For the Pentagon, does it need to taper down to exact fit the supply side of the unit, or can I keep it rectangular and just center the ~7x30 opening in the 11x36 back panel?

  5. creativedestruction | | #5

    Sorry this is off topic but the second means of egress from Bed 2 shouldn't be through a garage. I realize Texas may not mandate the IRC, but if it did this definitely wouldn't be allowed. It's a death box and a depression box.

  6. Deleted | | #6


  7. ATXMatt | | #7

    Looks like I can add a fire sprinkler to that room and call it good.

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