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Community and Q&A

HVAC Closet for Return Plenum

kermit49 | Posted in Mechanicals on

Ducted mini-split on a 2100 sf one level, zone 3 good enough house.

A vertically mounted Fujitsu 12k mini-split in it’s own “HVAC closet”.
Has supply plenum above it with duct distribution above that.
Mini-split return entry point has a 90 degree return plenum with filter box at entrance. Closet door is short, permitting wall and hallway return grill below door.

After hallway return air passed through grill is it ok to have a separation between the wall grill and entrance to the filter box?  In other words, the closet space acts like a return plenum feeding the filter box not attached to the wall grill.

Thank you.

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  1. richmass62 | | #1

    is this a mini split or an air handler with a powerful fan?

  2. kermit49 | | #2

    Hi. A Fujitsu slim-duct mini-split that can be mounted vertically.

  3. Expert Member
    Akos | | #3

    Once you turn a closet into a return plenum, you need all material in there to be rated for plenum, this pretty much rules out anything wood. I think it is common in some areas to have louvered door as a return but I think it is a code violation.

    I think the simpler solution would be to have plenum run through the wall beside or over the door and put a filter grill on it. If you don't have space beside the door, you can always shorten the door and raise the bottom up. Build a mini wall under the door where the return can now run.

  4. kermit49 | | #4

    Hi Akos, glad you chimed in with good info on code issues and you are correct.

    The closet door is going to be short and be walled off below with a hallway grill.
    I like your simple solution and will plan on isolating the bottom of closet creating an all drywall plenum that gets hallway return from grill. The mini-split return plenum will go straight down and through a large filter box open to the "plenum" below.

    Great suggestion. Thank you.

  5. monkeyman9 | | #5

    Sounds like you did what I did. I made my own custom return plenum that turns 90 deg to the Fujitsu slim duct unit. So I have a box against the wall with the grille and then Triton filter housing attached to that. Then a 90 deg to turn to the Fujitsu unit. Def doable.

  6. kermit49 | | #6

    Thank you MonkeyMan. Your account is re-assuring. I have not done it yet, drawing up all the mini-split and ERV ducting system now. Akos reminded me to make the plenum code compliant.

    Since you are using this, any issues with air flow noise through grill/filter and into the plenum?

    Thanks again Monkeyman.

    1. monkeyman9 | | #7

      The noise is fine. I'd say the return is louder than the supply, but it's not bad. Maybe like a box fan on low is a good way to describe it. I've seen many central AC systems have louder supplies and returns. If you can message me in here somehow, I can email you a few pictures for ideas.

  7. kermit49 | | #8

    Hi Monkeyman. That's good information thanks.
    I am revising my return design now, will need to check if it meets code.

    A shop made return plenum drops down a bit before turning 90 deg facing the wall entry screen.
    The end of plenum houses a vertical filter tray. There is room for vertical filter removal.

    The exposed dirty side of filter faces the screen but away from it.
    Entry air will obviously fill the closet area surrounding the steel plenum and filter.
    I think this will satisfy code. Seems to be the same as fully ducting return from mini-split to wall and drawing from the hallway.

    Will look and see how messaging works here.


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