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Return air plenum on minisplit

raul4817 | Posted in General Questions on

So after going back and forth with my engineer we settle on a solution for my return air side. Just curious if anyone has any advice or suggestions to this solution

I’ve planned for a central return in the foyer that is almost dead center and has the doorways to all 3 bedrooms. 

The idea is to come off the unit return 5″x30″ with a filter rack that increases to 10×30 to accommodate the filter. This would be left open in the condition attic with a simple square or rectangular duct pass through from the space below. Similar to how fujitsu has in the install manual, although this is probably considering the unit is in a closet.

My concern is most have recommended oversizing the filter, but there would be no bends or turns on the return ducting, just a box off a box essentially.  If this is feasible I’d like at least a 1″ filter perhaps 2″  

Fujitsu provides a simple filter for this application but not totally sold on it so this was the solution we came up with.

Will this pass through exchange enough air from the space below? Not just attic air.
 
My designer approved this setup just wanted ask the community here with some real world experiences.

Raul

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Replies

  1. user-2310254 | | #1

    Raul,

    I will give your post a bump to take it back to the top of the Q&A.

  2. raul4817 | | #2

    Thanks Steve,

    Here is a pic of the filters provided by fujitsu. How many have actually utilized these.?

  3. Expert Member
    AKOS TOTH | | #3

    The stock filter is fine if you are using the unit for cooling only. With that, eventually the coil will need cleaning.

    You do want a proper filter.

    Most 2" furnace filters are rated at 0.2"WG at 500FPM. For a mini split you want closer to 0.1" pressure drop, so that is around 250FPM. So your 10x30 filter is good for around 520CFM. This is probably fine for a 9k head. 12x36 might be better. 1" filter is definitely too restrictive.

    1. raul4817 | | #5

      Akos,

      This Unit will be for heating as well.
      So the 1" filter is more restrictive than the 2" ?
      This unit is an 18k btu. with around 554cfm on high.

      I was trying to keep the width of the return to 30" since this is close to what the actual size of the return coming off the unit. With that being said I am flexible on the height. 10"X30" was the plan but I could easily make this 12x30, 14x30, 16x30, pretty much whatever height. My thought process was to keep it simple for the sheet metal fabricators.

      Raul

      1. Expert Member
        AKOS TOTH | | #7

        2" filter is generally 1/2 the pressure loss of a 1" filter for the same flow rate. 4" is even better but hard to get a filter grill that accepts one. If you can fit a filter box with 4" filter, it is the best way to go.

        The simple way to get a return on the mini split is either:
        -Get a reducer made from 30" to 24" and connect to a standard cold air return boot. You can then use either a filter box or a filter grill. There are a lot more "standard" options when you work with 24" size.

        -Get a length of 24x8 rectangular duct, cut it to around 36" length and put end caps on both ends.
        On the 8" side make a cutout for the inlet size of the ducted unit.
        On the bottom 24" wide section make a cutout for the filter grill / filter box bellow.
        This is slightly more restrictive than the first option but doesn't need any custom bending.

        P.S. Check out this for filter size/flow rate. A higher MERV filter will be more restrictive, so make sure to check.

        http://www.dwyer-inst.com/PDF_files/Priced/DF8_cat.pdf

  4. MattJF | | #4

    The simple Fujitsu filters are sufficient in most scenarios for protecting the equipment/coil.

    If you have any aspirations of actually removing particles from the air, you need to go big. It is reasonably debatable if a minisplit should be an air filter, but it certainly can be.

    https://www.phius.org/NAPHC2018/Think%20Little%20Slide%20Deck%20-%20NAPHC%202018.pdf

    Plan for a 2" filter as it is generally the sweet spot for cost/performance. I would also use a filter grille, or at least the frame from one, for mounting the filter. It is easier to get a good perimeter seal with gasket or tape with that configuration than a slide in filter.

  5. raul4817 | | #6

    Matt,

    Thanks for that link. I have ran read that before but it has been months. I did purchase a filter grill 16x25. I will try to use as you said at least the frame from one. I could probably have the sheet metal guy fabricate me a variation of one and add my own gaskets.
    Raul

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