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Community and Q&A

Locating a Minisplit Head Near HVAC Ducts

drew74davison | Posted in Mechanicals on


I’m seeking some input/support for a project I’m undertaking.  I have a bonus room above 3 car garage (~312 sf) where I want to add a ductless mini split.  Room has been retrofitted with spray foam insulation (floor, knee wall, ceiling) but is still not comfortable in winter and summer.  I’ve attached a drawing.  My preferred placement of the head unit is on the interior wall (door wall) as it provides the best routing for the line-set (in terms of least bends/turns and least opening of walls/floors to install) to my preferred location for outdoor unit which is rear exterior of house).  Other options would either require lots on bend & turns and/or routing the line-set on front-facing exterior wall(s) of house which is aesthetically undesirable.  On the interior door wall there is a cold air return duct about 7′ high.  My questions are 1) could I mount a wall head where cold air return is and move cold air return down the wall vertically close to floor (I don’t mind that this wall would not be useable for furniture, etc) or 2) could I mount a floor head below the cold air duct.  My concern is loss of heat/cooling from the unit being drawn into cold air return.  Many thanks for any input.

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  1. Jon_R | | #1

    When a supply has proper velocity and direction, it can be placed very close to a return without excessive short-circuiting.

    1. drew74davison | | #4

      Thank you Jon. I presume a new unit would have proper velocity and direction is something that can be adjusted after install.

  2. Expert Member
    Dana Dorsett | | #2

    It should be fine to put the return lower on the wall.

    Some considerations:

    Be sure to run the Manual-J load numbers and don't oversize the ductless, or use a ductless that has both a very low minimum modulation AND high efficiency at minimum speed. Some have crappy COPs at sub-2000 BTU/hr, others are pretty good. eg:

    The Mitsubishi FS09 runs great at <2000 BTU/hr across temperature with COPs north of 3 in heating, north of 4 in cooling:!/product/34427

    LG's LAU/ LAN090HYV3 has a nice comfortable very low minimum modulation but with sub-2 COPs at that speed (sub-1 at minimum speed when at 17F or cooling in heating mode)!/product/31912

    Since there will be some amount of heating/cooling from the ducted system (even with the register closed) think about where the supply register is located. If it's on the far end of the room it's not a problem, but if located near the ductless there may be some unintended feedback causing the ductless to short cycle even when right sized.

    1. drew74davison | | #3

      Dana - Thank you for the guidance. Their are three supply registers in the room (two at mid-point of 26' walls and one under window at exterior wall) nearest supply register would be ~13' from the location of the mini split.

      1. Expert Member
        Dana Dorsett | | #5

        >"...nearest supply register would be ~13' from the location of the mini split."

        Sounds like it's not a problem then. If the supply register louvers can be adjusted to throw toward the far window rather than toward the mini-split it would even help a bit with air mixing in the room when the mini-split is running at minimum blower speed.

        1. drew74davison | | #6

          Dana - one clarifying point: would preference be a wall unit up high and relocate cold air return lower vs floor unit and leave return where it is? I don't have strong aesthetic preference so would default to best solution performance wise. I understand that floor units may be better for heating-dominated climates (house is in Michigan).

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