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MXZ-8C48NAHZ – over shoot heat and cool in shoulder seasons

Mark B | Posted in Mechanicals on

My HVAC system was installed by Doc Rusk in Cincinnati, OH, which was a diamond Mitsubishi contractor when they designed and installed the equipment.  Due to Mitsubishi being purchased by Ingersoll Rand / Crane, Doc Rusk dropped the Mitsubishi line all together so they could keep their Carrier business.  They say there is no way around the problem identified below, and they have now gone incommunicado due to dropping the Mitsubishi line (although they kept the $60,000 I paid for this system).

But first, my equipment:

  – I have a MXZ-8C48NAHZ outdoor unit, with two indoor branch boxes: PAC-MKA51BC
  – branch box 1 is connected to: 3 – MSZ-FH09NA wall units
  – branch box 2 is connected to: 1 – MSZ-FH09NA wall unit; 2 – MSZ-FH06NA wall units; and 1 – NVZ-A18AA7 air handler

The MSZ-FH06NA and MSZ-FH09NA wall units all use the normal (simple) MSZ-FH series remotes (this means each wall unit’s fan runs 24/7), while the NVZ-A18AA7 uses the PAR-33MAA wall remote set so the fan (and the unit) modulates on and off as needed.

We selected this system so we can completely shut off various rooms in our house.  For instance, on branch box 1, one or two MSZ-FH09NA wall units are frequently turned off.  On branch box 2, both MSZ-FH06NA wall units are virtually always off.

In the fall of each year, the room with the branch 2 wall units over cool, and in the spring of each year, they over heat.  We previously had the air handler set to continuous run, but that resulted in the hallway and bathrooms supplied by that unit over cooling or over heating.

The outdoor unit is really two separate units stacked on top of each other.  If any indoor unit is calling for heat or cool, both outdoor units’ fans are rotating.  The HVAC contractor says due to head pressure considerations, branch box 2 will keep enough indoor units accepting Freon so that the outdoor units’ combined  head pressure doesn’t get too high.  They say this is what causes the over heat and over cool problems.

It seems to me that it would make more sense, when very little cooling or heating is needed, for just one outdoor unit to run versus both units.  That would cut the head pressure consideration in half.  Alternatively, if that can’t be accomplished, why can’t the head pressure be dissipated by running the indoor unit that is calling for cool or heat as needed, but then dissipating the remaining head pressure by slightly opening the Freon flow to both branch boxes and all remaining indoor units?

Also, if anyone knows of another Mitsubishi contractor in the Cincinnati area who is sophisticated with respect to Mitsubishi multi-head installations, I would appreciate any recommendations, as Doc Rusk has dropped us like a hot potato.

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Replies

  1. Expert Member
    Akos | | #1

    The outdoor unit does have two fans, but it is still a single refrigerant system. This means you are limited to the modulation range of the unit.

    If you look at the specs your unit:
    https://ashp.neep.org/#!/product/31983

    The minimum capacity in warmer weather is between 1 ton to 2 ton, this means that when your smaller heads are calling for heat or cooling, the excess refrigerant needs to go somewhere and this is typically bypassed through the rest of the units, causing the issues you discribe.

    This is a common problem with multi split installations especially when they are grossly oversized. Since it takes a very large or an extremely leaky house to need the full 54000BTU capacity of this unit, I would guess this is probably at least 2x oversized for your house, which makes the problem even worse.

    The one thing you can help is configure the units to fan off once they hit their setpoint. This way you get less cooling and heating through the zones where refrigerant is bypassed through.

    http://ld3.melsup.com/sfiles/Application%20Note%203048%20ME%20-%20How%20to%20turn%20off%20indoor%20unit%20fan%20when%20set%20point%20is%20met.pdf

    You might also want to get an HVAC tech to check that the zones are wired up properly. Sometimes the installers mix up branch box connections that can create similar problems:

    https://www.greenbuildingadvisor.com/article/184112

  2. Mark B | | #2

    Thank you for your analysis.

    Our house is a new local LEED 4 platinum house. The part of the house that uses this system is 3,950 sq. ft. We are in climate Zone 6. Our house has less than 1 air turnover per hour (with the ERVs off). We have never had a utility bill higher than $300 per month (the total house is 7,800 sq. ft. with 10’ ceilings and our electric is $.12 per kWh).

    The problem occurs when the outside temperature drops below about 82F. When we turn off the rooms that use the Mitsubishi system, that part of the house is about 3,000 sq. ft. (which includes the 550 sq. ft. safe room which is completely below grade). Given your comments, the 1-ton minimum cooling must be too much cooling when the rooms are turned off and the outside temperature is under 82F.

    Of course, since the rest of the house has an equal sized traditional Carrier high efficiency system, it is also likely oversized. Uugghh!

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