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Mitsubishi 8-Zone M Series, Climate 2B or HOT

CK1105ATX | Posted in General Questions on

Hello, long time stalker, first time poster.

I am in Central Texas and have an 8 zone Mitsubishi, MXZ-B48 Outdoor, 3&5 port branch boxes, all GE wall mount indoor units 15/12/12/6/6/6/6/6.

When first installed everything was 90% amazing.  The rooms that were cooled by this system were cold and dry.  The condensate lines would pour out water.  The only issue we had at the time was that the bathrooms and hallways were hot, especially right after showering.  I installed 6″ jumper ducts with inline fans but it still was very uncomfortable in these areas.  My contractor(father in law) decided we should put in a central system to handle the bathrooms, hallways, and the main living area.  A 2 ton heat pump system was installed, and it works great.  

Going into the 3rd year with the system, we began noticing that the smaller bedrooms(6K heads) we very humid, and the 2 rooms with the 12K heads were not getting to their setpoint.  

My father in law is a 40 year HVAC guy, but is not incredibly versed in mini’s.

This is what we have done so far while working with Mitsubishi Tech support.  We have not seen any positive results and are getting frustrated.  

1. Recovered all refrigerant.
2. Cleaned all coils/blower wheels/drains/drip pans
3. Pressure test w/ bubbles and sniffer to 550, no leaks
4. Pulled Vac and triple evacuated 
5. Weighed in exact amount of  Virgin 410A required

System is still not cooling well but is doing some funny stuff.  When the indoor units are set to COOL and the setpoint is pushed all the way down to 59, I am only getting a deltaT around 13.  When I push the button on the indoor unit to place it into emergency cool I get a deltaT around 16, when I use dip switches on the outdoor unit placing it in test run, I get a deltaT of 21.  This is the same on all the indoor heads.  

Mitsu Tech support asked me to go through the entire system by flipping DIP switches to see if all the wiring was correct/if one head was cooling when it shouldn’t/essentially checking to see if an LEV was stuck.  They warranty replaced the main board on the outdoor unit and this did nothing to remedy the situation.  

Does anyone have any ideas what could be happening, outside the box thinking is appreciated.  Very frustrating given how amazing the system worked the first 2 years.  

Also I have had 2 “Diamond Dealers” on the property and quickly realized that I knew far more than they did, one was literally googling on his phone.  He just started dumping 410 into it a pound at a time(part of the reason I started from scratch) The other admitted to me that he was the senior tech and only had training on minis with a 1:1 ratio and not multi-splits. Problem is these are the only 2 diamond dealers in the area.

Any help is greatly appreciated!!  Thank you for reading.

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  1. kjmass1 | | #1

    How big is the house? 6 tons of cooling is a lot. Single floor ranch?

    I'd guess the additional heat pump is taking the brunt of the load, so the bedroom units are cycling/way oversized and not dehumidifying. I'd start with turning off certain heads so the units that are running, are running 24x7.

    Not sure why the 12k head can't hit it's set point. A single 12K head on the second floor of my 1900sf home in MA can cool the entire house almost by itself. During a week long 95-100s heat wave I'll turn on an additional 7k.

  2. joshdurston | | #2

    What is the indoor entering air temperature at the delta's you mentioned? VRF systems are not like conventional HVAC. The oudoor unit modulates the fan and compressors to maintain refrigerant pressures, the electronic expansion valve will modulate to maintain a target superheat, which means if the entering air temp is low the temp delta will be low as well and vice versa.

    The commercial LG systems that I'm most familiar with, reset the suction pressure based on outdoor conditions, basically giving you colder cools when it's hottest outside, as it cools off the suction pressure rises for efficiency.

    Perhaps there is an issue with your outdoor temp sensor? Might be worth metering out the resistance to confirm it is accurate.

    Also, has the been work on the house that could affect inflitration/pressurization? Fireplace, rangehood, bathroom exhaust, for the ducted heat pump unit you installed, does it have properly located and sized returns. Does the problem exist if you shut off the ducted heat pump?

    Confirm condensate lines/pumps are clear and functional.

    Check the room to room pressure delta's with the heat pump fan running. You should be in single digit Pascals.

  3. CK1105ATX | | #3

    Thank you for the response. The house is a single floor about 2000sqft, I should have mentioned that when we have been testing the system with Mitsu Tech Support, the central system has been off.

    Attached is an outline, showing the system. Sorry it looks like a school child made it but I threw it together this morning for this post.

    What is strange is that when I put both 12K units into Test run on the indoor units, the DeltaT never gets better than 16 degrees regardless of load.

    We no longer turn on the 15K in the great room or the 6K in the kitchen, which leaves us with 12, 12, 6, 6, 6, 6 which equals 48K(exactly matching the outdoor). The 6Ks in the small rooms used to cool the rooms to 69 degrees in about 30 minutes and hold that temp all weekend. Now they take hours to get to 69 but the humidity in the room is over 70%, once setpoint is reached the deltaT is 7 degrees. I checked deltaT on another install with the exact same components and the deltaT was 30 once the setpoint was reached.

    Sorry if my rambling is hard to keep up with.

  4. kjmass1 | | #4

    Ideally maybe you add another register to the small bedroom bottom right, use those 2 6Ks to swap the 12Ks in the family and gaming room (probably oversized anyways)? Lose the kitchen 6K and then you'd have 4 6Ks to be able to downsize to a 2 ton outdoor unit? What's the insulation like?

    1. CK1105ATX | | #6

      KJMASS, Insulation is R15 in walls and ceilings. Problem with reworking the system is that Mitsu has moved from the M-Series MXZ-B to the new MXZ-C product line. Which is not compatible with my indoor units.

      Also, I am trying to find out what happened. The system was incredible for the first 2 years.

      1. kjmass1 | | #8

        Gotcha, makes sense. Sounds like a refrigerant issue but with no leaks and the appropriate 410A amount you should be able to cross that off. I'm assuming you measured all the linesets, rise etc.

        If you are still under warranty keep trying to escalate your case.

  5. Peter Yost | | #5

    Hi CK -

    Sure seems to me as though you need whole house dehumidification; you can't rely on your mini-splits alone to manage the latent load. But hey, I am not an HVAC guy so I have asked my contact at Mitsubishi to weigh in--stay tuned.


    1. CK1105ATX | | #7

      Peter Yost, I have looked into whole house dehumidification. Might consider it at some point. I just dont understand why the system is not working the way it did for the first 2 years. Mitsu tech support and the tech support at the supply house both agree that there is something very wrong. However nobody can pinpoint what the problem is.

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