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Community and Q&A

Problems with Multi-Zone Mitsubishi Minisplit

VTBalla34 | Posted in General Questions on

Hey all, first thanks for all of the great discussion on Mitsubishi systems.  GBA is pretty high in search rankings whenever these systems are mentioned and the feedback for the threads I have read over the past several months has been very informative.

We recently purchased a house in southern Virginia (Climate Zone 4A) that has had some inconsistent cooling issues but this heat wave has really brought them to light and exacerbated whatever is going on.

I am hoping the community can offer their expertise to help me get to the bottom of a) why one of my zones overshoots the AC set temp and keeps blowing AC air continuously and b) why one of my zones works only intermittently


The setup details are as follows:
-MXZ-4B36NA outdoor unit
– SEZ-KD15NA indoor units (3 total, each with short supply duct runs)
-MHK1 controller for indoor units (3 total)

One zone, on the main level, supplies a bedroom (with full bath & closet) as well as a half bath and the laundry/mud room (5 total supply vents, all at floor level).  The controller and return are in the bedroom, within 5 feet of one another.

Another zone, also on the main level, supplies the open concept living/dining room and kitchen (4 total supply vents, all at floor level).  For what it is worth, the living/dining room has 30 foot vaulted ceilings that open up to a loft, which is over the kitchen area.  The controller and return are in the living room, within 5 feet of one another.

The final zone is on the upper level and supplies a bedroom, bathroom, and the loft area that overlooks the living/dining room below (3 total supply vents, all installed in the ceiling).  The controller and a return are in the bedroom, within 5 feet of one another.  There is a second return in the loft area.

The equipment for the two main level zones is installed in the basement ceiling (the basement does not have HVAC).  The equipment for the upper level zone is in the attic.


Now with that out of the way, I have two issues.

The first is that the zone for the main level bed/bath/closet/half bath/laundry continuously cools, even after the set point has been reached.  I have it set to 73 and the t-stat usually sits at about 68 in there with 45-50 degree air blowing from the register.  This would be true even when the other zone t-stats are set to 68 and showing 70-74 actual temps and blowing air not nearly as cold.

What could cause this one zone to be so much colder than the others and not turn off?

The second problem is that the other zone on the main floor (for the living/dining/kitchen) only seems to work intermittently.  A good chunk of time the T-Delta is Zero with supply matching return (68-72 range).  This occurs even when the other main level zone is blowing 50ish and set point already reached.

I can sometimes turn the t-stat off and/or turn everything off at the breaker and let it reboot, and have the troublesome zone start blowing cold air, but that is obviously not a good solution.

The upper level zone seems to function mostly fine.  That area just gets hotter than I would like because a) it is higher than the rest of the house and hot air does its thing and b) there are windows in that bedroom that take direct hit from the afternoon/evening sun.  It will usually get to the 68 degree desired temp by the time I go to bed.


So I know this was a long introduction post.  I appreciate anyone that was able to read this far.  I am not the best DIYer out there but I am comfortable with most troubleshooting, running a voltmeter, changing out components in the condenser (changed a couple of start capacitors and fuses myself in past systems I’ve owned).

From my reading, it sounds like I may have a freon level/pressure leak issue that may be causing the system to operate strangely, potentially a defective contactor for the continuously cooling zone, or maybe a defective head unit for one of the two zones preventing proper operation.

Any advice is most appreciated and thanks again for the helpful references.


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  1. VTBalla34 | | #1

    For anyone that might have been interested in this, I had a Mitsubishi Diamond contractor out here last week and we troubleshot the system together since one of his techs wasn't available. We discovered that the two zones on the main level had been cross wired meaning the refrigerant lines and the electrical lines did not track together. We rewired Zone A to Zone B and vice versa at the outdoor unit and that took care of the one zone always being too cold and the other zone not being cold enough.

    Amazing the previous owners of this house managed to live here for almost 10 years without discovering there was an issue (and that the original installer so so inept they couldn't even manage to run electrical cables to the right zones!).

    This didn't solve the entire system problem however as the three zones didn't work very well when all 3 were running. Two of them running would be mostly fine, but still not super cold air. He came back a few days later and ran a diagnostic test that ran each zone concurrently at full blast and added refrigerant until he was getting a 20 degree t-delta across all the coils. This took about 5 pounds of refrigerant before he buttoned it back up.

    No problems since then, even on these 90+ degree days we are having. Hopeful the inept contractor also just didn't charge the system fully at initial install rather than a leak, obviously, but things seem to be good for now.

    1. Patrick_OSullivan | | #2

      The one suspect thing here is that while I'm not an expert, my understanding is that the only way to get a correct charge in a minisplit system that is low (but you don't know how low) is to empty all the refrigerant, and then recharge with the correct amount for the total lineset length.

      If this procedure had been done, it would have provided the chance to pressure test and pull a vacuum, which would have answered the 'why is it low' question one way our another.

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