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Mitsubishi Mini Split Blower Motors Turning Off

b_d | Posted in General Questions on

I had a new house built in 2019 and had a Mitsubishi Mini-Split system installed to use for heating and air conditioning.  The house was completed and I moved in at the end of May 2019.  The system has never worked properly and I have spent the last 2.5 years trying to get the problem resolved with no success.

The system consists of an outdoor compressor unit part number MXZ-3C30NAHZ2 and 3 indoor wall units part number MSZ-GL06NA, part number MSZ-GL12NA  and part number MSZ-GL18NA.

It seems to me that the system may be oversized for my application but I don’t know if this is what may be causing the problem.  My house is only approximately 800 square feet with 8 foot ceiling height.  The house is extremely tight and very well insulated with energy star rated windows.  The GL18 wall unit is located in a living room/kitchen area approximately 500 square feet.  The GL12 wall unit is located in a bedroom approximately 150 square feet.  The GL06 wall unit is located in a bathroom approximately 80 square feet. 

The problem I am having is as follows:  In the AC mode I can run 1, 2 or all 3 wall units and the blowers run continuously regardless of whether the units are calling for cooling or just in standby mode.  The blower speed may vary depending on the fan setting but they never completely turn off unless the wall unit is powered off.  This seems to be working correctly.  When in the heat mode, if I only have 1 wall unit running, it doesn’t matter which one, the blower runs continuously except for when the unit reaches the temperature setpoint.  When the unit is calling for heat the vane is in the wide open position (vane set to auto) and the blower is running at the speed selected either auto or preset speed.  When the unit reaches the temperature setpoint, the blower will turn off completely, the vane will move to an almost closed position, and the blower will restart at a slow speed.  This is what I refer to a standby mode.  The only other time the blower will stop running completely is when the system goes into defrost mode.  The problem happens when I run more than one wall unit at the same time.  They are all set to heat mode.  It doesn’t matter which combination of units I run either 2 or all 3.  If the house is cold and I have all the wall units running on full trying to bring the house up to temperature, the blowers will run continuously.  The problem shows up when the house is at temperature and I am just trying to maintain the set temperature.  The blower motors will randomly shut completely off.  They may remain off for a random amount of time, several seconds to several minutes.  This will happen randomly in any of the wall units.  It doesn’t matter if the wall units are calling for heat (vane wide open) or in standby mode (vane almost closed).  My understanding of the operation of the system is that the blower motors should always run continuously since this is how it circulates air through the system to monitor the room temperature.

Since the system seems to operate normally in AC mode, I did not notice the problem until the 2019 heating season began.  I first contacted the contractor who installed the system.  He came out to check the system.  He agreed with me in that it sounded like the system was not operating properly.   He connected gauges to monitor the refrigerant pressures and monitored the system operation for about an hour and was unable to find anything wrong.

As the weather got colder the problem seemed to get worse and was much more noticeable.  I made several attempts to contact the installation contractor with no success.  I contacted Mitsubishi Customer Service multiple times and spoke to several different people and although everyone agreed with me that this was not normal operation, no one had any idea what the problem could be.  I got through the winter of 2019/2020 with the system operating this way. I only ran the large wall unit to heat the entire house.  I only used the other wall units occasionally if needed. It did keep the house heated but it was not operating properly and the rooms where the wall units were not operating were several degrees cooler than the main part of the house.

The following heating season 2020/2021 when I began having the same problem, I contacted Mitsubishi Customer Service and was told I would need to contact a Diamond Contractor for service.  I contacted A local Diamond Contractor and after several calls back and forth they told me that the problem was related to a recall on the outdoor compressor unit because of a faulty reversing valve.  They said that Mitsubishi was replacing the entire outdoor compressor unit under the recall.  They told me that since they did not install the system they would have to charge me for the difference in the labor charge since Mitsubishi only provided $275 to cover their labor cost for this recall service and the actual labor charge would be much more.  I contacted Mitsubishi customer service to verify this information and was told that this information was false and that there was no recall.  They also said that the problem I was having was not related to the reversing valve.  By this time the 2020/2021 heating season was just about over.  Again I was able to get through another heating season by using only the larger wall unit to heat the entire house.  I would only turn on a second wall unit occasionally if needed.

When the 2021/2022 heating season began and I was still having this problem, I was able to contact the original installer who came out again to check out the system.  He was able to duplicate the problem I have been having, agreed that this was not normal operation, but had no idea what the problem could be.  He contacted someone who he said was a Mitsubishi rep. who told him that it sounded like a faulty control board in the outdoor unit.  After about 3 months, he was finally able to get a new control board which was covered under warranty.  On January 10, 2022 he came out and replaced the control board.  He stayed just long enough to make sure the system was functioning but not long enough to find out if it solved the problem.  When he left he guaranteed me that this would fix the problem.  Within an hour after he left the problem with the blowers randomly shutting off was back.  Absolutely no difference from before.

At this point, I am extremely frustrated with this system.  Everybody agrees that there is a problem with the way the system is operating but nobody can tell me how I can get this resolved.  This has been going on now for 2.5 years.  Way too long.  I need to find out if there actually is a problem and if there is, how can I get it fixed.  Nobody I have talked to has any understanding of how this system is supposed to function and why the blowers would just be randomly cycling on and off.

I would appreciate any help from someone who actually has some understanding of how this system is supposed to function and what the problem could possibly be.

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  1. Expert Member
    Akos | | #1

    Strange, normally the blower does run all the time and you have to specifically configure it to shut off. You can go through the application note bellow and configure it to run all the time:

    P.S. A 30k unit for a house that small is way oversized, a 6k wall mount in bathroom is beyond silly oversized. A 12k for the bedroom unit is probably 4x to 6x oversized as well.

  2. b_d | | #2

    Thanks for the quick response. I was thinking that the system is oversized but this is what the heating contractor recommended and installed. I live in NH and it can get very cold in the winter. This is the reason the contractor said he installed such a large system to make sure it could provide enough heat. Could this oversizing be what's causing the problem with the blowers cycling on and off? The system seems to maintain the temperature within 1 degree of the setpoint at all times. I'm trying to get a basic understanding of how the blower motors are controlled and what would cause them to randomly turn off and on.

    1. Expert Member
      Akos | | #3

      On almost all wall mounts, the blower should be running all the time. It will slow down once setpoint is reached but I haven't see any that turn off unless specifically configured to do so.

      Since you have plenty of operating data, you can go through the calculations here to figure out what your actual heat loss is:

      A multi split will probably be around a COP 2 to 2.5 in your climate.

  3. b_d | | #4

    I know the system is oversized but because it maintains the temperature within 1 degree of the setpoint and doesn't overshoot the setpoint temp, I don't think this is the problem. The jumpers that would enable the blower to turn off once the setpoint temp has been achieved have not been removed. If only one wall unit is powered on, the blower will run continuously. As soon as a second wall unit is powered on, the blowers will randomly turn off and on. It doesn't matter if the unit is calling for heat with the vane wide open or in standby mode with the vane almost closed. This makes no sense. Nobody seems to be able to figure this out. What I need to find out is what controls the blower. Is it dependant on the temperature? What causes the blower to turn off when in defrost mode? I think by getting a better understanding on what actually controls the blower, might give me a clue as to what the problem may be.

    1. Expert Member
      Akos | | #5

      Blower should not randomly turn on/off. I would suspect that you have either a wiring issue or the unit is picking up some EMI interference.

      Wiring is pretty easy for the installer to check, I have seen cases where the wire with a bit of insulation was pushed into the terminal so the screw was clamping on insulation instead of the wire causing intermittent contact. Could also be that one of the wires is simply loose.

      If the wiring was installed before any drywall, it is also possible that an errant screw is partially shorting out the wire. Hope this is not the cause as it is very hard to find.

      Unlikely, but if it is picking up EMI, you can try putting some clamp on ferrite beads on the wiring to each head. Something like this might work:

      A bad ground can also cause a lot of issues. Check the ground going to the outdoor disconnect, at the outdoor unit and at each indoor head, make sure it is actually grounded in each location.

  4. b_d | | #6

    I don't think it's a wiring issue since the units communicate over the AC supply lines and an intermittent wiring issue would also cause power issues which I am not seeing. I don't think it's an EMI issue either since there is no RF sources in the area and I don't have any problem running only one of the wall units. Problem only shows up when running more than one and it doesn't matter which ones. Could it be a low refrigerant charge, not being able to supply more than one unit? What about the power board which powers the compressor? The inverter system should power the compressor at a variable speed depending on demand. If this isn't working properly maybe the compressor just can't keep up when more than one unit is running

    1. Expert Member
      Akos | | #7

      "units communicate over the AC supply lines" Yes and No. The units have a dedicated communication wire that use one of the power lines as a reference. This is why you need a 14-3 run to each head, two power, one comm.

      Low charge almost always causes insufficient output. The indoor head will run but put either no or little heat. Blower on the indoor head should never randomly shut off unless in defrost.

      I doubt it is the outdoor control board as the indoor heads get power directly from the main power feed going to the outdoor unit. The outdoor control board powers the compressor and outdoor fan and communicates with the indoor units through the comm wire I mentioned earlier. The control board inside the indoor unit is what drives the blower and vane motors.

  5. b_d | | #8

    I will check the wiring when I get a chance. If it was a communication problem due to wiring issues, I would think that I would have problems when using the AC in the summer but the blowers run continuously with no problems when in AC mode. There is a separate power board in the outside unit which powers the compressor. I'm thinking that maybe there could be a problem with that since it controls the compressor speed depending on the load. Maybe it just can't keep up when running multiple units.

  6. jwasilko | | #9

    The blowers on the heads will shut off during defrost.

  7. evantful | | #10

    B_D, when you assess the room temperature relative to set temperature, where are you getting the numbers from? Do you have Kumo Cloud adapters for each head or Mitsubishi wired remote thermostats?

  8. b_d | | #11

    I have several thermometers throughout the house to monitor the temperature.

    1. evantful | | #12

      But they are not directly connected to the Mini Split heads, correct?

  9. b_d | | #13

    No. They are not connected to the mini splits. I use the wireless remotes to control the mini splits.

    1. evantful | | #14

      This is the most important clue I believe:

      "If the house is cold and I have all the wall units running on full trying to bring the house up to temperature, the blowers will run continuously. The problem shows up when the house is at temperature and I am just trying to maintain the set temperature. The blower motors will randomly shut completely off. They may remain off for a random amount of time, several seconds to several minutes."

      I suspect nothing is wrong with the unit or is not low on charge. If it was low on charge you would see issues spring up more so trying to get up to temperature (I had this very issue with my units due to the Mitsubishi Diamond Elite Dealer not realizing both my line sets were 27 and 28ft, and anything over 25ft needs additional refrigerant). It's operating fine under load.

      Unfortunately I suspect that the unit the installer spec'ed is so over sized that it's causing what you see.

      I have two 1:1 Mitsubishi units, a FH12NA and FH09NA. These are installed in a 1955, 1100 sqft Ranch, pretty leaky house. R-11 in the walls, maybe r-25-30 in the ceiling. Uninsulated basement with one corner being an at grade walk out.... so pretty mediocre at best. I'm in Climate Zone 6, upstate NY (probably just a few degrees warmer than you in NH). So 21k total between the two units, and they handle my house without issue well into the negative single digits and teens.

      So with your house being 800 sqft and tight, I suspect you may have only needed 15k of capacity maybe even just 12k.

      What you are seeing with your blowers stopping is what I see when I get into March and I forget to turn off my 9K unit as the day warms up. In heating mode, they get above set point and turn off their blowers for a few seconds or minutes at a time. They do not do this in cooling mode, they will run the fan continuously.

      I suspect the temperatures your head units are seeing are different from what your thermometers are reading. I have KumoCloud modules on both my heads which allow me to read what the head unit is getting from it's own temp sensor and normally it's a number of degrees different from what another thermometer says elsewhere in the room or nearby.

      By restricting the output to the one head, you maybe be able to reduce the total CFM output of the system to a level that it can not blow past set point for the one unit. (again what it sees at the head)

      Certainly you have a multi-split unit so that adds a different variable, but I suspect occuam's razor might apply here in regards to the system sizing.

      1. b_d | | #15

        Makes sense. I guess I'll just have to live with it the way it is. That's probably why I have been having such a hard time contacting the original installer. He probably knows he screwed up in sizing the system.
        Thank you all for your help.

        1. PBP1 | | #16

          Unfortunate, I have MXZ-3C30NAHZ2 and 3 indoor SEZ units (ducted) (for Zone 5, 2100+ sq ft, sufficient for below 0 F). On my thermostats, wireless controllers, I see "wait" every once in a while, during which the blower shuts off (I also have them all connected to Honeywell RedLink), things explained in the links below. In cooling, the condensate pump runs all the time, a bit of an annoyance.

          As to wait: "If zones are set to different modes, the first indoor unit to call for heat or cool will take priority. Other zones go into standby, and “Wait” will be displayed on the screen." However, when all three SEZ are in the same mode (e.g., heating) "wait" still appears.

          But maybe not applicable for wall units? Check out the thread?

          Matthew Kelch | May 23, 2019 04:06pm | #11
          I do not see these options as avilable on the MHK1 when used with my MFZ-KJ09NA or MSZ-FH09NA.

          Aun Safe | May 24, 2019 06:13am | #12
          As noted above, those settings are only available for Mitsubishi's ducted units (e.g., PEAD, MVZ, PVA series air handlers).

          Setting 125: When in Heat mode, once set point has been reached, fan 1) stays on, or 2) turns off
          Value: 1 (default) – fan stays ON once set point is reached
          2 – fan turns OFF once set point is reached

          Setting 127: When in Cool mode, once set point has been reached, fan 1) stays on, or 2) turns off
          Value: 1 (default) – fan stays ON once set point is reached
          2 – fan turns OFF once set point is reached

          1. Expert Member
            Akos | | #17

            Could the "wait" happen in any case when the setpoint is satisfied and no head is calling for heat or cool? With an oversized setup, this is likely to happen pretty often.

  10. PBP1 | | #18

    I'll take a closer look, but "wait" does not happen very often and, fortunately, not oversized (HVAC guys have a longstanding relationship with the energy consultant/HERS rater). I think I've seen some instances of "wait" when I manually adjust to above set point while another head is in service/calling for heat. At set up, they explained the 130% sizing limit of total air handlers ratings wrt rating of ASHP and said that the calling/handing off of the individual air handlers would work fine to meet overall demand. The system has been running since March 2018 w/o complaints (CZ 6), only modification was to install a larger sq ft filter box for the 15k air handler. The power usage versus outdoor temperature plots seem fine.

  11. b_d | | #19

    Tried an experiment today. Temp outside was 9 degrees this morning. The 18k wall unit in the living room / kitchen was running all night set to 65 degrees. Temp in room was 64 degrees. Other units off. System was in the middle of running a defrost cycle. Raised temp on 18k unit to 67 degrees. Waited for defrost cycle to finish. 18k system was now supplying plenty of heat. Turned on bedroom 12k wall unit and set temp to 67 degrees, temp in room 63 degrees. Turned on bathroom 6k wall unit and set temp to 67 degrees, temp in room 63 degrees. Both units almost immediately started to supply heat and blowers in all units were running. Vane and blowers in all units set to auto. After about 10 minutes I checked all units. 18k in living room / kitchen vane open blower running supplying plenty of heat. 12k in bedroom vane open blower off inside coils/fins cool to the touch. 6k in bathroom vane open blower off inside coils/fins cool to the touch. Approximate line lengths 18k living room / kitchen 25 feet, 12k bedroom 60 feet, 6k bathroom 35 feet. I know the system is oversized and maybe the blowers were shutting off at times to prevent a temperature overshoot but that was not the case this time. All units should have been running on full heat since the setpoint was 3-4 degrees higher than the actual room temp. This system is driving me crazy. Is there anybody out there that can help me figure out what is wrong? Any help would be greatly appreciated.

  12. PL | | #20

    B_D. It was great to see your post. I too have been going crazy over our Mitsubishi unit that is a MXZ5C42NAHZ manufactured in April 2019. YOU ARE NOT ALONE! I have been going absolutely nuts with this unit that has a distribution module to 5 indoor heads. However, my issue is with the cooling. Not one unit operates consistently, and I have 3 years of emails going back and forth with the installer requesting that it be fixed. This summer 2022, the Mitsu rep came out and ran all the heads on high for about an hour, said there was no problem, left and the units stopped working. I run mine in Dry mode and I could go on for hours, but they have not run correctly since day one. In Dry mode, they are to run continuously at low temps to dehumidify. One does, another cycles, and 3 others I am lucky if they run cool 25% or at all. I am at my wits end with the excuses and visits. I would have never thought Mitsubishi a Japanese company that prides itself on quality would ever have these issues. Let me know if you ever resolve it or at the very least know that you are not the only one. By the way it took three years to find someone online that had a similar issue as mine.

    1. NewMXZ5C42 | | #21

      I don't know if you are still working the issue or have learned more...
      I just got a MXZ 5C42NA3-U1 with 5 indoor units (1 GL24, 1 KL09, 3 KY06).
      There are issues with cooling (GL24 cooling okay - turn on others - GL24 stops cooling) also issues with heating that I'm trying to understand using various tests. The contractor is working with Mitsubishi, they are a Diamond Contractor.

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