GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

One Indoor Unit in Multisplit System Not Cooling Air

kyva1929 | Posted in General Questions on


We had a new Mitsubishi multi split system installed early this year with one outdoor unit hooked to 5 indoor units.

Couple weeks ago we were turning on the entire system for the first time (we had a long indoor furnishing work done by ourselves) and found two of the 6k indoor units had problems:

1) One of the 6k unit does not produce cool air
2) while the other one would automatically shut off due to communication problem with the outdoor unit (per troubleshooting guide based on indicator flashing pattern).

The installer sent a tech to troubleshoot and fiddle a bit with the wire and got the unit with communication problem working. The unit that was not producing cool air was not fixed i.e. substantially warmer air compared to other units in cooling mode, and the room in which the unit is located in never reach target tempeature 74F.

The tech could not find any problem with the unit and right now we are at a lost in what to do. I appreciate if anyone with experience with heat pump system could possibly shed some light on the situation.

Thank you!

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Dana Dorsett | | #1

    A crimp/narrowing of the refrigerant line could potentially have that sort of symptom. A refrigerant line that is well beyond the maximum recommended length could too.

    Diagnosing these sort of problems via web forum isn't a very efficient way to go about it. If the installer's techs can't figure it out on their own they need to be going through it in real-time on the phone with Mitsubishi technical support staff.

  2. GBA Editor
    Martin Holladay | | #2

    Kin Chan,
    I agree with Dana. We're not going to be able to diagnose this problem over the internet.

    That said, your HVAC contractor is responsible for providing a system that works, so keep bugging your contractor. It's your contractor's problem, and right now you have a system that (if your report is accurate) isn't working. As Dana said, help from a manufacturer's rep from Mitsubishi may be needed in your case.

  3. kyva1929 | | #3

    Thank you Martin and Dana for your response.

    The contracted hvac companyt sent a tech to service our unit today and to our dismay there turns out to be a serious condensate drain line leakage issue.

    The tech fixed the original problem by properly wiring up the electrical(power) and communication wire. He also adjusted the condensate drain line hidden in line covers, and put insulation tape in certain exposed sections of the refrigerant pipe because we pointed out a slight water leakage problem in the basement where refrigerant pipe enters the basement and connect to branch box. When the tech left all five units were able to produce air at around ~5X Fahrenheit.

    We left the entire system on for a more throughout testing and left the house to shop for new furniture. When we return couple hours later, we found the other three originally working units stop producing cool air, after which we head to basement for investigation to find a dripping wet branch box, and an entire section of rim joist turned wet from water leakage where pipe enters. I am fairly certain that the water had damaged the branch box electronics which render the three units that are connected to the damaged branch box malfunctioning.

    I have sent the contracted hvac company an email stating the problem and demanding that they send an experienced technician to immediately correct the problems.

    I am thinking of first filing a complaint to Mitsubishi just to let them know of the issue since this company is actually a certified diamond dealer. I feel that a company with such low quality of workmanship and service would be producing lots more victims in the coming years. I would love to hear if there are additional resources available to me to get this mess corrected quickly. The house is located in Massachusetts.

    side story:
    We have not had good experience with this company when they were installing the system. Back when they were doing the installation they had installed the branch box in an outdoor location attached to the siding. This decision was approved by their supposedly senior installation manager. I soon found out on a Mitsubishi manual that branch boxes can only be installed indoor. They did not listen until a masscec inspector correctly pointing out the fault.

    Another issue was the installation tech's lack of knowledge in charging the system who claimed that it is not possible to charge the system in winter months due to cold weather, which causes inaccurate measurement of temperature/pressure of the system hence preventing charge being applied correctly. I had pressured them into sending an experienced technician who knew about the method of weighing in (which is supposedly the only proper way to charge a multi split system, if they ever read the installation manual).

    Couple things that I feel that they did correctly though, was purging the refrigerant line with nitrogen, pressure testing the system and using soap to check for leakage. The issues I have mentioned may be a problem with their junior people lacking knowledge and experience.

    I really appreciate if anyone can have additional inputs on this matter and can direct me to additional help.

    Thank you for your time and attention on this matter.

  4. Proteus427 | | #4

    Hi Kyva,

    I’ve got a question for you and anyone else that may have insight on this. I have a similar Mitsubishi split system with an indoor branch box so the 4 interior units.

    Does your system randomly purge or exhaust every few hours. It almost sounds like the entire system pauses, a gush of air gets pushed out of the system and then it slowly fills up and operates normally again. When I say gush of air it almost sounds like a jet engine comes on momentarily and you can hear it through the piping system.

    Thanks. Any thoughts would be appreciated.

  5. walta100 | | #5

    What you are describing maybe the noise the unit make when it is in a defrost cycle.

    A defrost cycle will happen about every 3 to 6 hours of run time depending on the weather conditions.

    What is happening is the indoor and outdoor fans turn off the unit reverses into cooling mode and the compressor operates at max speed taking heat from inside the home and using that heat to warm the outdoor coils enough to melt the frost that had accumulated on coils blocking proper air flow. when the outdoor coil is warm enough the system will return to heating mode and the fans will restart.

    This is a necessary function and part of normal operation.

    Note if your system has a leak, it will be under charged the system will defrosts more often.


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |