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New Install of Mitsubishi Heat Pump - Single Zone acting "funny"

EdSheridan Member Posts: 16
Hello all,

I am installing a series of Mitsubishi MXZ heat pumps and I am having a problem, which I am trying to diagnose. Heres the situation:

This is a Multi-Zone (3) MXZ-3C24NA with three identical 6k BTU ceiling cassettes.
The Cassette furthest from the outdoor unit about (46ft) is not cooling properly, the other units are operating without any problems.

After the install, I did two positive pressure nitrogen leak tests one at 320 psi low side operating pressure, and then after speaking with a friend of mine I did a second at 480psi (which is where my nitrogen gauge maxed out) both held for several hours, (I did not do an overnight standing pressure test and I do not have the precision of a digital manifold gauge.)

I did a bubble leak test on all my flare fittings, I found no leaks.

I did a system evacuation with a micron gauge down to 150 microns, but I was not able to get the microns gauge to level off entirely, The microns kept creeping up very very slowly, after about an hour and 30 minutes of slow creep, It reached about 450 microns from 150. I thought that this may be attributed to my evacuation setup which consisted of somewhat awkward arrangement of valve core removal tools and/or the length of total line set or maybe some residual non-condensables, since it it had been wet and humid.

I did a DSB and determined that I needed .42 oz of additional refrigerant based on my line set length, I weighed it in by breaking my vacuum with the refrigerant, then opening the service ports to allow the remaining refrigerant into the lines.

The first evening, after completing this, I let the units run despite the cool temps, to test the units. The Ambient temps were rather low in the 60's. When I came back a few hours later the evaporator coil on the unit giving me problems was iced over.

I know that this occurs when the saturation pressure falls below 32 deg (or right about 100 psi) The outdoor unit PSI on the vapor service port is about right around 150 psi with all three indoor units calling. Since I do not experience this with the other units. This seems to me to indicate that there may be a liquid line restriction? My understanding is that these units rely only EEVs not TXV or Fixed orifice designs. Since these are brand new out of the box, I suspect a problem with my install rather than the unit but what's the best troubleshooting order of operations and how do I determine what's going on here.

After a defrost of this unit overnight I re-ran all units together by pressing all the emergency on switches for cooling. I measured the outdoor vapor line temp at 48.2 degrees, the indoor units are blowing at 51.8 with max fan-speed. The problem unit blows around 75 degrees.

Thanks in advance



  • pecmsg
    pecmsg Member Posts: 4,965
    1st thing tech support will say is "Have you pulled and weighed the charge"?

    You broke the vacuum with virgin refrigerant That opened the service valve?
  • EdSheridan
    EdSheridan Member Posts: 16
    edited June 2023
    I broke the vacuum with virgin refrigerant from a cylinder. I weighed in .6 oz which is about .41 lbs.

    I was able to resolve this yesterday. - The freezing of the coil not in use was due to a wiring error, which was sending refrigerant to a unit which was not calling for it. shut it down, wired s1 to s2 on the problem unit and found it was connected to the wrong position on the outdoor unit. I rewired the connections and it's now resolved. Keep those wires labeled people ;)