Heating issue with Mitsubishi M-Series Mini Split (mxz-3b30na-1)

[aa7ae]

Hello everyone, I'd appreciate some help in understanding some oddities with my Mitsubishi Mini-Split.

I've tried twice to call Mitsubishi support, but unless you're a contractor, the person you get on the phone is only as good as the remote control, which is essentially useless.

Hence, this forum, which I'm hoping I can get a better understanding of what is going on here and why?

I have a two story 1400 sq. ft. house. One outdoor unit and three indoor units (M-Series 2.5 ton)

Outdoor Unit: mxz-3b30na-1
Downstairs Living space: msz-ge18na-1
Upstair Master Bedroom: msz-ge09na-1
Spare bedroom/office: msz-ge06na-1

Cooling mode in the summer seems to work great. I can control the fan speed, the temperature seems to work as expected, and it's a godsend when it's hot outside.

The problem is in heating mode.

Question #1: I can't control the fan speed. Doesn't do a thing when I try changing it. Is this normal in heat mode? I can do it in cooling mode no problem, turn it as high as I want and blow the papers around the room if I want. In heat mode, I get a beep, but nothing, acts like it's locked out of changes.

Question #2: Is it normal that the downstairs unit always runs the fan low speed in heat mode, but the two upstairs units keep cycling the fan on and off? This seems odd to me and I don't know if this is by design or indicates the main issue I'm having. Some people say it should be running all the time as the downstairs unit, others don't seem to know and the documentation doesn't say a thing about this.

The main issue: No matter how low I set the temperature in my master bedroom at night for heating, it will still cycle the fan on and off, heating the room which it shouldn't be. The temperature will be over 70F even though I want it to be only 65 in there. During the summer in cooling mode, it seems to work as expected. I can set 72F and it will be a solid 72F. So I don't think it's a problem with the thermister in the head unit. The fact the fan cycles on and off is the weird part if it's supposed to constantly run low speed, which is does in cooling mode, but not during heating.

The second issue is downstairs. Once in a while (usually mild weather), the unit seems to overshoot the set point during heating by several degrees. If I have it set to 70, it will crawl up to 75 and act like "runaway"; it doesn't want to stop heating. I have to either lower the temperature several degrees or turn it off for a few minutes.

I decided to install the MHK1 external wireless thermostat accessory to the downstairs unit to see if it would clear up the problem or make it more accurate. The issue of overshooting the set point and going to 75F is still occurring even with the new thermostat. If I knock the temperature down a few notches, seems to clear it up or get it to stop, so I'm not understanding why it's doing that occasionally.

Last year this kind of thing was going on, so I called the contractor that installed it. The sent out a technician that admitted she didn't really know mini-splits very well at all. She had to call Mitsubishi support and step through a bunch of weird procedures with the indoor unit remote to see if it would blink any error codes. No errors. But the tech support at Mitsubishi relayed an explanation to me that I still think is kind of poppycock and doesn't really address the issue. Explanation is, he said that because all three units connect to the same refrigerant line and they don't have completely independent valves, I'm getting something called "blow by". So he's essentially telling me, if one unit is on, the rest are on getting fed hot refrigerant, and that's the reason why my upstairs bedroom my still be heating even though I've turned the thermostat clear down to 55F or whatever the lowest setting is. I don't really accept that explanation for a couple of reason. #1, why would someone advertise a system as having independent room controls if it doesn't work as advertised? #2, why does it matter if hot refrigerant is running in the other unit's as long as the fan doesn't turn on? Isn't it reasonable to have a setting of like 68F downstairs in the living area and 65F in my bedroom and expect it should work?



Even with the downstairs completely off, it still doesn't work like I expect in my bedroom. Looking for second opinion.

So far I've left the master bedroom unit off and just program my backup baseboard electric heater as it works they way I want, but is less energy efficient.

Thanks in advance for your comments and discussion. I'm really hoping to get to the bottom of this, if there is a bottom! Or at least understand how this system is supposed to behave.

[pecmsg]

Get a Diamond Dealer to look at the entire install.

[Hrdworkingacguy]

Like it or not the person on the phone at Mitsubishi support is correct. The valves close down to a minimum position in both modes, it's just more noticeable in heat. The indoor coils become the condenser coil and the compressor has a minimum frequency when your capacity required is low the system will still be rejecting a certain amount of heat to each area. It's just how it works...I would recommend looking up the sequence of operations for your system in the manuals or go to www.mylinkdrive.com and find your model #'s and look at the documentation...The installing contractor should have been trained on how these work and gave you a run down ahead of time...

This is fairly typical operation on a multi-head machine...Wether it's oversized or not is another issue...