Mitsubishi Indoor/Outdoor Problem - One room never stops cooling - Problem Solved

SuperJ

pecmsg said:

So what happens when the board fails and a warranty claim is filed?

It's not a warranty issue. It's in Mitsubishi's documentation.
https://nonul.mylinkdrive.com/sfiles/Application Note 3048 ME - How to turn off indoor unit fan when set point is met.pdf

rick25s

pecmsg said:

So what happens when the board fails and a warranty claim is filed?

Since that is the actual solution supported by Mitsubishi Support, and I have a record of the call ticket#, the warranty is still intact, which is also stated in the ticket at my request.

As SuperJ said above, this is the actual fix. When my contractor was told by support that the MHK2 remote would fix the problem, it was expected the tech was to call support at the installation, to be given the jumper information, but he didn't do that. He must have forgotten, since it was almost 3 months later when came back for the install. He just installed the remote, which was just half of the fix that Mitsubishi Support intended.

ratio

Ah, I was bit by that problem on a Mitsubishi VRF install a few years ago. I didn't know mini split heads suffered from it too—good to know.

TAG

I think the problem starts with the setup -- you have 39k of load (max) on a 48K compressor. What's the turn down on that 48k unit ? It can not run and supply less than the what the 6k head wants put out. The head can't modulate. That's why with the multi units you must load the whole building space and match the compressor to the space -- and don't oversize. Then you use the tables and see the actual head outputs and match the compressor ... my guess it would have been much lower.

Remember -- the heads are really never off. They must have bleed in all the lines all the time. The multi work best when sized correctly for the whole house load and none are off -- open spaces obviously the best .. but doors open for AC as much as possible When you start turning them off -- some refrigerant still flows to all the heads .. but the bulk goes to the heads that are on and that is based on the minimum output of the compressor.

I think what was happening -- the compressor was producing enough bleed to over cool the room at that low level ... with the fan never off the flow was enough to overcool the room. You are now turning the fan off -- but the coil is still cold. The wall units are designed to never have the fan off.

We went into the weeds 5 years ago (me and dealer) with an engineer from Mitsubishi when I set up my first multi head hyper. I have since done two more .... as well as many singles.

In fact -- I'm doing 3 singles in my new house vs a multi because of the problems you are having -- the way it will be used will not have all three running and it causes problems for the other heads -- also the multi heads don't have the same outputs ..and I need some in a kitchen. When you need full modulation at any given head the singles have a much wider range

rick25s

TAG said:

I think what was happening -- the compressor was producing enough bleed to over cool the room at that low level ... with the fan never off the flow was enough to overcool the room. You are now turning the fan off -- but the coil is still cold. The wall units are designed to never have the fan off.

You are exactly right, especially what I quoted above. What was kind of a red herring was, that only that one room was having the problem, not there other room with 6k head.....But, the other room is a little bigger, and one more window, so it may have had more loss to absorb the 'extra' cool air coming in when the room was at temperature.