When I had my Mitsubishi systems installed, you could not have external thermostats with the high-wall units. They can not be retrofitted. I understand that on the current systems, you can install a separate thermostat at any time. I don't know when the break point was and they are expensive. You need a thermostat and an interface module. I've seen them for sale on ebay at times, but I found out from the sellers that they won't work for me after some dialog including giving them my serial numbers. Apparently, the models designations did not change, but serial numbers enabled them to determine if mine were compatible.

Some of the Mitsu systems can be had with IR sensors that blow toward hot or cold spots.

Ionized, thanks. Maybe I'm not finding what you're describing. I looked this up on the web, and the only gadget I could find was Mitsubishi's MHK1 wired remote. From what I can tell, that's not really a thermostat, it's a wireless remote that attaches to the wall and communicates though an interface box that wires to the indoor unit.

You can see the MHK1 "wired" remote described in the pdf linked below. It's not at all clear that it has its own temperature sensor. Anyone ever taken one apart to look?

BTW, the pdf below also confirms that the high-wall evaporator unit is supposed to be reading the return air temperature. If mine is doing that, it's ignoring it.

These are supposed to be the Lexus of splits. Mine sure cost like one!

I will say that this evaporator's fan is by far the quietest one I've ever run across (something I was looking for). However, a 15 degree swing in room temperature regulation is just not the kind of performance I expected from a high-end split.

I used Samsung splits in South Korea 7-10 years ago, and they worked FAR better than this. And if I understood one Korean's comments right, the cost over there was less than a quarter of what I paid for my unit, so it's even more maddening.

Is it possible that the temperature sensor in mine is bad? I found only one other report on the web from someone whose Mr. Slim's internal temperature sensor doesn't do so well, and it was a pretty mild protest.

Anyone else here have a Mr. Slim mini split? Does yours hold a reasonably consistent room temperature when outside temperatures rise?

Anyone here have one of the MHK1 remotes? Do you know for sure that it has its own temperature sensor?

Thanks again to anyone with ideas.

Here is a link that might be useful: Mitsubishi M-series Contractor Guide

I think I found what you're thinking of, the PAR21MAAUJ remote. It has a setting to use the evaporator's sensor, the remote's sensor, or both.

Here's the installation manual at a dealer:

http://ecomfort.com/PDF_files/Mitsubishi/mitsubishi_par-21maau-j_installation.pdf

Which doesn't tell you how to connect it to the evaporator!

According to this page however

https://www.acwholesalers.com/Mitsubishi/PAR-21MAAU-J-Deluxe-MA-remote-controller-M-Series-Requires-MAC-397IF-E/29954.ac

http://tinyurl.com/mqtyatb

You have to connect it through the MAC-397IF-E or MAC-333IFT-E interface, which apparently plugs in to the logic board inside the high wall evaporator unit.

Here's the MAC-397IF-E manual:

http://www.mitsubishielectric.com.au/assets/LEG/SG79Y415H05.pdf

Total cost of thermostat and interface is almost $550! Mr. Slim already cost more like Mr. Husky, and this is pushing him toward Mr. Tubby.

The $100 window rattler I used before had a simple mechanical thermostat, and it did a much better job of maintaining a consistent room temperature than this one does. Seems to me I shouldn't have to spend another ~20% to get reasonable performance from this split.

Here is a link that might be useful: Mitsubishi PAR21MAAUJ wired remote

Yes, that is it, but I was told that it won't work with my equipment.

Since your installer let you down, do you think it is time to get in touch with the US office? In order to do that, you probably need to give the installer every opportunity to make it right.

I have seven indoor units hooked to three outdoor. I've had them for two full cooling seasons. Only one of mine is on an exterior wall. It is in the utility room and gets the least use so I can't really say if I would have noticed the problems that you have. Recently, due to some work in my normal exercise area, I have been working out in there, but cooking consists of turning the sucker on before I go in, warming up with some stretching and then cranking it down while I am on the machine. I shut it off again as soon as I am done. I am not looking for a stable temp. I can't believe that your observations are normal. I do not have that kind of issue with my other units mounted on interior walls.

Being installed on an outside uninsulated wall is probably, or at least may be contributing to the temp swings. The temp sensing device is in the evaporator. It is possible that it is a bad thermistor and that is a simple thing to check...once you are into the board and can disconnect the sensor from the board and read the kohm resistance on the sensor. Mitsu has the readings or values for the sensor. It will likely take very fine wire test leads to get into the back of the thermocouple. Your dealer should be able to do this for you. Once you have the resistance value (temperature) you can compare it to the air temp surrounding the unit. You may want to consider buying a few cans of the right foam insulation and insulate behind the unit. It must sense only the interior air temp. Good luck

...and what size is the unit? How large is the space? What are the minimum and maximum input values on that model? I'm assuming it is an inverter (wvariable speed) heat pump

Those are large units. Are the spaces that big or do you have very poor air sealing and insulation? I presume that your equipment was chosen after heat transfer calculations were done.

If your equipment is well matched to your heat transfer, you should expect a space to take some time to cool from ambient. Variable-speed equipment can be mismatched to bring the temperature down quickly and still give good humidity control when the desired temperature is reached. Beware running the fan on high manually. The variable speed fan control is necessary to control humidity properly. It needs to slow as set point is approached.

This makes no sense,"Tech says that's how these new energy efficient units run. He says you gotta run them on max to start with or before the house heats up during the day or close the doors down the hallway to trap the cooler air in the living room".

I don't know why you would not expect it to take a couple of hours to cool down so I am puzzled by this complaint, If you are not home to have it cool in time, that is what the timer function is for:

"But - NO! We are running both of these units on MAX 59 degrees on a high fan to cool either room which takes a couple of hours."

"My electric bill is slightly higher " Have you corrected for cooling degree days?

tcnascar,
Here is my story which may be helpful for you.

A few years ago we had 2 outdoor and 5 indoor Fujitsu units installed. The were wonderful and cooled like crazy. Basically the room would instantaneously feel cool because of the cold air falling around you. I was really pleased because I expected to have to precool the room and that turned out not to be the case.

This year after running them a few times in the spring I was very disappointed in the cooling. I was kind of mad too. They cost a fortune and they only cooled a couple of years before needing servicing.

I had the installation company out to look at them. One they found a small leak on the exterior line so they fixed that and recharged. The other was fine. They checked the pressure on both. They measured the temp at all the indoor units and it was ice cold. But still the house was not getting very cool. I was disappointed.

We were also having light dimming problems all through the winter. My neighbors had our power company out 3 or 4 times to look at it. We measured the voltage coming into the house and it was low as well as varying.

One day the power company showed up at our door to let us know they were replacing the switches at our pedestal because of all the complaints about the lights dimming and the power would be out.

Low and behold, the very next day the house was ice cold again and has been ever since. I think the lower voltage was preventing the fans from running at the proper speeds which is why it was cold at the outlet but the house was not getting cool.

I do not think your problem is the mini-splits. I think it is probably the power coming to your house.

$.02

This post was edited by mic111 on Thu, Sep 25, 14 at 0:05

Flair is going to be offering a wireless thermostat that will allow for much more accurate control of any minisplit that has an IR remote control. We had similar experiences and built a simple to use device to fix this problem.

OP here. Surprised to see someone revived this thread after all this time.

Daniel, I'd appreciate it if you'd reply with details of the "simple to use device" you built to solve the problem.

I still get somewhat wide temperature swings (around +/- 3 degrees F some days). However, I've found two things which seem to have mostly cured the really extreme swings I described above.

First, I did install an external thermostat and interface. I was able to find them on Ebay at much less than list price, though they still weren't cheap.

That helped, but not enough.

For a while I was considering using the interface's on-off contacts with a simple old fashioned mechanical thermostat. But after a lot more search engine exercise, I found something surprisingly simple.

What made the larger difference was nothing more than switching fan speed from low to automatic. I had left it on low to minimize noise - that's the main reason I chose this Mitsubishi model, because it had a very low, very quiet fan setting. Apparently, though, the system's computer is too stupid to control temperature consistently without being able to control fan speed. Who would have thought it?

I'm reasonably satisfied with the system now, though at times I'd like it to be quieter. However, if I buy another mini-split, I don't think I'll automatically go for another Mitsubishi. I'll look at other brands for sure.

And I'd be interested in how you built your improved controller, Daniel.

Thanks.

Hi David,

Absolutely, and glad to hear you figured things out. Sparing the details on some of the other whizbangs and features, the main thing we did was ensure that the sensor itself is located in the location of the room that makes the most sense. It then uses Infrared to reconfigure the minisplit (or window unit) based on the sensor readings. A wired solution is usually cleanest for control but for many running a wire is difficult so we felt rolling with Infrared was the way to go. The other thing that is pretty cool is the incorporation of a bluetooth beacon so that you can optionally have it autoon/autooff/preheat/precool, a feature that many of the smart thermostats for central systems are just now really bringing mainstream. Taking it a step further, it tailors the preference to the specific occupant (my wife always likes her office warmer than me) rather a generic set point. For the mistubishi units some features are probably a bit less exciting since they really layer on the sensor ritz but for many of the other units out there, this easily makes them much smarter.

Cheers,

Dan

Random thought, considering the exterior wall installation and the sensor is in the unit, I have a clock with a wireless exterior temperature sensor. I mounted that sensor outside and in the morning when the sun doesn't hit it, the temp is accurate, but when the sun hits that sucker it says the temp outside is like 130 degrees when its only like mid 80s. Pretty sure if your unit is sensing 130 degrees it wont operate right :)

Very surprised they allow these to be mounted in exterior walls when the sensor is in the unit.

it seems like a lot of your problem can be tied to stratification of air by temperature. That is going to be a larger problem with poorly insulated structures. By increasing the blower speed, you mix the air in the room. I find the same thing in heating mode with mine. I have very cold floors in a raised house. Ceiling fans help a lot and so does bumping up the blower speed in the mini split. I prefer to do the former in rooms that have fans.

I'm thinking of adding an external thermostat to mine but after reading I'm not sure. The ac part is fine. The heating is up to 5 degrees warmer then the remote is set at. How do you get this infrared One?