# Temperature trouble with new Mitsubishi mini-splits



## hvactech126 (Nov 11, 2010)

Those units are designed to run a lot. They will modulate the compressor and fan speeds to a point where they maintain the setpoint and hardly every shut off. This is ok because the power usage is very minimal as the units ramp down. Also, if you are using the standard remote control then the temperature sensor is in the unit and in order for it to get a sample of the surrounding air it will run the indoor fan and may not be running the outdoor unit. You can get MHK1 thermostats for the units that will sense the temperature at the wall mounted thermostat instead of at the unit. The MHK1 thermostats can also be set to be programmable to change setpoints at 4 different times during the day.


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## jeffersly (Nov 4, 2014)

That's nice information but I don't see how it is relevant to my problem.


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## hvactech126 (Nov 11, 2010)

It has everything to do with your problem. You need the MHK1 thermostats.


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## beenthere (Oct 11, 2008)

jeffersly said:


> That's nice information but I don't see how it is relevant to my problem.


What your describing is typical of mini splits. As 126 said, you need to upgrade if you want it to work like you described.


126 is pretty sharp, I'd listen to him.


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## jeffersly (Nov 4, 2014)

beenthere said:


> What your describing is typical of mini splits. As 126 said, you need to upgrade if you want it to work like you described.
> 
> 
> 126 is pretty sharp, I'd listen to him.


OK, if you two don't mind, please explain some things to a confused newbie:

When we were pricing out the units the contractor told us we didn't want the wall thermostats because they were stationary whereas the normal thermostat is in the remote, so you put it where you want the temperature to be reflected. I then totally forgot about this, until when I first got the units and they were on Auto mode, they were switching back and forth from heat to cool quite rapidly. I called the installing contractor and he reminded me that it was because the temperature sensor was in the remote and I put the remote in the direct path of the air flow. He was right -- it was in the direct path of the air flow, and when I moved the remote elsewhere the problem went away. So the first thing I am confused by is the assertion that the temperature sensor is in the unit as opposed to the remote. Is this a change from the older MSZ-FE09NA models, maybe?

The other thing I'm confused by is why there would be a difference between Auto mode and Heating-only/Cooling-only. In Auto mode, currently, it never cycles to cooling -- it just does a good job of not heating the room far past the set point (no more than a couple of degrees). In heating mode, it does cycle off and on but keeps heating further and further past the set point (8+ degrees). I don't understand why a separate, wall-mounted thermostat would help with this. It seems to be some discrepancy between the ways the modes operate, but I don't know why that is, and I don't understand why a wall-mounted thermostat would make a difference here. Is the unit sensing the temperature differently in the different modes? And if so, why?

Thanks!


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## hvactech126 (Nov 11, 2010)

Your contractor does not know the Mitsubishi product. The room temperature thermistor is in the wall mounted head unit not in the remote. Also, does your indoor unit have an "i-see sensor" that looks like this indoor unit because it will act differently based on the windows in the room and which side of the house the unit is facing. It will also act strangely if the pass through hole behind the unit is not properly sealed and drafts are coming through from the outdoors and offestting the room sensor. I have seen some poor installations where that was the case.









here is you operations manual for a MSZ-FE09NA: http://usa.mylinkdrive.com/uploads/..._FE09-12NA_Operation_Man_SG79F408H02_6-11.pdf

AUTO MODE: The unit selects the operation mode according to the difference between the room temperature and the set temperature. During AUTO mode, the unit changes mode (cool-heat) when the room temperature is 4F away from the set temperature for more than 15 minutes.


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## hvactech126 (Nov 11, 2010)

8-6. i-see CONTROL OPERATION (MSZ-FE09NA MSZ-FE12NA)
The sensors constantly measure the room and floor/wall temperatures to automatically adjust to the set temperature by estimating the temperature actually perceived by a person inside the room (“sensory temperature”).
Advantages
· The air inside the room is conditioned quickly to a comfortable condition.
· The room will not become too cold or hot even when the air conditioner is kept on
for a long period.
· The air conditioner will not overcool or overheat, which means you can save on electricity.
i-see control operation is activated when i-see button is pressed with a thin stick in manual COOL or manual HEAT mode.
NOTE:
 i-see control operation is activated when the remote controller is first used following replacement of the batteries 
or resetting of the remote controller.
i-see control operation is cancelled when i-see button is pressed with a thin stick once again.
NOTE:
 If the conditioner is turned OFF without cancelling i-see control operation, i-see control operation is activated the 
next time the air conditioner is turned ON.
• When AREA setting is not activated, the sensing range of i-see Sensor differs depending on the installation location of 
the air conditioner.
Refer to "Remote controller in SERVICE FUNCTIONS".

Taken from the service manual: http://usa.mylinkdrive.com/uploads/documents/215/document/MSZ-FE09-18NA_Service_OBH542_4-11.pdf

You would do yourself a lot of good to read all manuals from the manufacturer.


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## jeffersly (Nov 4, 2014)

I admit that I hadn't read the non-operations manuals from the manufacturer, although I have thoroughly read the operations manual for my unit (http://usa.mylinkdrive.com/uploads/...MSZ-FH09-15NA_Operation_JG79A806H02_03-14.pdf) which is frustratingly vague on a lot of details. The service manual seems to have more detail, and I will read it -- I didn't realize it would contain the same information (but more detailed) as the operations manual as opposed to simply information for servicing.

You've given me some interesting things to think about. However, I still am not seeing any explanation for the behavior in my original post -- why, given a particular setpoint and all else being equal (auto fan speed, auto air direction, i-See sensor on/off -- on the model I have it does operate in Auto mode as well as Cooling/Heating, etc.), Auto mode would do a good job of keeping the temperature *while never switching heating/cooling operation mode* but Cooling-only/Heating-only goes way too far.

Thanks!


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## jeffersly (Nov 4, 2014)

Sadly, looking at the service manual I immediately see a discrepancy with the operations manual.

In the operations manual it says:

"During AUTO mode, the unit changes mode (COOL↔HEAT) when the room temperature is 4°F (2°C) away from the set temperature for more than 15 minutes."

In the service manual it says:

"COOL mode changes to HEAT mode when about 15 minutes have passed with the room temperature 2°F (1°C) below the set temperature.
HEAT mode changes to COOL mode when about 15 minutes have passed with the room temperature 2°F (1°C) above the set temperature."


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## beenthere (Oct 11, 2008)

The manuals are always written by the same technical writer.

The manuals are never written by an engineer. Its always someone that went to college on how to write technical manuals. But they seldom know how the item operates. They just copy what an engineer scribbled for them.

The service manual is usually the most accurate one of the manuals.


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## jeffersly (Nov 4, 2014)

I wrote Mitsubishi and they did indeed confirm that the service manual is correct.

That's very useful to know for normal operation. I've been trying to think if as a result it has indeed been switching to cooling mode when in Auto (regardless of me not having noticed it), which is why the temperature is kept in check. But I can't come up with a scenario where that works.

In heating mode, it could simply *not engage heating*. It's almost like it's forcing itself to actively heat every X minutes regardless of the setpoint. It knows it's too hot, too: if I flip it over to Auto with the same setpoint, it instantly recognizes it's too hot and starts switching over to cooling.

I do hear it running in Auto mode as well at times when it's not actively heating, but this is explained in the previous post as the unit running air through itself so the thermistor can sense the current temperature.


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## Ben25 (Jun 14, 2014)

Whoever installed your unit definitely didn't know what they were talking about. We normally recommend that you use either heat or cool mode, (or any other mode) but not auto. 
I would make sure the hole behind the unit is well sealed, if you haven't already.


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