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Remote thermostat for Fujitsu mini-split ?

davidsmartin | Posted in Energy Efficiency and Durability on

I recently had a Fujitsu mini-split (AOU15RLS3H with ASU15RLS3) installed in a 24*36 foot room in Vermont. It is working well except that the temperature in the room not far from the unit is usually about 5 degrees below the setting on the remote. I have found two ways to get the temperature to the desired level: (1) Increase the setting on the remote or (2) set the fan not to automatic but to high. Neither of these solutions is ideal.

(Note that it is not in “economy” mode, the people sensor is off and the fan louver is pointing down.

It appears to me that the problem is that because the temperature sensor is in the unit itself instead of on the wall, the unit comes on and heats the air near the unit then shuts off or goes into almost an idle stage. The problem is not insufficient capacity, the problem is that the unit seems to think that the room is warm enough when it is not.

One solution might be to install a wired thermostat if the temperature sensor is in the remote. Can I do that?

Or is there a better solution?

Or am I missing something that should be obvious? And why do they put the thermostat in the unit instead of on the wall as in other heating systems? Is this guy correct?

Thanks for any suggestions,


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  1. davidsmartin | | #1

    Is this good advice?

    Q: Should I set my mini split system to the same temperature that I used to set my old heating system?

    A: No. Think about where the temperature is being measured. Your old thermostat was measuring the temperature about four to five feet off the floor. But, your ductless mini split system will measure the temperature on top of the indoor head which is mostly likely seven or eight feet off the floor. Remember, heat rises so your thermostat will register a slightly warmer temperature because it is higher off the ground. Most New England homes have a temperature variation from floor to ceiling of three to eight degrees. So, the best way to set the temperature on your mini split system is to ignore the temperature setting and adjust the temperature until you are comfortable.

  2. charlie_sullivan | | #2

    Fujitsu does sell remote temperature sensors. I don't know the details about whether they are compatible will all models, or whether you have to change settings to get it to use the remote sensor, etc.

  3. tenbob | | #3

    I just installed one of these myself; However, I installed the wired remote thermostat (they're not cheap!). I've found that the unit, even with the remote thermostat had a tendency to overshoot and exhibited a behavior similar to what you described until I set the "house insulation setting" to high insulation. After that, it tracks very well. As I understand it, the logic implemented in the unit (i.e., the default), assumes that any change in temperature requires drastic measures to compensate; in the high insulation mode, the unit doesn't attempt heroics....
    I also have a very high ceiling in the room where I've installed the unit with a loft space; it is very much warmer as you go up the stairs compared to where the remote thermostat is installed.

  4. GBA Editor
    Martin Holladay | | #4

    Many people have reported the same problem. As Bob says, the wall-mounted thermostat solution achieves good results (if you are willing to pay for the thermostat).

    Two other points:

    (a) Setting the fan on "high" is definitely the way to go if you want to maximize efficiency.

    (b) To maximize the airflow rate -- a desirable goal for efficiency -- adjust the louvers so they are slightly below horizontal, not aiming down as you now have them.

    For more information on these issues, see Ductless Minisplits May Not Be As Efficient As We Thought.

  5. PatrickCampbell | | #5

    I am not sure about the advice to just set it higher.
    In my experience, if I set the set point to say 75F, in order to achieve 70F on cold days, it means that it will actually reach 75F on more mild days. Obviously, you do not want to have to keep tinkering with the set point, depending on the outside temp. You should be able to set it and forget it. I'm thinking a remote thermostat is a must for these units, unless they are placed low to the ground.

    A newer Fujitsu 15RLS3H in Vermont here. I'm going to e-mail our installer to see about the remote thermostat.

    How do we set the insulation mode?? I don't see it mentioned in the owner manual.

  6. tenbob | | #6

    I have the same unit (15RLS3H); the settings are described in the installation manual that came with the unit. If you go to the Fujitsu portal, you should be able to find it.

  7. davidsmartin | | #7

    Fujitsu tells me that if I install the wired remote thermostat then I will lose the capacity to set the thermostat below 60 degrees (the feature they call "minimum heat" that I use when the house will be vacant for a week or more).

    Now I wish I had gotten the floor unit or at least put the wall unit a foot or so lower. But I understand that moving the unit down after it is installed is not an easy task. Expensive lessons!

  8. STEPHEN SHEEHY | | #8

    David- do the wired thermostats have the energy saving feature that the remotes have? When set, it senses a lack of movement in the room and lowers the set temperature by several degrees, boosting the temp when someone moves around. If the place is vacant, you might end up in the low 50s, with a set temp of 60. I'd check with Fujitsu.

    Your problem with the location probably being too close to the ceiling seems pretty common. The manufacturer is sort of stuck, in that locating the indoor unit a foot lower would put it at an awkward height in many homes. Even an additional 6" lower would place the bottom of the unit at about 6' above the floor in a house with 8' ceilings, creating a head bonking hazard in some locations. I wish the manufacturers would address the issue in their installation manuals, because most house do have some potential locations where the lower height would work.

    One of my units, 9RLS3H, is only about 6" from a ceiling, but the ceiling is sloped, so I don't seem to have the problem you do. The room stays pretty much right at the set point on the remote. The other unit, 12RLS3H, is about 18-20 inches below a sloped ceiling and also keeps right at the remote set point.

    Try playing around with the temp setting, the fan setting and the vane setting and see if you can get to an acceptable result. You are right, moving the indoor unit would not be easy.

  9. PatrickCampbell | | #9

    Folks please confirm with your installer. Mine said the Fujitsu wired remote "control" is just that, a remote control; it is NOT a remote thermostat. Sounds like a waste of money unless you wanted to change your temp, out of the line of site of the head unit.

  10. davidsmartin | | #10

    By default, the RVNUM remote leaves the temperature sensor in the wall unit. However, there is a setting that changes the sensor to the remote.

    Part of the problem at Fujitsu is that documentation is weak and that installers often do not know the details. The website indicated that another remote did not have the "minimum temperature" function but was silent on the RVNUM. Only after I bought the remote and saw the manual was it apparent that the RVNUM also lacked this function.

    It was also unfortunate that my installer was unaware of difficulties that could arise from mounting the unit close to the ceiling.

  11. PatrickCampbell | | #11

    Thanks for the reply - have you moved the thermostat to a remote wired location with success then?

  12. davidsmartin | | #12

    Patrick --

    After buying the wired thermostat and discovering that it would not permit me to set the temperature below 60 degrees when the house is vacant (minimum heat feature) I decided to return the remote. My fallback position has been to run a very small fan (8 watts) to direct a bit of cooler air toward the unit. That seems to help a bit but is obviously not the ideal solution.

  13. PatrickCampbell | | #13

    Is there any way you could point me to where it might say that you can set the RVNUM "remote" to act as a local thermostat?

    The fan idea may be simpler ... I'm not sure if it wouldn't make just more sense for me to just put an efficient Panasonic bath fan in the wall, ducting cold air up from the floor level to the top of the unit.....?

    Also, Dana D. has mentioned that the unit will actually be more efficient, the lower the Delta T between the head input air temp and outside air temp. So might even improve efficiency and provide some helpful air mixing this way, too.

  14. davidsmartin | | #14

    Unfortunately I was not able to find the manual for the RVNUM online. If I had, then I would not have ordered it and then returned it. And now I wish I had scanned the manual before I returned it. However, I do remember that the manual included instructions for selecting which temperature sensor should be used, the one in the remote or the one in the wall unit. It might have been a dip switch that needed to be changed, as the default was to continue using the the sensor in the wall unit.

    As you can tell, I am very frustrated by Fujitsu's inability to answer questions about this remote -- I wasted many hours trying to get answers. Perhaps a Fujitsu representative can answer the questions.

  15. ComRef | | #15

    Hi David,

    This has become a concern with many of our clients who typically characterize the problem as "cold blow". The indoor unit will determine that less heat is needed and idle down, with the compressor cycling off and the indoor fan going to low speed. The impact is that the occupants will feel a draft.

    Last years model which was actually available for the last 3 years was the RLS2; a fantastic unit that did not have this operating profile. The S3 has different logic and operates much differently.

    The most effective solution that we have found through several experiments and under advisement of our very qualified and supportive distributor, Master Group Inc., is to change function setting #95. It is a setting that relates to the insulation factor for the home; high or standard. Standard will result in a unit that cycles much more while high will make the unit respond more slowly and run the compressor longer, resulting in a more constant state of operation: and slow steady heat.

    Your installer can make the change very easily.

    Dale Comeau
    Comeau Refrigeration Ltd
    Fujitsu Elite Dealer
    [email protected]

  16. davidsmartin | | #16

    Dale -- thanks for the suggestion. I will try it.
    Patrick -- I found documentation explaining how to set the unit so that it ignores the temperature sensor in the wall unit and uses the sensor in the wired remote. Look at the bottom of page 01-29 of this document:

  17. user-6239952 | | #17

    I have the same unit, 15RLS3H. My unit is short cycling so I changed that setting #95, and I do see it being a bit more steady - good result .But, I also want to use the remote sensor to really get it right.

    I have contacted Fujitsu and gotten some ambiguous, but clear-enough information that no remote sensor is available for the 15RLS3H. The only option, he said, was to buy a remote thermostat. They do call it a a 'controller' but it appears to be a full thermostat so it would meet my needs .I also need an interface and the Fujitsu person gave me two different part numbers, both of which were incorrect. '

    Then I contact about 3 'certified' Fujitsu installers plus the one who installed mine, and not a single one contacted me back in a week.

    I see the parts online for about $335 total, and I'm starting to wonder if I might just install the thing myself. It seems pretty straight forward. Anyone know how difficult the install is?

  18. kjmass1 | | #18

    Has anyone had success with the remote controller helping keep the mini split at set point? My unit is driving me nuts requiring daily adjustments depending on the outside temperature. I've tried adjusting functions for temp offset, as well as insulation mode and can't seem to get it consistent. How hard is the thermostat install outside of fishing the cable?

  19. user-6239952 | | #19

    Hi- Just following up on this. It's getting a little cooler here in Colorado and I am seeing some issues again with the unit keeping the temp steady. Right now it's set for 74 but the room is 71, for example. I have noticed that when the unit is working hard, in cold or hot weather, it's more accurate but when the temp outside isn't far from the desired temp inside, it's a wide range of accuracy.

    So, I'm thinking to install a remote thermostat but wondering if any has gotten it to work. I can't get a clear answer from anyone including Fujitsu or installers so will likely do it myself.

    There are now some pretty cool products which do the same thing, using the IR remote function. I assume these things just work like a universal remote - spoofing the real remote and controlling the temp that way.

    Great idea, simple, and cheap - but won't the beep be beeping every time the remote tells the unit to change the temp?? Annoying!

    Maybe I could open up the unit, find the beeper, and put some tape on it or just kill it somehow but that seems a little dicey!

  20. Expert Member
    Dana Dorsett | | #20

    The Fujitsu's wired & Wi-Fi "thermostats" are really remotes, and don't automatically behave as thermostats. They have local temperature sensing on board, but the mini-split head will continue to use the sensor in the head until it's programmed to use the fixed remote's temperature sensor. (I don't have the details on how to do that.)

    RLS3_Y_H heads are already Wi-Fi enabled, but Fujitsu sells both wired & IR Wi-Fi adaptors to use with older units.

    1. Deleted | | #21


  21. Calum_Wilde | | #22

    Bringing this one back from the dead. Has anyone installed a remote controller on a Fujitsu RLS3 wall mounted heat pump? I've read through the instructions for both the heat pump and the RVNUM and nothing shows where to access the connections inside the wall unit of the heat pump.

    1. kjmass1 | | #26

      Hi Calum,

      Homeowner here. I installed that model wired controller to an RLF1. On my model I had to install a board on the inside of the unit. Instructions were awful and my hvac guy couldn't get it to work. Paid a certified Fujitsu tech to fix and it works fine now. If I remember there are 3 or 4 settings that have to be changed to make it sense from the controller (I'm assuming that's why you are doing it). It made a huge difference for us and eliminated short cycling and overshooting on mild days. Being able to place it far away from the unit helps keep the entire floor at an even temp.

      A Sensibo IR controller will do essentially the same thing and is only $100.

    2. DarkNova | | #32

      Did you ever figure this out? I have the same question.

  22. gusfhb | | #23

    I have said this before I'll say it again. Minisplits are not designed to keep +- 1 degree. Part of their efficiency is not turning on and off, so, guess what happens.....

    The OP[err three years ago] had his minisplit setup in a completely inappropriate way.
    Minisplits can 'throw' air really well with computer designed vanes and fans, compared to a 'dumb' vent in the wall. Let them do their job.

    If you have one point source of heat in a large room, you need to let the air flow out, don't point it down so it makes a pool of hot[or cold] air.

    Really, only you can set up your system, and honestly, it can take quite a while to get it the way you want it.

    Size in relation to load has a big effect on how the unit keeps temperature constant, and of course the actual load varies with outside temperature. An oversized unit has difficulty turning itself down on medium temp days and not overheating[or overcooling] the room.

    The smallest unit available 10 years ago is too large for our large master bedroom. So we keep the fan on low and the vanes turned away from the bed. This works most of the time. The SO has her office in this space, and hot sunny days with computer and printer going, she may put it on high. If the vanes are pointed out toward the bed, it will end up getting shut off at night, and it gets stuffy.....

    The living room unit handles about 1000 sq feet, and I could have bought one size smaller, it generally stays on low speed, going to medium on hot days[we use these primarily for AC] or high if we come back from a trip and need fast pull down. This means to me, that the size smaller would have been sufficient, and in my opinion, modulate better.

    Again, it is setup in a way to get the air around the room, and it works, some of the space is actually behind it and it is reasonably well controlled.

    This unit will keep a pretty even temp, usually actually within 1 degree, but not without some level of manual intervention.

    I find the auto fan does not do so. I am willing to push a button once or twice a day to ensure domestic tranquility.

    And the reason I comment.......moving the thermostat has not solved the actual problem. You have changed where you measure the temp, so it will be warmer or cooler there, but you have not changed the balance of temperatures in the room.

    I think you will be just as unhappy with a remote thermostat if you do not set the unit up to move the air around the room in a way that will even out temperatures

    1. Calum_Wilde | | #24

      "I have said this before I'll say it again. Minisplits are not designed to keep +- 1 degree. Part of their efficiency is not turning on and off, so, guess what happens....."

      I'm not sure what you're saying here. Minisplits are designed to keep the temperature, where it's being measured, within a small range and they work most efficiently when they don't turn off and on.

      Everything else in this thread is about getting minisplits to not turn off and on when they don't need to. Your post is about evening out the temperature differences between different rooms or parts of rooms. That's not what we're discussing in this thread.

      What you've written is important and would make a great discussion, but it deserves it's own thread.

      1. gusfhb | | #25

        Actually, no they aren't, they are designed to let the temp drift in order to maximize efficiency
        From The OP
        ""It appears to me that the problem is that because the temperature sensor is in the unit itself instead of on the wall, the unit comes on and heats the air near the unit then shuts off or goes into almost an idle stage. The problem is not insufficient capacity, the problem is that the unit seems to think that the room is warm enough when it is not""

        this is what I am addressing

  23. Expert Member
    Dana Dorsett | | #27

    >...they are designed to let the temp drift in order to maximize efficiency

    That would be the exact opposite of all Fujitsu marketing statements, which put the emphasis on precise modulation to the sensed load to achieve higher efficiency. Even for their non-modulating multi-split heads they modulate the compressor around a range to match the actual output the compressor to the fixed output of the head at whatever blower speed it's running. This statement is found in multiple locations on Fujistu's website:

    "The Inverter component allows the outdoor unit to vary its speed and output to match the required capacity of the indoor unit. Thus, the Inverter model can achieve 30% more operating efficiency than conventional models and therefore, is much less expensive to run."

    In most of their brochures the have this or similar statement:

    "Constant Comfort through
    Inverter Technology

    Inverter technology is like cruise control for your
    heating or cooling system. Compressors only run as
    fast as they need to handle the cooling or heating
    demand. They can handle greater extremes in
    temperature, are smoother and more stable in
    operation, and reach the desired temperature more
    quickly than conventional air conditioners. "

    Older brochures used to show graphic representations of flat-stable room temperature with inverter technogy equipment comparing it favorably to varying /oscillating temperatures than comes from cycling.

    Having the sensor in the head may have the effect of some temperature drift, but that is not the design goal. Modulation is always more efficient than letting temperatures drift.

  24. josh_in_mn | | #28

    Some of the newer units have occupancy sensing, for example "iSee" on Mitsubishi. They use this information to allow the room temperature to be controlled to a lower or higher setpoint when the room is unoccupied. But even in this mode, I suspect the room temperature is controlled pretty accurately, within the limits of what stratification does to the actual room temperature vs the sensed temperature.

    1. Expert Member
      Dana Dorsett | | #29

      It's not entirely clear if or when occupancy sensors actually save kwh if used for setbacks. Most of the time setback strategies end up increasing energy use unless the setback period is quite long (a full day or longer.)

      The Mitsubishi iSee is designed to detect cool or hot spots, and attempts to even-out the temperatures in the room. It can also be programmed to direct the air either toward or away from detected occupants. While it can be used for a setback when no occupants are detected, that isn't necessarily going to save kwh due to the lower operational efficiency incurred during the recovery ramp. To save on energy use there has to be sufficient energy saved by the stagnating higher/lower room temperature to make up for that efficiency hit taken during temperature recovery back to the setpoint.

  25. gusfhb | | #30

    >>>>...they are designed to let the temp drift in order to maximize efficiency

    That would be the exact opposite of all Fujitsu marketing statements, <<<<

    marketing statements are well, what they are....

    The question is, are the units programmed for accuracy or efficiency? In my experience, which is may differ from yours and is certainly much more limited, they push for efficiency.

    There is a time of year[or of they day] where virtually any unit that is large enough for peak loads will be oversized. What do most of the units do in this case? They continue to run, for a period of time, determined by their internal programming, because this is the more efficient thing to do.

    A standard central heating system is programmed for accuracy. My hydronic system never shows any temperature variance, because it values accuracy over efficiency.

    This is certainly a tangential but related conversation to the OP.

    My point is that it is not the location of the thermostat that is causing the temperature variation in the room. Moving the thermostat in the situation described would result in the furthermost areas of the room being warmer, but the area closer to the unit being hot. My experience tell me that with proper airflow settings in large rooms you can get pretty even temps across the area with the internal thermostat.

    It is my experience, with 5 different high efficiency Mitsu units in 2 different houses over the last 15 years, that even after careful tweeking and adjustment, there is no set it and forget it place. Almost all of the units require at least occasional manual intervention, usually just changing the fan speed.
    It is my opinion that this has nothing to do with the location of the thermostat but is more or less inherent in the control strategy of the devices.

  26. davidsmartin | | #31

    OP reporting in, three years later:

    My installer really did not want to move the unit down six inches from where he originally put it, between two ceiling beams. But this week he did install the very expensive remote wired controller which also required a communications module to be installed in the wall unit. After installing the wired controller we changed the default settings so that the wired remote also senses the temperature and uses that temperature to control heating. We also made sure that the "high insulation" option was selected. I have set both the horizontal and vertical louvers both to swing as that seems to spread the heat around the room better.

    Now there is much less cycling of the unit -- with the outside temperature below freezing it only cycles off when it needs to defrost. And the room temperature is much closer to the thermostat setting.

    I had delayed installing this wired remote because I lost the ability so set the unit below 60 degrees when I go away for a week or so. However Bruce Harley suggested that I could almost achieve the same by setting the temperature correction factor to minus 8 degrees.

    Thank you everyone!


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