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Ductless minisplit location? Remote thermostat?

Trevor Lambert | Posted in General Questions on

I’ve read a number of complaints about ductless mini splits not accurately maintaining the set temperature. The most common reasons cited are the fact that the thermostat is contained within the unit, and the unit is too close to the ceiling, resulting in the air temperature sensed is higher than the ambient air in the room.

If a remote thermostat is used, does this mean you don’t have to worry about ceiling clearance (of course still observing installation instructions)? Conversely, if you keep the unit well away from the ceiling, does this mean you probably don’t need the remote thermostat?

I originally planned for the unit to go about a window beside the kitchen. This would put the top of it about 5″ below the ceiling (8’6″ ceiling height). I also have the option of putting it beside the window, which would allow it to be lowered about a foot. It would mean re-routing some wires and cutting another hole in the drywall, which is some work but doable. Obviously don’t want to do it needlessly. I’ll attach a drawing showing the options. The house is super insulated, and I haven’t noticed any air stratification.

The unit I plan on having installed is the Fujitsu 9RLS3. Is there a wireless remote sensor option for this unit? Running wires to a suitable location on the wall would be a pretty substantial job. I do plan on calling Fujitsu, but the other time I called them I got mediocre results. Their website is essentially useless for this level of detail. The installers in my area have little, quite possibly zero, experience with both Fujitsu specifically and ductless in general, so it’s my job to sort this stuff out in advance.

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Replies

  1. Expert Member
    Akos | | #1

    Trevor,

    I would put it in the more central location just to get better coverage. The mini splits without a remote sensor do tend to have some issues when heating, the way I look at it, is worry about how the room feels not what the set point is. If the set point needs to be a couple of degrees higher or lower, not a big deal. The Fujitsu has some hidden options for getting it to work with highly insulated houses which might also help.

    If this can't get you the comfort you need there are some aftermarket IR (ie Sensibo) controls with remote sensing that seem to be simpler/cheaper than Fujitsu's thermostats.

  2. Trevor Lambert | | #2

    The problem with dealing with it as you suggest, at least from what I've read, is that it's load dependent. That is, under certain conditions, 2 degrees higher will be right, but at other times it won't. I'd kind of like to set it and forget it.

    Is connecting the aftermarket control to the Fujitsu straightforward?

  3. Josh Durston | | #3

    I'll second the offset will always be changing, one day 2deg will be right, the next it will be 5deg and then 0deg.

    The efficiency is directly related to the temperature of the inlet air. Lower inlet temp in heating equates to higher efficiencies and capacities.
    Also, you want the unit to be serviceable for coil cleanings, excessively high mounts will make this tough job even more miserable.

    I think location trumps height though, I'd rather have the unit a little high but in the right location to serve its respective area, than low and ineffectively jammed in a tight obstructed corner.

    Instead of a remote thermostat, you can install a remote room temp thermistor. It replaces the entering air sensor, and should be a fraction of the cost of the remote thermostat. They usually come in a kit with a plastic housing and decent length of wire with the connector pre-attached. Some units even have a jumper you can clip to allow the fan to cycle completely off once the heating/cooling demand is satisfied if using a remote sensor/thermostat. If it's running off the internal sensor, it needs constant airflow to sample the air temperature.

    1. Trevor Lambert | | #7

      In regards to the location, the difference is only about 4 horizontal feet and it doesn't really bring any corners or obstacles into play. Intuitively, it seems like the 1 foot height difference is a bigger relative change than the 4 foot horizontal.

  4. Ethan Foley | | #4

    From my understanding, aftermarket IR controls with remote sensing don't change where the wall unit gets it's information from. Some of them have an internal formula for adjusting the setpoint on the wall unit higher than the displayed setpoint to accomodate the difference between what the wall unit is sensing and the remote thermostat is sensing. Units like the Sensibo just use the same IR sensor that the remote that comes with your unit does, so it has to have line of sight (or bounce off a wall, etc). I've never set one up, but I believe it's pretty simple. I would prefer the wired remote temperature sensor that changes where the wall unit gets it's information from.

    1. Trevor Lambert | | #6

      The Sensibo unit is sold by Fujitsu as an official accessory. From what I can tell, it does have a temperature and humidity sensor on board. From other sources on here, it also seems like you can program the RLS3 to use an external temperature input, whether it's hard wired or wireless. So if, and I guess that's a big if, ALL of those things are true, it should theoretically work. I was actually hoping that the supplied remote had an on board sensor for purpose, but that was pie in the sky kind of wishful thinking.

      EDIT: the above comment about Sensibo is wrong. I got it confused with a different brand. The official accessory sold by Fujitsu is Intesis.

      1. Ethan Foley | | #8

        That would be pretty neat, I know you definitely can't do that with the Mitsubishi units.

        The installation manual for the unit Josh is talking about is linked to here: Accessory UTY-XSZX https://portal.fujitsugeneral.com/files/catalog/files/UTY-XSZX.pdf

        1. Trevor Lambert | | #9

          That remote sensor is not listed as compatible with the RLS3 in the chart on page 69 of the line brochure:

          https://www.fujitsu-general.com/us/resources/pdf/support/downloads/halcyon-2018-full-line-brochure-02.pdf

          However, the same chart says the IR controller is also incompatible, and that is contradicted by other Fujitsu documents. I don't understand why they can't have their ducks in a row.

        2. William Morse | | #17

          Yupster, you definitely can do that with Mitz, maybe not all, but many. The Kumo controller and remote sensor are ~$200.

          Bill

  5. Trevor Lambert | | #5

    Josh,

    That sounds good. Where do I get one of those and how do I ensure it's compatible? Does Fujitsu sell one? I found something called a "dry contact wire kit" from them, part number UTY-XWZXZ5. Wait, never mind that - found a description in the line brochure and it's for something else.

    I found what I think is the OEM thermistor. I'm assuming I could just run wires to that in a remote location? Any additional resistance in the wires could be compensated for by calibration or just adjusting the set temperature.

    1. Josh Durston | | #15

      I did a bit of digging, unfortunately the only remote sensor I could find (besides the thermostats) was for the ducted units. For Mitsubishi and LG mini splits you literally unplug the return air sensor and plug in the cable to the remote thermistor.

      The service manuals should have a table that shows the resistance to temperature relationships.

      You might be better off to get an official Fujitsu thermostat with a built in temperature sensor.

  6. kjmass1 | | #10

    A couple points as I've spent more time than I'd like to admit researching all of this with my Fujitsu's.

    My 12RLF1 is mounted above a window and only has maybe 3" clearance to the ceiling. 1940s house so very limited options. Operates fine but temp sensing is not good, ours particularly bad because fridge cabinets creates a little alcove.

    Sensibo does work pretty well if you want to move the temp sensor somewhere else, but within line of sight. Unfortunately this usually means within the path of airflow, distance limited by IR, and you need to plug in which is by the floor so you could have all sorts of obstructions/furniture, animals, humans etc. Not ideal but can work. You can set a boundary so if the temp at Sensibo falls below say 70, turn heat to 72 and high fan. If temp hits 74, turn to 72 and auto fan. Again, not ideal but does work pretty well. One quirk is that it uses Celsius so not accurate within a degree.

    I ended up hardwiring their thermostat off of ebay Fujitsu UTY-RVNUM. Expensive. My head also needed a controller board which was difficult to install with terrible documentation as you know so I would recommend using a Fujitsu installer for that and fishing the cable yourself. Now it will sense wherever in the house you put the thermostat. A lot easier to change settings and for the most part it is set it and forget it.

    Not sure why, but my unit won't kick in to high fan heat if it drops below the set point. So I do find myself changing fan speed from auto to high depending on overnight lows as my unit is more for AC and shoulder seasons.

    Now I use both. The Sensibo app to control it via phone and the thermostat for the temp sensing. Works great. There is also some tie ins to Nest and IFTTT if you really want to play around.

    1. Trevor Lambert | | #11

      It appears like the RLS3 series needs an interface kit to go with the hardwired remote. A lot of money for a feature that should be standard, IMO. I'm tempted to just have it installed without all this stuff, then figure out how to hook up a remote thermistor after the fact. Either find the part number of the thermistor and buy it, or measure the value and buy a generic replacement.

      But you can confirm that the Sensibo does indeed have its own temp sensor, and you were able to program the Fujitsu to operate off that sensor temp instead of the internal one?

      1. kjmass1 | | #12

        Sensibo is a cheap solution that may work for you before going all in on the thermostat.

        Sensibo is an IR blaster, it replicates your remote's buttons- power, mode, fan speed, and set point buttons. The app allows you to do detailed schedules, control while you are away, etc.

        It does not "program" the Fujitsu unit at all. It just uses it's own software and sensors to send IR commands to the head unit for you. I've attached a couple screen shots.

        1. Trevor Lambert | | #13

          So in order to control the temperature of the Fujitsu, it sends a command to change the set point temperature? Like if the Fujitsu is set at 70F, but the Sensibo is measuring 66F, it will send a command to up the set point to 74F?

          1. kjmass1 | | #14

            Correct but don't take the fujitsu set points literally as there is a wide variation from floor to ceiling, and Sensibo's sensor will most likely be different from Fujitsu. And Fujitsu has a 2 degree difference in setpoints. I'd let Sensibo monitor your temp for a day and then adjust accordingly.

          2. Trevor Lambert | | #18

            Now that I have a Sensibo, I should clarify that this isn't exactly how it works. By default, all it does is exactly what the factory remote does, which is tell the indoor unit a set point on a schedule. The sensing and cycling is still done completely within the indoor unit. There is an additional feature called "Climate React", but it's not as smart or automatic as implied above. You can program in cut-in and cut-out set points; basically a crude IFTTT type control loop. Using this feature, I found that the mini split was cycling too often. It made more sense to just set the temp with the factory remote. I wouldn't recommend the Sensibo unless the only thing you want it for is controlling your mini split over the internet.

          3. johns3km | | #19

            I'm sorry if I wasn't clear- no third party implementation will be able to control how the minisplit modulates or cycles. What it will do is allow you to move the temperature sensing aspect to somewhere that makes more sense in the room, and not 8' high near the ceiling. It will allow for better temperature control, at the expense of cycling.

          4. frandel | | #20

            Does thE Climate React help maintain the room temperature better? Flair has a setting called Room Sense that will send different set points to the mini split if it's not doing a good job on its own.

        2. frandel | | #16

          Yeh, Flair does the same thing. They have a Puck that's a thermostat and sends IR commands to the mini split. It gets a better read on the room temperature.

          I use it to control my Lasko heater.

  7. BFW577 | | #21

    My Midea had dip switches on the indoor units board that could adjust the temperature sensor +2 +4 +6/ -2 -4 -6 degrees. Its a floor console unit that reads the temp 2 feet above the floor on the air intake. I set it +2 and it worked out perfectly. I think my Gree also has this setting.

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