Why Doesn’t My Mini-split Heat Pump / Air Conditioner Work?

I recently had an interesting question – a reader asked what could cause a Fujitsu mini-split air conditioner to cause the air to become *more* humid. In fact, they noted that the air became highly moisture laden and the house was just yucky humid.

I really scratched my head on this one because, from a physics standpoint, under “normal” conditions, this is impossible with a mini-split. Why? Because a mini-split system has an air handler unit in the house with the only connection to the outside (and outdoor humidity) is through a small hole in the wall where the electrical and refrigerant lines run. And yet it happened.

The questioner noted that multiple units were involved and that various parts of the electronics had been changed, and yet the problem persisted. He noted that he’d heard of a number of other people with the same problem. I admit, I was baffled!

Then it came to me. In fact, I had worked with an associate, helping them to track down this exact problem. While I can’t state with 100% certainty that the problems were the same, the symptoms are the same. In addition, I realized that my own home’s systems exhibited the same issues, but I automatically made the adjustments to make the systems work properly!

Here’s what’s going on…

In short – the fundamental design of mini-split heat pumps is flawed and requires a workaround to make them work properly. When they do, they’re wonderful, but if you don’t know these secrets, you may have a horrible time with yours, cursing the company and salespeople.

Here’s the secret – only an idiot would design an air conditioner or heat pump where the thermostat is located within the unit itself!

Let’s say my room is 75 degrees F and I want it to be 72F. All my life, I’ve set the thermostat to 72F and my air conditioner will run until the thermostat senses that the temperature in the room is 72F. Any installer locates the thermostat somewhere away from the cold air supply for the room. After all, if cold air blows on the thermostat, it will think the room is cold and turn the unit off.

Now look at the mini-split. How convenient. It’s a small (usually) wall mounted unit with a remote control. You intuitively set the remote to “auto” and 72F. The system comes on, runs for a while, then turns off, but the room is still nasty warm and humid. Any child could tell you that the air around the system is going to be colder than the air in the room in general. Sure, if you mount it at the ceiling level, like the manufacturer recommends, most of the cold will go down, and the warm air will accumulate at the ceiling, so it will work better. But then when you use it during the winter as a heat pump, what happens? Yea, right, the warm air rises, and now the system turns off prematurely again. So either you’re screwed in the winter, or in the summer, but no matter how you mount the system, the thermostat is going to tell the system to shut off far before the room is at the temperature you set.

To be fair, manufacturers sell external thermostats that you can buy for a couple hundred dollars more. But they really don’t push them. I assert that the units are fundamentally flawed when operated using their internal thermostat because they can not work properly under all conditions!!!

Now that I’ve got that off my chest, here’s how to get the most out of your mini-split air conditioner / heat pump.

First, ignore the numbers on the remote control, they’re meaningless. Pretend they’re written in a language you cannot read. The only thing to know is that when you press the up-arrow, the temperature will be warmer and when you press the down arrow, it will be colder. Got it – ignore the numbers.

Next, get yourself a cheap thermometer and place it somewhere useful in the room, not near the unit so you can monitor the actual temperature in the room.

Next, if you want the system to work well, use the manual fan settings. Pretend the “auto” setting doesn’t exist. Is the room a little warmer or a lot warmer than you like? Set the fan to medium or high. For air conditioners, this runs contrary to conventional wisdom which says you should never run the fan unless it’s needed. This is much less important for mini-splits, so forget you ever heard that.

Finally, turn the temperature down until the system is blowing out nice, cold air (assuming air conditioning mode). Let it run for an hour or so. If your house was humid, then it should be getting much less humid and you should see a good stream of water dripping out the condensate lines outside. If you don’t, then your system is defective in some way. But if you do see water, then it’s working.

After a little experimentation, you’ll find that you can reliably set the temperature and fan settings to keep your room comfortable. Typically, for moderate, typical spring weather where it’s humid and a little warm, then setting the system to 68F and medium fan works really well. It dehumidifies the house without making it too cold. The house will NOT be 68F, it’ll be more like 72F. Remember – ignore the numbers other than as a reference point.

When it’s hot and humid out, I usually crank the fan to high, leaving the system at 68F but sometimes I have to go colder. Again, you have to experiment with what works for you.

Tips for better dehumidification:

If you find the house is humid but not tremendously hot, you can set the fan to “low” and set a cold temperature – 64F for example. This will force the unit to try to really chill the air, which is most effective at dehumidification but the air flow will be low enough that you won’t freeze yourself out. Experiment with the settings, keeping in mind that the unit won’t dehumidify at all if you don’t set it to an aggressively cold temperature. It simply won’t do anything but appear to run all day if the house is 75F and you set the air conditioner to 73F.

Give this a shot and let me know how it works for you. It definitely works for me. Yes, it’s a kludge but it’s logical and the science is sound.

145 thoughts on “Why Doesn’t My Mini-split Heat Pump / Air Conditioner Work?

  1. I was told that my Mitsubishi MSZ-FH15NA heat pump dehumidified in the WINTER when heating, not just in the summer when cooling. Seems counter intuitive. Could this be true?

    • I can’t think of any way that would happen other than the fact that heated air is lower relative humidity by virtue of the definition of “relative humidity”. But as far as the actual moisture content of the air, I don’t see how that could be.

  2. Ted, we had a Fujitsu slim duct system installed in June with a 9k and 18k units, and since the beginning, we’ve had a smell. It’s intermittent but occurs regardless of function and fan speed. The contractor has tried various remediations and brought a Fujitsu rep the last time. I think the smell is coming from the crawlspace where the units are installed on top of old rock wool insulation. They say there is nothing in the units and no leak in the limited duct work they installed. Any ideas?

    • Do you have a model number that I could look up to see the exact configuration?
      If there are any ducts between the units in the crawlspace and the air output into your living space, the system could certainly be sucking in the stale air and blowing it into the living space. Even something as subtle as the connection where the duct comes through the floor or wall often has air gaps where the stale air could be getting in. Plus, the air handling systems are never really airtight.
      Does the smell always come when the system is running or does it occur sometimes when it’s off? If you get a smell when it’s off, try sniffing around where the ducts for both air supply and return enter your living space to see if you can determine if the smell is stronger from either of these places.
      Sometimes, what can happen is that other fans in the house, like when a dryer, stove vent, or bathroom fan runs, in the process of blowing out air, it has to suck air in from the house. If there’s any gaps open to the crawlspace, these things can suck that stale air into the house.You can force the issue by trying this: close all the windows and doors in the house, so the house is tight. Then, with the Fujitsu turned off, turn on the clothes dryer and any other vent fans you have, like bathroom and stove. Then, go to the room where you get the smell. See if that makes the smell worse.
      Check out these things and you’ll be able to narrow down the source of the smell.

      • The outdoor unit is a 24k, the indoor units are ARU9RLF and ARU18RLF. Our third floor is finished but has crawlspace behind the walls. Both units have ductwork to and from the units. There is no smell when the units are off. It sounds like totally sealing the system is impossible. I’ve thought about putting smoke in the crawlspace to see if any is drawn into the system.

      • Where is the air filter installed? Do you have a central return? I would look there because that is usually the “weakest link” in the system since it has the strongest “sucking” force of the air into the system that will be distributed around the house.

        The smoke idea can work really well. I would try having the smoke concentrated near where the air return heads to the living space, as noted above – that’s where bad air is most likely sucked in.

      • it can be illuminating to pull off those grates and see what’s happening where they attach to the wall. Sometimes the installers didn’t even use duct work and just use the wall cavities. Even if they do use ducts, often there are big air gaps so that any pollutants inside the wall cavity can be sucked right in.

      • Fujitsu had some recommendations for rigid elbows, but it only made it worse. We’ll check it out with smoke. Somehow we need to eliminate potential causes. When this started, we only had the smell with the fan on high after a cooling cycle.

  3. http://i0.poll.fm/js/rating/rating.jsThanks for this blog post. I’ve got the reverse problem – we live at altitude (over 5200 feet) and have a problem with the air being overly dry with humidity levels as low as 15%. The unit works great in summer, this fall we have run a humidifier to help our new 5wk old baby adjust to the climate and seems like the unit randomly stops heating, even when the outside temps are not very cold, say 35F. The humidifier will get the room to 40% humidity or so. Will over humid air cause the heat to shut down? It seems like it heats better when there’s less humidity, but 40% doesn’t seem “high” enough to trouble a unit like this? Ideally we’d like to maintain a humidity of at least 35% for the next few months until the baby’s a bit older.

    • that’s a great observation. Most likely the unit is going through what is called a defrost cycle. they do this periodically when the temperature is in the mid-30s down to the mid to low twenties. under normal humidity conditions, the outdoor unit will build up ice on the coils so periodically the system has to stop running heating and reverse operation to heat up the outside coils and melt off the ice.
      As for your specific question about indoor humidity affecting the operation, there should be no worry. 40% is certainly within a normal range that should be no problem.

      • Thanks for respondingTed. In the meantime I had also tried your tip about setting the fan to medium rather than auto and… the unit seems to heat the room much more consistently!! It does seem to go into a defrost cycle, but the room temp doesn’t drop substantially during the defrost cycle so I think the issue before was with the warm air collecting near the top of the unit and fooling it into thinking the room was warm, with the fan always on medium the air circulates and the temps are what we want. Thanks again for this article, we’re much more comfortable now!

      • So glad that it’s working out for you! These really are great when they’re working properly. But I think that they really messed up the ‘auto’ mode.

  4. two months ago bought a Daikin model FTK12NMVJU (inverter model, seer 19 efficiency, cooling only) and it is pretty much a waist of 3500 canadian dollars – I have the dreaded humidity problem – the daikin blows humid air back into the room. I also have a high end window AC Friedrich model cp10g10a (non inverter but a simple 100% on or 100% off model and seer 13 efficiency) which has non of those problems. After experimenting with daikin and friedrich i came to conclusion that the problem is the in way inverter AC functions. Although Daikin is 12000 btu model it can modulate between 4400 and 13000 btu when cooling, if 4400 is still to cold it starts cycling (turning the compressor on and off) and this is when the problems start. Since that blower is has to be always on when the compressor switches off the moisture from the the coil (which is not too cold at 4400 btu) somehow is not drained correctly and is blown back into the room – i could see the relative humidity go from 55% (which is already not too great) go to 62% within a matter of 15 minutes. Daikin can not reduce humidity lower 55% while the friedrich has no problem going below 55%.
    Condensation from Daikin is pretty much always a steady, mediaum speed drip, while friedrich on a humid day can turn into a small trickle. Also although friedrich is only10000 btu it cools better than 12000 Daikin. I was told by the installer that Daikin needs to be operated differently from friedrich in that Daikin needs to be on all the time as opposed to turning it on only when it is already hot, but the problem is that when it cools the space enought and starts cycling on and off, i will get a large spike in humidity. At this point as people already mentioned my only option is Dehumidification mode (low blower speed and very cold coil) and manually turn it off when it gets to cold. Very very disappointing and it seems to be a design flaw (inefficient condensation removal from the fins of the coil) or a it is just the way inverter functions.

    • That is a shame and unfortunate.
      One thing I would note, and I’m not trying to make an excuse for the unit – just provide some possibly useful ideas…
      If the humidity in your room rises that fast, it is highly indicative of a serious humidity issue with that room / your house. This is most likely due to humid air infiltration coming in through leaks around windows and doors. A second possibility is humidity coming up through the floor, if the house is over a humid basement or crawlspace.

      In a tight home, the humidity should stay low for quite some time after the AC turns off. It take a lot of moisture to raise the humidity level of the air in your house up appreciably. OTOH, it only takes a bit of an air leak to infiltrate all that humidity, negating the effect of your AC.

      • The interesting thing is that if i turn the Diakin off, it takes long time for humidity to go up, but if i let the blower work after the compressor starts to cycle on and off this when humidity spikes very rapidly. It seems like the humidity comes from insufficiently dry coil, And as i other people pointed out i did not have the same problem with 2 year old Friedrich which was supposed to be replaced by Daikin.

      • Id have to run some calculations to determine how much water would be required to increase the humidity. That actually could be informative. Can you give me estimates of the dimensions of the room (length,width, and height) along with the room temperature and humidity before and after. With that, I could calculate how much water would have to evaporate.

      • 29×19 with 8 ft ceiling – living room, kitchen and corridor one big open space with two doors leading to two bedrooms for a total of 900sf. The temperature was 24.1 with humidity 55% and after 23.9 with humidity 63% after Ac reached the set temperature and compressor stopped working.

      • the air that is coming out from Friedrich is 38 and the air from Daikin is 56.5 and both were on maximum cooling. Daikin has Powerful button which according to manual gives you the maximum cooling the unit is capable of.

      • That 56 degree coming out of the one unit seems a bit high comparatively. If that were a central system, giving maybe a 20 degree temperature drop, that would be normal. But mini-splits often have considerably more drop (colder air) which would help extract moisture from the air more effectively.

        The one trick with these mini-splits is that their charge is extremely critical. I was told by my tech that they don’t even let them put gauges on them because the amount of refrigerant that can be lost during measurement is enough to throw the system out of whack. While I think that’s a bit extreme, if your installer wasn’t top-notch, there’s a good chance that the amount of refrigerant in your system is not exactly to spec causing poor performance.

    • Coincidenally I saw a Daikin split in a Speedy autoglass office recently which had all this black mold on the fins and deflector blades. I wonder if this is also a humidity issue? I went home to check my fujisu inverter split and was relieved to find no mold. Upon inspection I did notice fine dust deposits on the internal fins. After three yrs, should the internal fins be cleaned? Would an air pressure hose be adequate to do the job? BTW- I have routinely cleaned the filters.

      • best to clean it yearly. I recently did mine using a compressor and an air nozzle and it worked pretty well. though now I’m also on a bi yearly service plan with my HVAC company and they dismantle it and do a proper cleaning.

  5. We purchased 9 of these units they have all failed within one year don’t buy anything from Fujitsu they sell garbage!!!! They provide no customer service most HVAC guys know nothing about these units. I think that they should refund anyone’s money who purchased these piles of junk. A failure of all nine units!!! within one year. Perhaps Fujitsu should be brought up on fraud charges and not allowed to sell any of their products in this country

    • I agree. Recently had two put in. If they are not on constant fan they will never reach the set temperature. Had one that they replaced the outside compressor then the inside unit and finally had to put in a remote thermostat to get it to keep the room at a constant temperature. Even then if you set it at 78 it may go to 80 or to 75. Junk. I had Sanyo units before and they were wonderful. Also these units are not so quiet when the compressor turns on. Would never ever have another Fujitsu in my home.

      • My Fujitsu just broke as well after just one year in the middle of a heat wave. I’m so dissapointed. I have no idea who to trust, AC guy said whole new system!

      • That should be within the initial warranty period. If a product breaks within the first year, it is clearly: a) a manufacturing defect; or b) installation error.

        It is most likely an installation error that led to the refrigerant leaking out. This is quite common among the new systems as they all use much higher pressures.

        If your installer will not fix/replace the unit at no charge, contact the local Fujitsu office and explain your situation. If you do not get satisfaction, write them a letter, explaining exactly when you purchased the system, when it failed, the name of your installation company and how they said it failed. Since it is such a new system, tell them that you insist on having the system repaired or replaced at no cost because this would only occur due to manufacturing or installation error.
        Copy the letter to the BBB so as to start a paper trail.

  6. https://polldaddy.com/js/rating/rating.jsI purchase a Fijutsu Multiport system with 5 air handlers. AOU45LXF2 is the main unit, the air handlers are,
    ASU18 RLF in the family room
    ASU15RLF in the living room
    ASU7RLF for the dining room, laundry room and bedroom

    The only one that works consistently and cools the best is the one in the bedroom.

    I had the complete outside unit replace after 90 days, it kept coming up with an error message and shutting off. They replaced the whole unit, before the unit was replace and the new unit, the system acts the same way.
    If we only have the family room unit on , which gets the most exposure the unit will blow out 65 degree air when it is 90 -95 outside.
    When you turn on any other units, that unit will rise up to blowing out 77-80 degree air.
    Doesn’t make any difference if all the units are on, if all are on, all blow out 78-80 degree air while the bedroom one is blowing out 64 degree air.
    Now if I turn off the main unit (18) in the family room, wait about 5 minutes then turn it on, after about 2 minutes it will blow out 65 degree air for about 10 minutes and then start going up back to 78-80.
    Since the outside unit was changed, logically it can’t be the outside unit.
    The installation contract check the levels of refrigerant and says they are OK. He has given up and can’t figure out what is happening and now no longer calls me back. It will be a year and starting our hot season and it would be nice to get them all blowing out 65 degree air.
    We set all the controls at 64 and fan at high. Never use the auto.
    Any ideas? What should I do now? I will have to call another contractor since the first one has given up.
    Thank you for any information.

    • We have had the same thing happen to us. They replaced the outside unit then the inside unit and it would never ever keep running long enough to cool the room. Three different engineers from Fujitsu came out and finally one said try a remote thermostat. Well it now stays running and the air coming out is in the low 50’s where before it would come out at 50 then go to 65 and then off. I would nerver ever again put a Fujitsu product in my home. We had a Sanyo before and the thermostat was on the remote not on the head and it ran very well. Who but an idiot would but it on the head. Thank goodness we had an installer that was relentless in getting it fixed

      • Well after a year of dealing with this issue and having had 3 other HVAC techs over to assess the issue, no one could figure out what the problem was. The original installer quit returning my phone calls and split town. SO here I had to spend hundreds of dollars and the aggravation on this Fujitsu system and was ready to rip it out and replace it with anything else but a Fujitsu. The last tech that came, went over the system inch by inch and figured out what the problem was. It was an installation issue where the original installer, incorrectly connected the return at a wrong port. So one unit’s return was actually matched with another rooms return. Once he changed that as well as fixing some horribly made flares which surprisingly were not leaking the unit worked fine. So an installer error cost me all these headaches. So much for Fujitsu certified installers, you get a bad apple in the batch now and then, I am just glad it’s finally fixed.

    • It may be defective. Has the installer come out to see if they think it is working?
      Mini splits work different from big central Air conditioning units. They can ramp the speed up and down so as to maintain a steady temperature rather than turning on and off. It’s it possible that this it’s happening? Ignore the other thermostat. They might just be reading differently. On the other hand, if there’s a huge difference, like you set the temperature to 70 and the temperature goes down to 65, then something may be broken.
      My best advice for now it’s to have the origin installer come out and see if it’s working properly.

  7. Thanks for all the great info. I just had a large 5-zone Fujitsu system installed in Northern California. We have two wall units downstairs for 1200 SF and one ducted system upstairs for 4 bedrooms / 2 baths (leaving 2 unused ports on the outdoor unit). The downstairs units are working great (thanks to your advice re: ignore the numbers and avoid the auto setting), but the ducted system upstairs seems really too weak, at least on the cooling side. Is this surprising, given that we are trying to cool about 1100 square feet? I suppose we could use one of the remaining ports to add either a wall unit in the master bedroom, or one of the “cassette” ceiling units. Do you have any recommendations in this regard?

    • In most cases, 1100SF probably is too large for one unit. Of course, it depends on the specifics – size of that unit, insulation of the room, windows, etc., but in general, I would likely use two heads, on opposite sides of a space that large. If nothing else, that would help to keep the temperature more even across the space. Otherwise, I’d be concerned that you’d have a large temperature gradient across the room, making it comfortable in one area but too hot in part and too cold in others.

  8. We had a mini split heat pump installed in our duplexes with one wall unit in each duplex. We were told that each wall unit would operate independently in each duplex. We haven’t been able to figure how to make that happen. From my research it looks like this system should be for one house, two different rooms, not made for separate living quarters. Any advice?

    • It shouldn’t matter that they’re in different units, they don’t know the difference. Each inside unit should have a remote control that allows you to set the temperature for each space.
      If you can’t figure it out, check with the company that installed it. They should be able to show you exactly what needs to be done.

      • From reading about some of the hidden dirt or mold buildup issues re. indoor units, it occures to me that I should check to see the condition of my flywheel or fan. My question is: how does one remove the cowling of an 15RSL3 unit? Do the ends come off? If so, which end?
        Also, as my unit elicits no visible signs of dirt buildup or odors, should I leave it alone?
        Incidentally, since conforming to your blog recommendations of cooling on manual fan settings and setting thermosatat at lowest temp, our humidity levels last summer were not an issue.
        Thanks in advance, for any advice offered.

      • If you shine a flashlight up through the bottom veins you can see the fan element to see how dirty it is. I don’t know that I personally would attempt to dismantle the unit to clean it out, you might be better off with a Shop-Vac or compressed air or a combination of the two. And leave the thorough cleaning up to a professional.

  9. Hi, i have a 3+ year old mini split. The AC works, but heat does not. I replaced the mother board, and problem persists. I made sure the freon was at the proper level (having read that low freon could affect the heating capability). My local service guy has no idea. Do you think the problem would be somewhere in the inside unit or the outside unit? I can buy another unit at an estate auction for cheap. Thanks for any help!

    • When the heat doesn’t work, does the air come out cold? Is it possible that the reversing valve is stuck?
      One of the pros who contributed to this discussion may be able to give some more tips.

  10. After reading all the above issues I wonder if I should get a mini-split. I have a mobile home in Santa Ana, CA. The central air and gas furnace runs too loud as the unit is right at the living room. I want to replace the AC and gas furnace with the mini-split to solve the noise issue. Is it worth doing so? Please help.

    • Well, you’d need at least a couple to cover the area. The noise from these comes from the blowers and can get noisy on high. But generally, they’re pretty quiet. From a reliability standpoint, they largely seem as reliable as the installer. I think the first generation of super efficient units made the coils too thin and they were more prone to leaks. I’ve had two units with no issues and one location that had problems, got replaced, then the replacement had problems! One Fujitsu and one Mitsubishi.

      • The minisplit will be much quieter and more efficient. However, you could get a free standing heat pump for under$500 per unit. These are easier to manage than a window unit and much cheaper than a mini split. At their lower cost, it would take years for the mini split to pay for itself and if it broke down, the replacement would be less than the repair bill for a mini split.

  11. Just an FYI, my dual mini split would not heat. It would just skip over the setting, but cooled okay. Called the repair person, it was the battery in the remote!

    • Thanks for sharing this information. That’s a surprise! Good thing you didn’t pay for a service call just to learn that! Hopefully, this will help others avoid this type of problem.

  12. hey T.D. mine has the same leaky problem with the wall unit how do i get to the drip plate or drain pan to see if it is clogged there at that part of the drain.

    • The answer involves simple grade 12 physics. Your problem has to do with not having an airtight envelope around your house. Either you keep windows and doors ajar or your house has large air gaps that allow warm moist air into your house. A typical AC unit cools warm air which then condenses. This creates water on the cooling coils on the indoor unit which then collects in the drip tray and drains to the outside. This process will dehumidify your house under normal conditions. If you allow warm air into your house by opening windows at night, this warm air will cool down and invoking Dalton’s Law of partial pressures, the cooled air becomes relatively more humid. Don’t feel bad…We made the same mistake in our first year with our Fujitsu. It runs so efficiently, that we weren’t concerned with cost, so we often keep doors and windows open.

  13. T.D…. you’re a genius. My story:

    I had a Fujitsu 12000 BTU mini-split, single unit put in two years ago. I moved into the house near the end of July, and there wasn’t much time to shop around, and staying without A/C was not an option. Various sources told me the Fujitsu was the way to go.

    The installer advised to “just leave it on auto, and let it do all the work”. It didn’t take long for me to see that “Cool” was a much better option than “Auto” in summer, but I left the fan on Auto. And it cooled… somewhat. On the most blistering days, the house was comfortable. But on less hot days… it was OK, I guess. My family would come over and ask “Dude, is your A/C on?”

    Then the first winter came, and I used it in “Heat” mode. It is supplementary to my electric baseboard heaters, so I’m happy with whatever it puts out. Cleaned the filters regularly, no issues.

    Come next summer, switched to “Cool” and away we went… until one day I noticed some brown streaking on the louvers. Looking with a flashlight, I also noticed moldy-like deposits in the airway, and on the blower wheel. PANIC! This was shocking to me because the filters were never, ever full to the brim with dust. Either the dust was bypassing them or something else was going on.

    So I called in a company to have it cleaned with the whole drip bag and pressure wash and everything. It cost $180.00 CDN. Hey, at least I learned how to take the unit apart. Could the “Auto” fan setting have contributed to the deposits on the blower wheel… such as when the fan stops, and the drip tray has water in it?

    Then I read this blog post, and a whole bunch of things started to make sense to me. You’re right, T.D., for the amount these things cost, they really don’t educate you very much on the best way to use them for your particular home.

    So anyways, since reading this post I’ve been using “Cool” and fan setting 1 and 2 (out of 4 possible), and my house has never been more comfortable. I use an independent temp and humidity sensor, and the humidity has gone down by at least 5-10%, since switching the fan mode. Okay, so I may be using a little more electricity, but I think the comfort is worth it. My next electric bill will tell the story when I compare it to last year’s.

    Now, accumulation on the blower wheel seems to be back, but nowhere near as bad as least year. Soon, I’m going to take the unit apart and attempt to clean the blower wheel myself.

    Thanks again to T.D. and everyone else who posted… it’s not easy to find proper info on these types of units.

      • I have 2 units that for some reason only come on for a minute at s time. Thrn go off anf the economy light snd timer light come on. As much as ive tried with the remote to cancel the timer and eco mode neither work. Please any info is helpful. My other 2 u its in the back of my house ate fine.

      • That sounds like it could be some problem with the control board. Perhaps a call to the manufacturer or installer might help.

  14. This article is somewhat correct. It is correct the temp settings are useless. However, if you run the fan set to medium or high, you will turn the room into an over humid swamp.

    The fact is thesw units do NOT dehumudify. It is a byproduct of how they are designed. As they cool the fins in the head unit, water condences on those fins. The fan then blows that water RIGHT BACK IN THE ROOM. You can test this yourself, set the fan to run, get a humidistat with a remote sensor and hold it in the airflow. It will register 100% humidity levels.

    Since the cold air it blows out can hold less water, you basically get a unut blowing water into the room. This will cause wood floors to pop, windows to condensate, and can cause mold.

    There is no solution to this, it is a byproduct if the design. Water will never flow off the fins faster than the fan blows it out into the room

    In my opinion these systems should be recalled and refunds issued as they are clearly a defective design that can cause unsafe conditions in a customers home

    • You are partially correct. Unlike central AC systems where you don’t want to set the air handlers to run constantly because that tends to re-evaporate the water off the coils, with mini-splits, the air distribution is generally overly conservative/too low for adequate circulation. Running them full time on medium to high permits better air distribution. You can easily see how much water is being extracted by monitoring the condensate line.

      I’ve used these systems for many years and have dialed in the performance for myself and clients and found that this type of operation yields far better cooling and dehumidification than the factory “auto” settings.

      You may very well measure 100% humidity if you measure 45F air coming from the unit. That’s because the very same air entering at 75F and 50% humidity will be 100% relative humidity at about 56F. Consult a psychrometric chart and you will see this. It’s an function of how relative humidity is defined. In fact, even if the unit is extracting substantial amount of moisture at 75F, say reducing it to 35% RH, it will still be 100% RH at 45F.

      Again, simply monitor your condensate lines. If water is coming out of them, then the unit is dehumidifying. In both Fujitsu and Mitsubishi units that I have monitored, ample water is extracted and rooms are left very comfortable with these units when operated in this manner. The main problems I have seen/diagnosed is when people use conservative settings, like 72F and “auto” fan combined with units mounted low in the room (contrary to the manufacturer’s recommended placement guidelines). In these cases, these systems act like they’re not working and result in “swamp-like” conditions.

      • I’m researching the mini split system for our ductless ranch home, a very solid mid-sixties era brick and plaster house. I need to make sure that the system will cool and/or heat to 66 in the bedroom, 72 for the rest of the house is okay in the summer. Just as important, can the heat be restricted to no more than 65 during the winter? We like it cold when it’s cold outside when we’re dressed for the central PA winters. My preference would be for the 60s all the time, but don’t want to pay for the required AC. We generally have our heat set at 55 in our bedroom in the winter. I’m worried that the heat pump won’t allow the room to get that cold. I appreciate learning from this article to force the system to do what you want it to and not use Auto.

      • Unfortunately, your questions can’t be answered without knowing more about your home. A proper installation would require a “load analysis” where they examine your home, it’s insulation, air tightness, etc. and compute an estimate of how much heating and air conditioning it might require under different circumstances. Even this is somewhat guesswork.

        I’ll give you anecdotal information – I have a 9,000 BTU Fujitsu heat pump in my master bedroom which is very well insulated with triple glazed windows on the north side and double glazed windows to the east. Lots of insulation in the ceiling. That unit has no problem maintaining 66, which is what we set it to every night for sleeping. During the winter, we have it on heat at a similar temperature, but it rarely runs because we get heating from other parts of the house.

        Many (but not all) homes with mini-splits have central heating of some sort and use the mini-splits for supplemental heating and cooling, allowing the type of spot-conditioning that you’re asking about, just perhaps not in that full range.

        I just checked my Fujitsu remote control and it won’t let me set the heating to below 60, so I’m not sure what to advise there.

      • TD, you are right on w your explanations. I too have been using auto since installing last spring and was used to turning down remote to 70 to provide best results. Cooincidentally I recently turned the remote to 68 and the fan on hi-medium in order to cool the back bedrooms too.
        The whole house seemed much more comfortable even though not that cold the next morning.
        I also agree that a built in thermostat seems counter intuitive. I wish it had been explained before I installed. Perhaps w Bluetooth or some other tech we may have remote thermostat resolution in the future? I’m sure these manufacturers are aware of this short coming. Thanks again.

      • Just purchased a 9k Mitsubishi split unit for master bedroom under 300 sq ft. Decided that since kids are now gone we can reduce heating and cooling the remainder of the house (4000 sq ft) and provide wife a more comfortable temperature while she goes through her female changes, not because of any issues with current whole house units. We have immediately experienced the humidity (62-72 RH) problems you mentioned. Called “Diamond” installer who came out and topped off coolant in unit and told me to reduce temperature to 65 from 69 and increase fan speed. He and I also checked the drain which seemed to be working. This briefly reduced the RH but during the night came back, actually next morning at 72RH. Called installer to inform the problem continued and he told me it was an infiltration problem & id need to run my whole house unit more frequently. I currently set the whole house units at 77 during the night and for this time of year in south Texas it seldom comes on. I’ve done what you suggested in turning down temp to the split a few more degrees @66 and reducing fan speed. This seems to have reduced the humidity to 57-58 but still not the range I’d hoped. Oh, I also used the dry setting which reduces it a bit but the room becomes a freezer and this end up being a poor option. Anything else you might suggest as I’m not confident the installer understands the issue as he has not run into it before? Almost feel like I’m doing something wrong!

      • It is possible that the installer is correct and you have so much air infiltration that the system can’t keep up with the added humidity. Is it humid there outside? What are typical nights like (temperature and RH)?
        I know with those settings (66F, medium speed), the system in my bedroom works great. But I’ve replaced all my windows and the room is very tight, so climate control is easy.
        If you have a dehumidifier, you might have to use that in addition to the mini-split. I would consider that a fairly extreme case, but it sounds like you’re setting the system properly.
        I would definitely check the room for areas where humid air could be coming in. Recessed lights? Leaky windows? Damp crawl space underneath? Shower not venting humidity well? Lots of house plants? There’s got to be a reason the humidity is so difficult to flush.
        Also try reducing the temperature a bit more and see what happens. It’ll probably be too cold, but it’s worth a shot.

      • Yes sit is very humid in south Texas but we are not the height of the season yet. Interesting that my central unit brings humidity down to 56 in bedroom …its only when I turn on the split it goes up. Wouldn’t infiltration affect both systems? Otherwise seems like I just wasted $3300 plus the cost of a dehumidifier.

      • One difference with the central system is that it’s pulling down the humidity of the entire house. The mini-split is just doing the bedroom, so if air from the rest of the house is circulating into the bedroom, then it would have to dehumidify that air too.
        When we use ours, we keep the master bedroom closed, so there’s minimal air/moisture exchange between the main house and the bedroom. Could that have anything to do with it?

      • Unfortunately, We do the same thing….close the doors to bedroom and bathroom. I’ll probably end up purchasing a dehumidifier so I can appreciate the cooling as advertised but I find this workaround annoying. I saw nothing about needing to do this or possible issues in my research or discussions with the dealer prior to making this investment. I think you may be correct in you assessment earlier in that there is an inherent design flaw or known conditions that customers/installers have not been educated on.
        Thanks for your attempts at helping!

      • Lower temperature with high humidity is really challenging since the air conditioning won’t operate if the temperature is already low. I expect the system will work much better at dehumidification when the temperatures increase and the air conditioner has to run longer to keep it cool. the mini splits should be better in general than the central system but probably there’s just too much humidity in the house as a whole for the mini split to deal with it while running minimally.

      • I am having a real problem with my new mini split. It will not stay running long enough to cool or heat the room on auto. Not much better on constant fan. They have replaced the outside unit as well as the inside unit and it does the same thing. It cycles on and off every two minutes. Do you think a remote thermostat would make it work. Or any suggestions would be helpful

      • That’s strange – I’ve never heard of one of these short-cycling like that.
        I’ve always had to set the fan to “medium” or “high” settings and set the temperature several degrees cooler than you actually need it in the room. For example, set the temperature to 67 and the fan on high and see what happens. Your room will most likely not cool to 67, but you’re forcing the system to run hard so that it can actually cool the room to a comfortable level.
        Try that and see if it stays running and cold air comes out and the room cools. If that works, then the system is ok.

        Yes, a remote thermostat can make a big difference. But first try my suggestion to ensure that the system actually works when you force it to work like this.


      • https://polldaddy.com/js/rating/rating.jsI ran it on 70 cool high fan. The rooms was at 69.4. At 12 noon the room was at 74.3. When I use auto fan I have to set the room at 64 to get it to maintain a temp of 74. Won’t the high fan use a lot of electricity compared to auto fan. I bought these for energy efficiency. They have now replaced the outside unit, the inside unit and blown out all of the lines and replaced the freon. Do you think they are working as they should? Thanks

      • The blowers are quite efficient. The main thing I’m trying to help you determine is if the system is working at all. That’s why I want you to try extreme settings. Try fan high and temp of 64 and tell me what happens. In my great room, I have to run like this to keep the room at 72 because we have lots of windows and poor cathedral ceiling insulation.

  15. Can anyone help me with this issue?

    My son’s Fujitsu remote has been set on 64 COOL (A/C setting) all day. We have a Honeywell thermostat in there that still reads 72 degrees. Why? Why won’t it cool the room? The installer told us it’s because the outside temps are between 60 – 65 degrees which isn’t conducive for any A/C system, including a split system, to cool the air inside the room. Is this true? We’ve tried all kinds of settings, Fan, Cool, etc. Nothing seems to do the trick.

    • As long as the indoor temperature is above the thermostat’s set temperature, the air conditioning system should cool. I use a Fujitsu to keep it at 66 in my bedroom every night, and most nights recently it’s been 40-60 degrees out and it works just fine! Your installer doesn’t know what they’re talking about.
      Do you know what temperature the air is coming from the unit? It’s possible that the blower is only running slowly. Try setting the fan speed to high and see what happens. Does the system work when it’s hot outside?

      • We’ve had a weird spring: 80-degree days and 40-degree days. Ironically, when we want the system to cool, it blows out warm (not hot) air. When we need the heat, it blows out cooler (not cold) air. Fujitsu tech support claims this is a refrigerant issue. We’ve had the installers here 5 times and without and they leave with a temporary solution that doesn’t last. We’ve since called another HVAC company and are waiting for a response. When the temps outside are moderate, everything is comfortable. But when temps outside go to extremes, the unit seems to fail.

      • very strange! Sorry it hear that. Hopefully, the new contractor will have better luck. It does sound like a refrigerant issue. But the operation sounds just weird. Have you ever measured the temperature? Sometimes it feels that way so I always like to get an objective measure of temperature since feel can be deceptive.

  16. Fujitsu units run DC fan motors which are extremely efficient and should be circulating air flow when in use. If the HVAC dealer would have spent more time educating homeowners on the theory and how a ductless unit works and the importance of circulating the air in the space humidity would not be an issue. The complaints you are finding are just that (or improper sizing). Obviously the salesmen/technician did not properly instruct the consumers on the proper settings of using the unit. And to argue your “on the wall thermostat”, many times ductless split units are installed in porches or commercial computer rooms. To add this thermostat in these environments is a terrible idea as temps vary and there is not a good location for this thermostat. Fujitsu could monitor temps through the remote BUT what happens when the remote is lost, batteries run dead, or left in the sunlight? I have personally sold and serviced endless amounts of ductless split units, most Fujitsu, and have not witnessed one complaint like your findings, as all my customers have been educated how these units work.

    • Unfortunately, not the case – this is definitely not a case of consumer misunderstanding. The ECU (electronics control unit) algorithms often don’t handle room conditions very well.
      Under ideal conditions, the units work like a charm. However, I’ve seen a number of situation where the system simply fails to perform well resulting in uncomfortable conditions with inadequate temperature and humidity control – it’s a fundamental problem with the way mini-splits are designed with integrated environmental monitoring.

      As one who as designed and programmed similar systems, I can state definitively that there are limitations to the way such systems work. Because the sensors are built into the wall unit, they can only monitor local conditions at the wall unit. If, for whatever reason, the conditions at the wall unit differ from those of the room in general, the system will fail to properly control the temperature/humidity in the room.

      Mitsubishi has tried to get around this by building in a “electronic eye” sensor that reads temperatures in the room itself and adapts to this. But the Fujitsu units are limited to sensing conditions at the unit itself.

      There are circumstances where the head units cannot be mounted optimally. In those situations where air-flow is not exactly as the mini-split designers intended, the control systems work sub-optimally. It doesn’t mean the systems are defective, but they do need to be used in ways that differ from the instruction manuals. One cannot simply set them to “auto” and a given temperature. In these situations, you have to force the system to circulate and set a temperature lower than desired (assuming air conditioning), in order to get proper room conditioning. This is a fact of life with these systems but a fact of life that the manufacturers and installers often don’t properly understand or tell the users. The result is uncomfortable and unsatisfied customers.

      • My point exactly- You are comparing a ductless split system to a conventional forced air system as so do your examples. I am not arguing with you statement above. Point of the matter is Fujitsu units in general provide more accurate heating/cooling and humidity control when the fan is circulating. Air circulation provides added comfort and for these units, better control. HVAC manufacturers and contractors recommend air circulation (even on a forced air system) due to changing environment conditions. Yes, a customer could witness excess humidity/temp if not running the fan which can be magnified if the unit is over sized. IF the unit is sized correctly and it is an inverter driven compressor this becomes less of a issue. Ductless splits rely on air circulation (picture a wind tunnel) to provide extremely efficient heating, cooling, and humidity control. This delivery method is very different from a traditional forced air system and consumers should be educated on the difference and how to get the most out of their Fujitsu unit. Have you attended a Fujitsu F.A.S.T training seminar? If not I would strongly urge. This is all explained in detail and they can answer all your concerns.

      • Andy, I’m not sure I understand you on this: “Yes, a customer could witness excess humidity/temp if not running the fan”
        The fan in the unit must surely run whenever the compressor is running. What do you mean by “not running the fan?”

      • There is a setting on the remote for all Fujitsu units to allow the indoor fan to run when the outdoor compressor is not. This is what the industry calls fan or blower circulation. The indoor fan is always on, filtering, and providing consistent temp and humidity levels.

      • It is the same common settings as selecting from heat to cool or temp up or down. There is no factory default fan or temp setting for that matter. A good installing dealer will explain to the consumer of these important settings and what affect they have on the system.

      • There is a defacto default setting which is “auto”. This setting however runs the fan on such a slow speed that it doesn’t adequately circulate air. This is why I’ve had to advise people to force the fan to medium or high and set a lower than desired temperature. Only then does the system provide the desired level of air conditioning and dehumdification.
        So if one considers “user education” to be telling them to ignore the manual, then yes, this is a user education issue.

  17. Throwing a new question into the mix… Recently had a 6 zone Mitsubishi minisplit system, for heating and cooling, installed in my home. One thing I am noticing is that there appears to be a minimum thermostat threshold of approximately 72 degrees when operating in the heating mode. The units do not heat continuously, they circulate the room tempered air for the most part and only kick in now and then to keep the rooms at ~72, as evidenced by separate thermometers in each room, no matter if I set the thermostat to 60 or 70. There are three different btu sizes installed; 6K, 9K and 15K and they all exhibit this same issue. In practicality, I can not keep the individual rooms less than 72 degrees, (regardless if the outside temperature is 60 or 30). Has anybody here experienced the same thing, or do you know if there is a way to adjust the seemingly hardwired heating threshold? Thanks!

    MSZ-GE06NA (x4), -GE09NA (x1) and -GE15NA (x1)

    • That’s interesting. I haven’t experienced that with either the Fujitsu or the Mitsubishi units. So if you set the temperature to 68f and it’s 40f outside, it still warms the room to 72? If so, that’s not right!
      If nobody else has an explanation, I’d urge you to ask your installer.

      • We experienced something similar and the only thing that changed was first, we reset our remote controls (hold down little pin button). And now we run the fan on high – not auto. It’s helped.

  18. So happy I stumbled onto this thread. I had a Mitsubishi Mini Split (MUZ-D30NA & MSZ-D30NA) installed a little over a year ago. I’m here in Central Florida and use it for a room (25×15) that was once a porch and converted to a room. When we bought the house, we had the open wood beams (flat roof) enclosed with drywall, added significant insulation, and had all 6 windows replaced with higher end double pained windows. Under the right conditions, we get a horrible musty smell in the room. Sorry for the length, but I am going to share the email that I just sent to the installer for the big picture of our situation. Any comments or thoughts are much appreciated!

    We still have an issue with the smell that occurs under certain conditions with our Mini Split. The recent cleaning, although good probably for our overall health, was not the fix all. The problem that I have perceived from day 1 (an oversized unit), still remains to me like the culprit. Please note I have spent countless hours researching this problem. I have tracked it via a spreadsheet during a typical day and can reproduce the problem at will. I am asking for more help as I live in this room all day. I am hopeful that 1) most importantly, we can find a resolution for my problem, 2) it deepens everyone’s knowledge of this “situational event” that will help you better help other customers in the future. Please note that I even went out and bought a dehumidifier as a test this week. If it weren’t so noisy, I would consider using it as a band aid fix.

    Issue statement:
    Our problem centers around a foul smell during certain conditions. Here is an example that will trigger the problem: The outside temperature is 80 and we want to cool the room to 76. The unit will cool it down to 75-76 degrees and all is well doing so. But now that the inside temperature is at 76, it’s just a matter of time before a pungent / moldy odor occurs. It’s so noticeable that our separate air purifier notices it, lights up, and kicks into high gear. Our digital thermometer shows the room humidity higher than it was before the unit cooling the room down. If it’s 90+ degrees out and the unit is running to keep it 75 or 76 degrees, there is no odor. Again, when the change in temperature (delta) from outside to inside is minimal it happens like clockwork. Hence, the issue is worse at night, when it’s 78 out and we want the room at 75 or 76 degrees.


    Switching the unit to dehumidify mode, clears the smell.
    Dropping the temperature, clears the smell.


    We can never set the unit in auto mode (setting 1 on the remote) , where it can keep it at a comfortable 76. It will sit there in a low throttle and stink up the room.
    In cool mode, we have to lower the temperature, and everyone freezes.
    In a typical night, we set it on 76. When it gets to 76, after a short time, the room starts smelling. I then flip it to dehumidify mode, it drops the humidity, drops the temperature, and the smell goes away … but we freeze. I turn the unit off until we warm up, then restart the whole process again. Likewise, I can drop the temp to 74 for a short time to help combat the smell. We have done this for the past year and I am exhausted.
    I dream of the day of being able to set it at 76 and just forget it.

    Other important notes:

    The issue is not that the unit is dirty and omitting a smell (irregardless of the temp). We had it cleaned and there is no change.
    If I run my separate dehumidifier at a target of 45% humidity, I can run the AC around 76 and it does a pretty decent job of keeping the room fresh.
    If the issue was that I have an overly humid room, why is there no smell when I enter the room in the morning with the AC off all night? The humidity in the room shows 47% on an average morning. When I run the AC for a short time at 76, the humidity rises to over 50% and the room starts smelling.

    • your heat pump is way over sized for the room its in, the model number indicates to me 30 000 btu, you would be happy with 12000 maybe 18000 btu/hr.

    • That’s quite a mysterious problem. Usually odor issues occur on a much longer scale. Having it happen in a matter of minutes or hours means the source of the odor is there all the time but something about the specific conditions is bringing it out.

      One item should be noted. You probably are aware of this, but I’ll reiterate it for others reading this.
      – If you lower the temperature of the air but the actual amount of water doesn’t change, the humidity goes up. This surprising fact is due to the definition of “relative humidity” (RH%) which is what humidity gauges measure. Because of this, when any air conditioner drops the room temperature, the air will have a higher RH% until the air conditioner condenses the water out of the air.

      It’s quite surprising that even dehumidify mode causes the room to get too cool. When I run dehumidification mode, it runs the fan so slowly that you can hardly feel the air movement. Is the system located in such a way that where you sit is directly in the path of the airflow?

      Back to the odor – high humidity air does draw out odors, so I’m not surprised that the odor would become more objectionable under those conditions. But again, the odor would have to be present in some of your building materials – in your walls or under the floor in the insulation maybe? And the poor dehumidification is creating just the right conditions to “release” the odors.

      You note that your air purifier senses the stale air and comes on? Can you tell me the make and model so I can look at the documentation? It would be interesting to know what it’s capable of detecting.

      What’s under this room and what is the construction? In most cases, I’ve found odors come from a damp crawlspace under a room, where there is not an adequate air barrier between the crawlspace and the room. This is common construction here where you might just have a tongue & groove flooring set over joists with fiberglass in between and a dirt floor below. Anything like this exist in your room?

  19. The notion of inaccurate temperature control may account for a lot of problems, but there could be other things in play when humidity control is poor. I go back to the problem that many people have with humid basements. If you don’t have one, you know a few people who do. Some are legitimately caused by a high moisture source like a dirt floor or high water table, but most that I come across are caused by air leakage. People love to “ventilate” their damp basements to “dry them out”. Basements are not inherently damp, but they are inherently cool. In summer, the floor is 55 or so (New England) and the lower walls are cool as well. If you take humid summer air and cool it to basement temperatures, it gets very damp and musty. The summer sun on the house creates a stack effect that just sucks tons of air in through every basement leak and opening.

    The same thing, to a lesser degree, will happen in a leaky house that is air conditioned. While it’s true that the mini split is a closed system, the house may not be. The summer sun will create a strong stack effect in a poorly insulated drafty house, and the air will change quickly. Outside humid air will be cooled and the relative humidity will rise. Condensation in the mini split can only remove a portion of the vapor, and as it is loaded with latent heat, it will be less effective in lowering the sensible heat.

    Because many splits are single split systems, people tend to treat them as spot air conditioners, trying to cool just a portion of the house as an oasis against the summer heat. They frequently open windows in other parts of the house that they are not trying to cool. Unfortunately, those windows allow moist air in which becomes more humid (relatively) in the mini-split cooled room.

    Just like with heating, you need to reduce air changes and keep the house closed to air condition effectively. Otherwise, the latent heat in humid summer air will overwhelm a single mini-split.

  20. My later model Fujitsu heat pump has a ‘Dry’ mode or dehumidificaton which probably works in the same manner, ultra cold temp, slow fan speed mode.
    It’s not the heat pump’s “fault” for the lack of dehumidification as it doesn’t even bother to check that, nor could it probably control that without altering temps to an undesirable level, similar to most basic home HVACs systems that would operate the same way.

    • Running the system in Dry mode was one of the many tevhniques we tried over the entire 2014 summer. It did reduce the humidity, but would not stop cooling when the temperature reached the set point. I’m told by our contractor that thrre is no shut off festure in the Fujitsu umits, as there is in the Mitsubishi unit.

      • So, under those conditions, the air conditioning requirements of the room had to have been quite low. In that case, the workaround would have been to set the fan speed to “low” and set the temperature to a temperature close to the desired temperature, maybe 2-3 degrees cooler to compensate for the “cold pooling” effect we’ve already discussed. Then, the unit would have stopped air conditioning after the room cooled down.

        It would definitely be advantageous if these units had humidity sensors (which are dirt cheap these days) so they could optimize the operation when needed primarily for dehumidification.

        Dick – I’m going to write a little article touching on the humidity issue you sent me – where the indoor humidity is higher than the outdoor humidity. There’s a good reason for that.

    • I believe Dick noted that he tried the ‘dry’ mode and that didn’t really work effectively for him either.

      Dehumidification is a by-product of effective air conditioning. This is why, historically, installers have been very concerned about properly sizing the air conditioners to the requirements of the house – ideally, you want the system to run for long periods at an output that is “just enough” to keep the house at the desired temperature. This is one reason that mini-splits can be so great – their inverter drives allow the compressors to run at a variety of speeds, tuning the cooling output to the load.

      The problem, based on my multi-year observations of my Fujitsu RLS units, is that their control systems do not deal well with less-than-perfect installation locations. In order to get them to operate as one would wish, you have to “game the system” by overriding the automatic settings and running them manually – set the fan to the desired speed and turn the cooling down a few degrees colder than you actually need in the room.

  21. Ted,

    The a/c only unit was installed about 2006. Impossible to know for sure, but it may have had the thermostat in the remote. Do you know if that’s the way mini splits were originally designed? Could it be that, in the interest of reducing costs, manufacturers went to the current design of incorporating the thermostat into the unit. ( some engineer probably got a big bonus for coming up with that cost savings).
    Do you think that using a remote that has the thermostat imbedded would solve my problem without going through the process you described above and which I probably won’t remember?


    • The remote thermostats I’ve seen are separate units, wired to the main system, like this

      I’m afraid that I can’t comment on the older systems. If I were to hazard a guess, I’d think that the control systems may have differed between the units. Having designed some (unrelated) control systems, I can say that it can be a non-trivial problem to handle the varying installations and usage conditions, especially in a system like this where you’re asking the system to “guess” at the temperature conditions in the room based on a sensor that’s so close to the source of the cooling. In fact, the new Mitsubishi has a “moving eye” that is supposed to scan the room temperatures so that it can better control the operation of the main system.

      I think the wired remote thermostat has a good chance of operating properly. I wish it had a humidity sensor built in too, but maybe that’s not necessary.

      I’d recommend you try the test first of running the unit with the fan on medium and a temperature lower than what you’d think, maybe 4 degrees below the actual room temperature that you’re shooting for. Check to make sure that there’s a lot of very cold air flowing out. Then monitor the humidity over the next hour or two to ensure that the humidity is dropping. I want to ensure that the systems are capable of working properly and that there isn’t actually some weird issue that’s preventing them from working at all.

  22. For what it’s worth, I’ve had none of these problems; my heat pump was set to 68F yesterday, and the thermostat in the adjacent room measured 69F this morning. Works ok for me. 🙂
    I’m still not quite clear on how you decided that the co-located thermostat accounts for the strange humidity problem, though?

    • Eric – the problems I’ve seen multiple times with these units is that the temperature surrounding them does not reflect the temperature in the room. Unless the air flows exactly right, you get a lot of cold air near the unit that simply causes it to ramp down the cooling/compressor.
      This could be considered an “installation problem” but it’s really a poor design. The unit should depend on perfect air flow in its vicinity in order to function properly.

  23. i do not recommend running any mini split lower than 72 degrees, you risk freezing your inside coil then leak refrigerant( also my suspect why units are running and not working)., you complain about the stat sensor being in the coil, but there is reasoning behind it., i find usually there is 2-3 degrees diff between coil and room temps, but every house and model are different

    • Ted,

      Thanks for that in depth explanation. Do you have to do the same thing with your Mitsubishi unit?

      In a prior home, we had a Mitsubishi mini split, but it had only the a/c function, no heat. It always performed exactly like most a/c units. Cooled very quickly to the temp set on the remote thermostat and then came on as needed to maintain that temp, while also keeping the humidity at a comfortable level. Never had to “game” it.

      At the time we bought that unit, there was a $400 premium to get a model that also provided heat, which we did not need. We are also now only interested in the a/c function from these Fujitsu units, but I was told that there was no price differential any more so we went with the heat pumps. Do you know if they still manufacturer a/c only units? Would we have avoided the problem we have experienced if these were a/c only from either manufacturer?


      • Dick – after my Mitsubishi was installed, replacing the Fujitsu, it did in fact behave the same way. I admit it is poorly located for air conditioning due to physical constraints, so I’ve purchased a remote thermostat for it that will be installed this spring.
        Interesting that you’ve had different results from the two units though there certainly could be other factors involved. You would think they both install the temperature sensors in the return air stream.
        I’ve got sensors on mine, one located in the input and one on the output. I’ll have to actually analyze more aspects of the settings vs. air temperatures etc.
        I’m doubtful if heat-pump vs. a/c only would make a difference. There’s certainly more moving parts, so to speak, in the heat pump, but it would be very surprising if the would be defective on multiple units.

    • I’m sorry Jim but I totally disagree with this statement as I’m sure the manufacturers would.

      Many people run air conditioners cooler than 72 without a problem and I’ve never heard of a freeze-up issue. I run mine regularly in the 60’s in order to get the room air comfortably dehumidified and down to the low to mid 70’s.
      Note that this is in a large living room, where I really don’t expect the unit to be able to handle 100% of the cooling needs. In our bedroom, the system is installed “by the book” near the ceiling, and it works quite nicely when set to 66F nearly every night of the year, keeping the room somewhere between 67-69F. Not once have I had a freeze-up issue with the coils, even under very humid conditions.

      • The prior a/c only unit was in a guest bedroom so we didn’t run it very often, but when we did, the temps would be set at 66 or 68 and the unit would take the room to that temp very quickly and keep it there. Never had a problem with it freezing up.

      • Would the potential freezing issue be as a result of low fan speed?
        We had a dehumidifier that began freezing up when low on coolant.
        Could this also be a factor?

      • I don’t believe the low fan speed would cause freezing, at least it hasn’t in any situations I’ve experienced. However low refrigerant is often the cause of freezing of coils. Having low refrigerant. Having lower for refrigerant allows it to attain vapor state to early in the coils which causes them to freeze up

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