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.

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

  1. https://polldaddy.com/js/rating/rating.jsHi, Last April I had a Trane Minisplit 18,000 btu 16 seer installed in a 3/4 glass enclosed room to replace a 20 year old motel a/c/heater that was supposed to save me heating and cooling costs. The room houses an endless pool that is covered, so it does not put off a ton of humidity. When uncovered for swimming, the windows are opened. With the motel unit, the humidity was never an issue. With the minisplit, the humidity gets up to about 75%. I had to install an inline dehumidifier with a pump, and my energy costs went up for the summer about 50%. . It’s really turned out badly. Now in winter, I have it set to 61 degrees. I live in the South, but it’s been really cold here–in the teens. However, yesterday it was in the 30s and high 20s at night, and the temp in the room did not move. It is not heating. I’ve had two errors, one being H3, but after popping the breaker, they resolved. So, currently the room is only heating to about 54 degrees. My electric bill almost doubled from last year. I have a thermometer/hygrometer on the opposite side as the unit. The room is about 10×26. The installer is coming out on Monday, but basically he’s at a loss, Maybe this was not the best place to put a unit like this, but it was his recommendation and very expensive. I will try your fan recommendation. Maybe the unit is a lemon. What kind of heating/cooling should I be expecting? I will also try to run the fan full timer in the summer as per your recommendation. Thank you for your help!

    • It really seems like the control software on all these mini splits is their weakness. For most efficiency, they try to run super conservative, like cars in Eco-mode -they can be unusable!
      I’d just go constant fan and colder temperature by a few degrees.

      • Thanks. I’ll try that. What about the heat? Should it be able to heat the room to 65 when it’s poorly insulated and 25 outside?At what outdoor temp does it start having heating issues? Thanks. 🙂

      • Same for heating – you want to turn up the temperature beyond the actual temperature that you’ll need under certain conditions. The amount of BTUs needed to heat the room is impossible to tell without analysis. However, knowing the specifications of your old heater would help because that would tell us how much heat did work in that room.
        With an 18,000 BTU unit at low temperatures, it wouldn’t be surprising if it only output 12,000 BTUs. That depends on the exact specs of your Trane unit. But I wouldn’t be surprised if it didn’t have the capacity to heat a poorly insulated room of this sort.
        You can run simple tests – just turn the heat up to 75 on the unit and let it run full blast for a few hours. If the room gets warmer, then it is capable of adding heat to the room at these cold outdoor temperatures. It’s really that simple to find out.

    • all you can really do is set it down to its lowest temperature period for my units that is 60 degrees. let’s blow that and you start running other risks but if it allows 55 is probably safe also as long as there’s no significant sources of moisture in the there, which there probably aren’t

  2. my Bryant mini split works well in the summer. Now with these cold temperatures it will only blow almost cold air. I always have it set on 71F and the air coming out is 56F . The dealer claims it wound warm with this cold. But isn’t that where i bought it for? What to do?

    • most modern Mini Splits should work down to fairly cold temperatures. in no case should they be putting out 56 degree air, at the coldest it should be room temperature. if it’s actually putting out 56 degree air that means it’s running an air conditioning mode.

  3. I have this unit in my kitchen for a 900 sq ft. home and it’s the only source of heat that I have. Currently, I can’t get it to direct the hot air past the kitchen into the living room (it’s an open floor plan so no doors are blocking.) How do I adjust the temp/air flow to get the warm air into the other rooms of the house??? I’m freezing here, please help!!

    • That’s the challenge with these units. They’re really designed for single rooms. Beyond that, you have to count on natural air movement to carry the heat around the house.

      I have used “helper fans” to assist in circulating air around the house which helps somewhat. You can either get the little fans that clip onto a doorframe and push warm air from the kitchen to the next room, or just a normal fan, to pull cold air from the adjoining room into the kitchen. I can’t say that it’s terribly useful, but it will help a little.

  4. I have a unit that will not get warm…I have the remote set for 78 degrees, I was scared to go higher because I didn’t want mess up the unit….but I have more confidence of what the problem may be…..

    • Quite possible that it has a leak. I wouldn’t advise running it as you can destroy the compressor if the lubrication has leaked out with the refrigerant.
      On the other hand, are you trying to get heat during extremely cold snaps? Heat pumps can struggle to heat when the temperature drops below a certain point. Usually, around freezing, the heat output will be noticably less.
      If it fails to generate heat when it’s not so cold,then its time to call for service.

      • I will do that….but the connected unit located in another apartment runs fine…that is why I’m so frustrated……but thanks responding to me..

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