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.