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At night during the summer, we run the kids room mini split with the door closed, and I've noticed the humidity gets really high (measured at > 70%). This surprised me, as I assumed the AC would be removing moisture from the air - maybe not so much as to lower the relative humidity, but ideally not so little the the humidity increases substantially as the air cools. To check if the mini-split is actually removing any moisture from the air, I put a bucket under the drain line outside, and discovered that very little moisture is being removed.

I think the issue is that the unit is too large for the room (with the door closed). Here is what I think is happening:

  1. The unit turns on, cooling the room, and condensing some moisture on the coil.
  2. The room quickly reaches temperature. Very little of the water that condensed on the coil has made it to the drip pan. The compressor turns off, but the fan keeps running.
  3. The water evaporates off the coil and back into the room. The evaporation is exacerbated by the continuously running fan.
  4. Eventually, the temperature rises. Repeat 1-3.

One solution would be to open the door. Another would be to get a dehumidifier. Let's assume that those aren't acceptable solutions. Instead, I would like to see if I can get the mini-split to remove more moisture from the room.

My thought then is to cover some substantial portion (maybe half) of the coil on the mini split head so that 1) the unit has to run longer in order to cool the room, and 2) more water is condensing on a smaller surface area. I reason that both of these should increase the amount of water that reaches the drip pan and exits the house.

What are the main risks here? Or is there some reason this definitely won't work?

Edit:

  • The unit is a Mitsubishi MSZ-FH09NA
  • I am fine using more power to pull out the moisture. I assume that this will necessarily be the case.
  • I don't know if this model is capable of multi-speed operation. At least I haven't seen any mention of this type of user-programmable setting in the manual.
  • The unit does have a dry/dehumidify mode, in which the compressor runs continuously under low fan speed. It works great for dehumidifying. The problem is the unit does not have a humidity set point, so running in this mode all night would make the room unacceptably cold. (as an aside, if anyone is aware of a system that has either a) built-in hygrometer and dry mode with a set point, or b) even better, a dry/cool mode with simultaneous humidity/temperature set points, I would really love to look into such a product.
  • There's lots of good discussion around my hypothesized underlying cause in How to I eliminate the smell coming from my Mitsubishi mini split units?, though the focus is on the associated smell.
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  • What happens if you leave the door open? It sounds like the unit is oversized for the room to have the door closed. Some of the newer units have a dehumid function that prevent exactly what you described.
    – RMDman
    Jul 28, 2023 at 21:48
  • Is this mini-split a type that uses "Bang-Bang controls"? Yes, what you describe is a known problem with traditional bang-bang A/C system sizing... but this is deemed a deficiency of bang-bang, not a law of nature. One elegance of mini-splits is that they can run at a variety of speeds, so they can dial in a low speed and cruise there continuously, averting the problem you mention. Is your unit capable of that? Jul 28, 2023 at 21:50
  • Put the mini-split in dry mode. Nearly all of them have that. Of course we can't be certain that your make & model does because you need to edit that information into your question. It will use more power than cool mode (pulling water out of the air costs energy...)
    – Ecnerwal
    Jul 28, 2023 at 22:18
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    spray soap suds on the coils to make them shed water droplets faster, so that they don't stick and re-evaporate into the room. repeat periodically.
    – dandavis
    Jul 28, 2023 at 23:05
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    @Evan, we have 3 Mitsubishi units installed 2 years ago and experienced the short cycling you described (fast cooling but no dehumidification). We had some success with a 3rd party product called Cielo Breeze that allows control according to humidity level. We've tried other things since then but we're still struggling to get humidity below 57%. No longer using cielo breeze.
    – bugdrown
    Aug 6, 2023 at 2:55

1 Answer 1

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Humidity is actually relative humidity -- it's a ratio of the mass of water vapor in the air compared to the amount that the air is physically capable of holding. Warmer air can hold more grains/pounds/grams of water vapor. In the evening when the sun goes down and the temperature drops outside, this is the reason why we perceive a higher humidity outdoors. As the temperature in the room drops the relative humidity rises, unless that vapor gets condensed out of the air somehow. (You already know this of course, but maybe it's useful for other readers.)

I think your theory is correct: the room reaches the set temperature quickly, before much de-humidification can take place. Also the heat load on the room is presumably reduced because it's night time, and the humidity load in the room is increased because there are warm bodies breathing in there.

That system doesn't have a dehumidify setting..?

Your risks in trying stuff seem minor. Perhaps the worst risk is that reducing air flow on the indoor unit could cause it to form ice -- but a mini split probably has a temperature sensor on the coil and it'll refuse to ice itself. Even if it does ice, that's easy enough to resolve.

Reduction of the airflow makes sense. I'd try it. Maybe it's as easy as cutting a piece of poster board and sliding that in where an air filter element belongs.

You could try running a space heater in the room to cause the mini split to work harder to cool. I'm not sure I'd want to make a habit of that because of the operating expense, but for the sake of science for a night or two, it would be an interesting test.

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  • Ignoring cost, the heater test is an interesting idea. I would locate the heater on the far wall from the minisplit head so that you don't have a false temperature reading. Jul 28, 2023 at 22:49
  • +1 The space heater will absolutely work (at a cost). I've done it, once for a server room that had chronic high humidity, once in my own workshop running the A/C and a pellet stove at the same time to lower the humidity to get a longer tack time on polyester paint. I dropped the RH by 60 percentage points in under 2 hours.
    – MTA
    Jul 28, 2023 at 22:52
  • Thanks for the space heater suggestion. Seems like if I was going to add a second appliance, it would be a dehumidifier, which I assume would be a) more efficient and b) allow me to achieve a specific desired humidity. But the space heater would be quieter.
    – Evan
    Jul 29, 2023 at 14:07
  • @Evan Incidentally, a dehumidifier is not so different from a small air conditioner: it's a refrigeration unit with the evaporator and condenser in the same space. It dries the air, yes, and its operation produces some extra heat in the space too. It seems like the dehumidifier should be more energy efficient than mini split plus space heater - but I wonder by how much.
    – Greg Hill
    Jul 31, 2023 at 0:25

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