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Recently, I've been cut off of access to sun and fresh air to my apartment. It is completely sealed now, with no windows or sun to the walls or roof. Ever since, humidity has grown a lot, and mold is spreading everywhere.

The climate where I live is an "always humid subtropical highland climate". That means it rarely gets extremely hot or extremely cold. Because of that, air conditioners are not very popular around here.

My first decision was to look for a dehumidifier, but after some research, and after contacting a manufacturer, I decided it won't solve my problem, because the dehumidifier is made for hot climates only - it has suboptimal performance below 18°C, and will freeze and stop working below 13°C. A climate of 0-10°C is common during winter, when it also happens to be more humid.

So my next option is an air-conditioner, which could give me some fresh air besides dehumidifying. My apartment is ~30m² (~323ft2), and ~3m (10ft) high, and I live with one other person. Some guides written in my language show I'd need 18,000-21,000 BTU, but they seem to consider only hot weathers, which is not my case. A calculator in English considers cold weathers, and it tells me I need only 8,384 BTU, which is less than half.

So, considering I need an air-conditioner mostly for dehumidifying on a cold weather (~5°C/40°F), what BTU power would I need?

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  • Do you not heat indoors above 13°C in the winter? If you do, a dehumidifier should work fine, and be a much better choice than an AC for cold weather usage. – Nate S. Jun 27 '19 at 18:42
  • Hi! No, we don't heat! Not common where I live! It gets cold in the winter, but it never snows here. – Teresa e Junior Jun 27 '19 at 18:45
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    > does not heat > cold damp think I found your problem – Harper - Reinstate Monica Jun 27 '19 at 20:02
  • @TeresaeJunior, interesting. Are you okay with the fact that an air conditioner will make it even colder in your house, when it's already cold? ACs and dehumidifiers work on the same principles, and the only real difference is that an AC exhausts its waste heat outside, whereas a dehumidifier exhausts both heat and cold inside (so it doesn't change temperature much). Both are less efficient in the cold. – Nate S. Jun 27 '19 at 20:06
  • You might consider using a dehumidifier with a small space heater pointed at its intake, to keep it within its optimal temperature range. I suspect this would do much more to improve your comfort and air quality than an air conditioner would. – Nate S. Jun 27 '19 at 20:10
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"Dehumidifying cool areas" is a serious trick

It's a common problem in England too, where it's called "the damp".

The crux of the problem is warmer air can hold more water than cool air. 100% humidity at 10C may be 30% humidity at 25C. That's how air conditioners and dehumidifiers cool the air. If they chill air to 10C, it can't hold much water, so water condenses until it is 100% at 10C. Then it warms back up to 25C and is only 30% humidity.

However the refrigerant method breaks down at low temperatures, because it's hard to get much cooler than 10C without the evaporator coils freezing.

You have two ways to go from here. You can then heat the air (which dries it out), or you can use a dehumidifier that is efficient at 10C. The best bet for that is a desiccant dehumidifier. It uses desiccant (similar to the little pouches found in medicine bottles and consumer products) arranged in a disc. Air blows through most of the disc, allowing the desiccant to attract moisture from the air. In a small wedge of the disc, hot air blows through to dry out the desiccant for the next pass. This hot, humid air must be ejected outside, it cannot collect water in a bucket.

If you want bucket collection, you are stuck running a refrigerant dehumidifier inefficiently.

Heat alone is enough

The simplest way of dealing with cold-temperature humidity is to warm up the air. The warmer air holds more moisture, which lowers the relative (%) humidity level.

Using an air conditioner as a dehumidifier

Since they work on the same principle, an air conditioner won't be any more effective than a refrigerant dehumidifier. It simply disposes of the hot air externally, causing net cooling.

A BTU is the energy needed to raise 1 pound of water 1 degree F. A pound of water equates to a pint equates to a half litre.

Water's latent heat of vaporization is well known as 970 BTU (per pound, obviously). You need to move that much heat to vaporize or condense water. Now, in the HVAC business, when they say "BTU" they actually mean BTU per hour. But that gives us a rough idea of the dehumidification power of an air conditioner: a 9700 BTU air conditioner will wring out 10 pounds/hour, or somewhat less than that due to inefficiency.

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  • @TeresaeJunior, just the opposite. An AC will cool the room while dehumidifying, while a dehumidifier will heat the room slightly (though not much). And if the AC does have a heating mode, generally it will not also dehumidify when it's in that mode. – Nate S. Jun 27 '19 at 21:15
  • Dehumidifier and air conditioner are the same thing: evaporator, condenser, compressor, refrigerant. The compressor pumps heat out of the evaporator into the condenser. We call the system "air conditioner" when the condenser and compressor are outside the controlled space; we call it "dehumidifier" when those two are inside. So yes a window A/C running in the middle of the room (not in the window) will add heat to the room. A dehumidifier would, too. I guess a dehumidifier probably tends to run the evaporator colder and maybe passes less air compared to a unit optimized for A/C, however. – Greg Hill Jun 27 '19 at 21:52

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