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First, my calculations. Let's assume a sealed 5x3x2 meters room and ambient temperature of 20 C. Volume is 30 cubic meters. According to this table, at 100% relative humidity there would be 17 g of water vapor per cubic meter or 510 g in the whole room. Henceforth a 10% drop in relative humidity should require about 50 g of water to be removed from the air.

The problem is that I'm getting about 100 g of water from my dehumidifier every day and (according to the psychrometer I have) the room is still at 70-80% relative humidity like it was before I installed the dehumidifier. Psychrometer works well outdoors and there's no reason to suspect it. The room has no sources of water vapor, save for myself and a cup of cold water. The door is closed most of the time.

Am I wrong at math? Missing some other important aspect? Any ideas how to help the situation in general?

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  • What are the walls made out of? What is the floor and ceiling material? Are any made of materials that can absorb water (concrete, wood, etc)? A lot of materials will absorb and emit water vapor to reach an equilibrium depending on RH. A room that has been saturated will take a long time to de-saturate. Dec 30 '20 at 18:50
  • It's a fairly regular apartment in a concrete building, with wallpaper on the walls and a woodblock floor. Intuitively, it should not be able to absorb or release too much moisture - I would be getting mold all over the walls, not just on windowsills.
    – sigil
    Dec 30 '20 at 18:59
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    Short form: there is a source of water...
    – Solar Mike
    Dec 30 '20 at 18:59
  • Not an expert but psychrometers indoors need to be located where the recorded temp is not significantly affected by varying conditions (heat/cooling vents, etc) and should reflect the ambient temp in the room. Not sure if this helps but....
    – HoneyDo
    Dec 30 '20 at 19:02
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    Yup, it's because "The dehumidifier is actually a cheap desiccant-based thing". Life is tough. It's tougher when you're cheap. You just burned that money away, like setting paper notes on fire. How can you afford to burn money? And yeah, 10 litres is realistic. My dehumidifier holds about 8l and I can empty it 3 times a day when it's really going. Dec 30 '20 at 20:26
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Using calculations for a "sealed" room isn't going to work unless you've actually sealed the room. (Do not actually seal the room.) Otherwise, moisture will seep into the room around the door, plus whenever you open the door, as well as various other pathways.

A person generates (according to the Internet) 50-90 grams of water vapor an hour, so the dehumidifier isn't even keeping up with your output.

Now, the dehumidifier should be doing more than it is, especially at that level of humidity. You should be able to get numbers for your dehumidifier that tell how much water it can remove from the air at a given humidity level. It probably needs some cleaning or repair.

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  • Thanks for setting me on the right path. I googled this up and yes, I have seriously underestimated the scale here. 100 g of water is really nothing.
    – sigil
    Dec 30 '20 at 20:02
  • Any dehumidifier I knew got about a gallon per day ( roughly 4,000 g) Dec 30 '20 at 21:13

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