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I have a brand-new 5000 BTU Frigidaire window air conditioner, and it seems to work fine except that it doesn't reduce the humidity in the room.

I turned it on last night, and the temperature was 81 °F with 63% humidity, and when I woke up this morning it was 72 °F with 70% humidity.

I thought that dehumidification was necessary in how air-conditioners worked. How could an air conditioner cool the air without removing the moisture?

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Are there sources of humidity in your home? Cooking? Showers? – Steven Jun 27 '13 at 20:10
@Steven It's in a very small bedroom with the door closed. There's a bathroom right outside, but my roommates are out of town so there were no showers or cooking overnight. There is another small air conditioner in the living room which does successfully lower the humidity, so there would've been no external source of humidity (except outside). The humidity in the living room is around 45%. – Jeremy T Jun 27 '13 at 20:13
Were you in a heavy sweat all night? – Michael Karas Jun 27 '13 at 21:11
If water was dripping from the condensate drain on the outside, it was lowering your humidity. – TomG Jun 29 '13 at 0:12
up vote 4 down vote accepted

Don't forget that the percentage humidity is RELATIVE. Cold air cannot contain as much moisture as warm air. For the same amount of water in the air, the relative humidity will increase as the temperature drops. By consulting a psychometric chart, I see your overnight run removed 19 grains of water per pound of dry air. If you rewarmed the morning air back to 81 deg F, the relative humidity would be about 52%. Or, if you cooled the initial air to 72 deg F without removing any moisture, you would get a relative humidity of around 88%. So despite the higher relative humidity reading, moisture was removed.

The moisture removed is a function of the temperature of the evaporator coils in the A/C unit and the volume of air flowing through it. As bib pointed out, the unit may not have been actually running that much, reducing the volume of air passing through the coil, and thus minimizing the dehumidification effect.

If you continue to cool the same air and avoid introducing additional moisture, the humidity level will eventually be reduced. It will take some patience.

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Thanks for the answer. Did you mean grams per kilogram? I thought you might have meant grams, but grams per pound is a weird unit. – Jeremy T Jun 27 '13 at 21:58
grains of water per pound of air -- US and Imperial Measure 7000 grains per Lb. -- 1 grain = 64.79 milligrams – Fiasco Labs Jun 28 '13 at 1:49
Sorry for the confusion, (thx FL for clarification) imperial psychometric charts use weird units but they use Fahrenheit. SI charts make more sense, but use Celsius. I was too lazy to use a calculator :) But since you asked, I got one out. 19 grains/pound = 2.7gm/kg. Perhaps 75gm water was removed overnight. Also don't forget a human body contributes heat and moisture to most air by it's mere presence. It is an important factor in calculating A/C loads in buildings. – bcworkz Jun 28 '13 at 18:39

Air conditioners need to be sized to the room they are in. The dehumidification process takes some time, so if you have an air conditioner that is more powerful than the room calls for, the compressor shuts off before the dehumidification takes place.

A 5000 btu unit is considered appropriate for a room 100 to 150 sq. ft. That is a pretty small room, but if your is smaller, that might explain it.

Also, be certain that the external air vent is closed, the baffles on the sides fit the windowframe tightly and that there is a foam filler between the raised sash and the outer window. If these are not done, moist outdoor air could leak in.

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