A dehumidifier removes moisture from the air. The usual technique used to remove the moisture is to condense the moisture onto a cold surface.
Anyone who has poured a cold glass of iced tea on a hot, humid summer day knows that moisture will condense on the glass. When air cools, it loses its ability to hold moisture; in the case of the cold glass, the moisture in the air condenses right onto the glass. If the glass is left on a table long enough and if the air is very humid, a significant puddle of water can form. You may have noticed the same phenomenon in any air conditioner. The moisture in the air inside the room condenses onto the air conditioner's cold coils. If it's a window unit, the water drips out the back of the unit onto the ground.
A dehumidifier is simply an air conditioner that has both its hot and cold coils in the same box. A fan draws the room's air over the cold coil of the air conditioner to condense the moisture (which normally drips into a bucket). The dry air then passes through the hot coil to heat it back up to its original temperature. That's all there is to it!
If you have a room that is air conditioned, it should not need a dehumidifier -- the air conditioner should be doing the dehumidifying for you.
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