Dehumidifiers remove water from warm, humid air, working something like an air conditioner in that they move the warm air past two coils. Moisture from the air condenses on one set of coils much like an air conditioner's evaporator coils. The water drips into a reservoir or through a hose to a drain. The other coil warms the air, which is blown into the house or room. Although a dehumidifier does not cool the air, it makes it more comfortable because it decreases the humidity. A dehumidifier does not function well under about 65 degrees.

Air conditioners dehumidify the air during the process of cooling it, so dehumidifiers are needed primarily where air conditioning isn't available, such as in non-air-conditioned apartments or houses and uncooled or damp areas such as basements.

Unless it is designed to drain automatically, a dehumidifier should be emptied fairly often during hot and muggy or damp weather°™sometimes as often as once a day. Consider a dehumidifier's reservoir capacity if you are buying a new one: larger capacity reservoirs work more efficiently and have to be emptied less often than smaller ones.