If your home doesn’t have air conditioning, it may rely upon a dehumidifier to reduce uncomfortable, muggy humidity in room air. A dehumidifier uses refrigeration principles the way an air conditioner does, but it doesn’t cool the air—in fact, it warms it slightly.

A dehumidifier is a box that contains cold evaporator coils, warm condenser coils, a fan, and a reservoir or drain to catch water (see the illustration below). The fan blows moisture-laden room air over the cold coils, where water condenses and drips into the pan or drain. The drier air then blows across the warm coils and back into the room.

Although a dehumidifier does not cool the air, it makes it more comfortable because it decreases the humidity. But a dehumidifier does not function well under about 65 degrees F.

A dehumidifier’s fan must have electric power, the evaporator coils must contain a proper charge of refrigerant, and the controls must work for the dehumidifier to operate.

In the same way that a thermostat controls a heater or air conditioner, a dehumidifier is cycled off and on by a humidistat that measures a room’s humidity.