How dehumidifiers work

Humidity is typically defined in terms of relative humidity, which is the amount of water vapor present in the air compared to the maximum amount of water the air could hold at that temperature. In the summertime, the ideal relative humidity indoors is 30 to 50 percent; In the winter, it should be between 30 and 40 percent so that condensation does not form on the inside of windows. Dehumidifiers can keep the indoor relative humidity at its optimum level, controlling musty odors and protecting furniture from water damage. While many dehumidifiers will run at temperatures as low as 41 degrees Fahrenheit, they are most effective at 65 degrees or higher.

Dehumidifier capacity is measured by the number of pints of water it can extract from the air in a 24-hour period. Typical capacities range from 30 to 70 pints, but some units can remove more than 100 pints of water. For a large basement with 1,400 square feet, you'll need a 70-pint unit. A 30-pint unit may be sufficient for a 600-square-foot area. However, for an area that is very wet, you may need a larger model.

Dehumidifiers bearing the Energy Star label meet energy-efficiency criteria set by the U.S. Environmental Protection Agency (EPA). These models remove the same amount of moisture as standard models while consuming 15 percent less energy. If you consult the list of certified dehumidifiers at the Energy Star website, you'll notice that larger-capacity models generally have higher efficiency ratings. For this reason, ConsumerReports.org generally recommends choosing a larger-capacity dehumidifier (60 to 70 pints per day) if you plan to use it regularly. Editors also note that larger models can be run at a lower fan speed, making them quieter.

Here are some other things experts say you should consider before buying a dehumidifier:

  • The collection container: Most dehumidifiers include a container for the condensed water. Unless you've hooked up a hose for continuous drainage, you'll need to empty the tank when it fills up and the unit automatically turns off. A larger container will need to be emptied less frequently than a smaller one; however, it will also be heavier and harder to carry when full.
  • The controls: Some models allow you only to select a basic humidity level, such as normal, dry or very dry. Models with digital controls, however, allow you to program in the specific relative humidity that you want. Users tend to prefer displays that also show the current humidity level, so you can gauge how well the dehumidifier is working and how soon you can expect to reach the target humidity level.
  • Automatic restart: This feature will turn the dehumidifier back on after a power outage -- a nice convenience if your power supply is unreliable.
  • Frost sensors: If the temperature drops below 65 degrees Fahrenheit in your basement, the condenser coils can ice up and become more inefficient at removing moisture from the air. Most new dehumidifiers will turn off automatically if this happens. Some machines include an automatic defrost feature, which prevents ice from building up.
  • A timer: This feature allows you to turn the dehumidifier on or off automatically. You can schedule it to run only at times of the day when you aren't around to hear it or during non-peak utility rate periods.
  • Your space: For optimal operation, dehumidifiers need good air circulation. Most units have a top exhaust and can be placed against a wall. If you choose a dehumidifier that exhausts out the back, however, you'll need to keep it away from walls and furniture. If you are planning to use a hose for continuous drainage, then make sure the dehumidifier can be placed close to a floor drain. A long hose is unwieldy and can pose a tripping hazard.

Back to top