The basement can be a dark and scary place. But is yours dank and musty, too? That's where a dehumidifier comes in. It's the solution when humidity levels leave rooms feeling stuffy and damp, or cause a drippy film of condensation on the windows (particularly double- or triple-paned windows). Other problems that come along with waterlogged air are rotting wood; doors or windows that are difficult to open because of warping; and in some cases, allergies among members of the household. These are all reasons to go shopping for a dehumidifier, either for use year-round or just in the muggy season.

Aside from avoiding nasty problems, a dehumidifier can make your home more comfortable, and save you money on your air-conditioning bill, since drier air feels cooler, allowing you to set your thermostat a bit higher. In fact, dehumidifiers operate on roughly the same principle as air conditioners (or refrigerators), though to different effect: Moisture is drawn from the warm air and condensed on chilled coils. That moisture then drips into a collection tank. The air, after being passed over heated coils to warm it up to room temperature, is then released back into the space.

Since the machines run almost continuously, they can use a great deal of power. So it pays to look for the most energy-efficient model you can find. According to Energy Star, the best models use about 20 percent less energy than standard units, thanks to more-efficient coils, compressors, and fans. These aren't details you can see, but there are ratings to guide you, as well as other, more obvious details that go into a power-saving model.

Here's what to look for when shopping:

Up to the job: choosing the right size
The capacity of the model you buy will depend on two things-the size of the space you want to dry out and the level of moisture present. So before you head to the store, be sure you're armed with the room's measurements. Most dehumidifiers will feature a chart right on the packaging to guide the shopper. Typically, it's one developed by the Association of Home Appliance Manufacturers (AHAM), and includes wetness levels from moderately damp to extremely wet, along with square-footage ranges. Using the chart, you can get an idea of the machine's moisture-removing capability, in pints per 24 hours, for a given area.

If you're faced with a choice between two sizes, it's probably best to go with the larger-capacity model. A dehumidifier that is too small is likely to run and run without doing its job effectively.

BELLS AND WHISTLES: FEATURES TO CHECK FOR

Timer: Look for a model that includes this feature. It will allow you to set the dehumidifier to turn on only at night, for example (when air is dampest and off-peak electricity rates apply), rather than running continuously.

Washable filter: A filter that can be removed and cleaned periodically helps you avoid clogs and keep the machine running efficiently. This can also save you money, as you won't need to buy new filters as often.

Anti-frost sensor: If the temperature occasionally drops below 65 degrees in the space you are dehumidifying, it's smart to buy a product with this extra. It will automatically turn the unit off if it senses that the temperature has fallen below a certain point, preventing frost buildup on the coils. Frost on the coils is a major energy drain, as it can cause the machine to run continuously without effectively removing moisture.

Built-in humistat: This feature senses the relative humidity (RH) in a room and, once the desirable level is reached, cycles the machine on and off to maintain it, rather than running continuously. This will save energy and make less work for you (without it, you'd need to buy a separate humidity sensor, and turn the machine off and on manually). According to Energy Star, the RH indoors should be kept between 30 and 50 percent for optimal comfort and mold prevention. People with allergies might want to aim for the lower end of the range.

By the numbers: gauging efficiency
When you're comparing models, keep an eye on the number known as the energy factor (EF). This represents how many liters of water are removed from the air per kilowatt hour (kWh) of energy consumed. The higher the EF, the more efficient the unit will be at removing moisture.

Also, of course, you should look for the Energy Star label. This program, sponsored jointly by the Environmental Protection Agency and the Department of Energy, promotes efficient appliances and products. Dehumidifiers that have earned the Energy Star rating use 10 to 20 percent less energy than standard models. Specifications vary according to capacity: A 40-pint-per-day dehumidifier, for instance, must have a minimum EF of 1.5 to qualify; a high-capacity unit, which will generally remove 75 pints per day or more, must have an EF of at least 2.5.

Once you have a general sense of the size and features you want in your dehumidifier, you can do one last bit of research before making a purchase: Check the Energy Star website and review the product list. You'll see every Energy Star-approved model on the market, and can compare them by energy factor. Armed with that information, you can decide what your smartest option might be.

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