A dehumidifier can make your home more comfortable and keep it free of mold.

During the humid, languid days of summer, an air conditioner can make your house feel less like a steam bath and help prevent the unpleasant, musty smell that's a sign of a damp home. But during cooler weather or in your basement or crawl space, a dehumidifier is the best way to reduce humidity levels.

Dehumidifiers do more than just enhance your comfort. By keeping humidity at the proper level, these machines will prevent a host of problems. Left unchecked, a household relative humidity above 50 percent can breed mildew, dust mites, and mold that exacerbate allergies and asthma. Mildew and mold can thrive on and discolor cloth or natural-wood furniture and grow on clothes, books, and other items stored in your basement or elsewhere. An increasingly common household problem in this country, mold can also cause drywall and wood to rot, resulting in tens of thousands of dollars in structural damage.

While preventing mold requires also properly sealing and ventilating your home, a dehumidifier can reduce potential for its growth.

High humidity could especially be a problem in new homes. "New construction is even more vulnerable to mold because homes today are made mostly of paper-faced wallboard instead of wood and plaster, which are more mold-resistant," says Lew Harriman, a humidity consultant and member of the American Society of Heating, Refrigerating and Air-Conditioning Engineers.

Using a dehumidifier in a cool, damp basement or crawl space is ideal for treating humidity-related problems. The dehumidifier uses a fan to pull room air over two sets of refrigerant-filled coils, one cool and the other warm, wringing out moisture along the way. The water that condenses out of the air typically drips into a bucket inside the dehumidifier. Most units can be connected to an external drain, eliminating the need to empty the bucket.

You'll find a few different brands, including well-known names like GE, Kenmore, LG, DeLonghi, and Whirlpool. Our past tests indicate that there are few real differences among brands, so concentrate on dehumidifying capacity, measured in pints of moisture removed per day. A small-capacity unit will handle about 25 pints daily, while a very large one will pull up to 75 pints of moisture out of the air. Price range: $100 to $300 or more, with many selling for about $200.

Whole-house systems are available; they generally run $1,100 to $3,000, plus installation. Although these dehumidifiers, available as portable units or add-ons to your existing central heating and air-conditioning system, are costly, the high amounts of moisture they remove allow you to run your air-conditioning less frequently. Manufacturers claim they can handle homes as large as 3,000 square feet.

Most portable dehumidifiers are similarly equipped, but there are a few features worth searching out. Look for:

A bucket or tank that is easy to empty, with comfortable handles and a cover that allows for convenient removal and carrying when full.

A washable air filter, for easy cleaning.

A hose connection, which lets you divert the condensed water directly to a floor drain or sump pump, so you need not empty the bucket. That is handy for when you go on vacation in the summer or for use in a second home. Remember that if you use just a drainage hose, you are relying on gravity to move the water to the drain or pump. Models from DeLonghi are equipped with condensate pumps that can send water 8 to10 feet upward and out a basement window. Whirlpool sells an add-on pump kit designed to work with most dehumidifiers on the market. The company claims it can send water 20 feet vertically, although it comes with only a 10-foot hose.

An on/off switch, which is more convenient than adjusting the humidity controls, or humidistat, to turn the unit on or off. Electronic humidistats make selecting a humidity level easier. (Indoor relative humidity should ideally be kept between 30 and 50 percent.)

Easy-to-read controls. Many models use electronic controls, some with digital displays. A timer can save money by allowing you to set the unit to run overnight during cheaper off-peak electricity hours. If the temperature in your basement is typically below 65° F., look for a dehumidifier that performs at low temperatures and has an antifrost sensor to turn off the unit if the air temperature drops below 65° F. Whirlpool has several models that it says work at 38°F.

Wheels or casters. Dehumidifiers can be heavy.

Buy the largest-capacity dehumidifier you can afford. The added cost of enhanced capacity is worth it. A larger-capacity unit will work quicker and more efficiently than a smaller unit, and it will be able to handle a greater range of humidity levels with little or no added noise.

Consider energy efficiency. Dehumidifiers can consume more than $150 a year in energy, so look for models with the Energy Star seal, which are 10 to 20 percent more efficient than non-Energy Star units. For more information on energy efficiency, visit GreenerChoices.org.

Know when to shop. New models appear in late spring and early summer, with most sales and in-store promotions occurring June through August. You might find closeout deals in the fall and early winter.

Place the dehumidifier in the most humid spots of your house, such as your basement or crawl space. You can place top-venting units along walls, but place others at least 12 inches from walls or furniture.

Close any doors and windows to the room.

Empty and clean the dehumidifier bucket regularly to prevent dust and mildew from accumulating. Cleaning the filter (if equipped) and coils regularly will keep your unit running well. Remember, you'll want humidity levels to be less than 60 percent all year long, not just during summer. In cold climates, the air won't hold a lot of moisture but water vapor can still enter the home through the foundation in contact with wet soil. Ground-based moisture and moisture in the air are also present in warmer climates. So, check the relative humidity in the living areas in your home as well as basement or crawl space, and use a dehumidifier to reduce elevated levels.