Dehumidifiers: What To Look For

How dehumidifiers work

Humid indoor air is uncomfortable, causes damage to your home and possessions, and promotes the growth of mold and dust mites. Humidity is usually defined in terms of relative humidity, which is the amount of water vapor present in the air compared to the greatest amount of water the air can hold at a given temperature. The optimal relative humidity indoors is 30 percent to 50 percent in the summer, and 30 percent to 40 percent in cool weather. Dehumidifiers can maintain the indoor relative humidity at the desired level, controlling musty odors and protecting furniture from water damage.

Dehumidifiers remove excess moisture by using built-in fans to blow the humid air over a refrigeration system with cold and warm coils. This causes condensation, which then drips into a collection tank. The remaining dry, warm air is blown back into the room. Dehumidifiers slightly raise the air temperature, as opposed to air conditioners that dehumidify air as they cool it. Many dehumidifier models are made to vent out the back of the unit so warm air isn't blown directly into the living environment when you'd least want it on hot summer days.

Experts say dehumidifiers are most useful indoors at temperatures as low as 65 degrees Fahrenheit. Temperatures below that may cause the unit to ice up, in addition to simply working less efficiently and wasting money. If you need dehumidification at lower temperatures, look for a model that's Energy Star-certified down to 42 degrees or 44 degrees Fahrenheit and has an auto-defrost function that prevents ice formation by shutting the unit off below that temperature.

Sizing is based on the condition of the area before dehumidification, as well as on room size. The most popular size is a model with a capacity of 30 pints to 50 pints (see sizing chart below).

Dehumidifiers can greatly improve the comfort of a room, basement or entire house. Experts say you should consider the following when choosing a dehumidifier:

• Determine the dehumidifier capacity you need. See the recommendations by Association of Home Appliance Manufacturers in the chart below. Experts recommend buying a larger dehumidifier than you need, as it will work more quickly and efficiently.

• Get an Energy Star-certified model. These save on energy costs in the long term. Most models larger than 40 pints are Energy Star-rated.

• Consider storage tank capacity. All dehumidifiers include a collection tank for the condensed water. Unless you've hooked up a hose for continuous drainage, you'll need to empty the tank periodically. The size of the container varies, however, from model to model. If you want to use the dehumidifier in a remote location and continuous drainage isn't an option, look for a tank capacity of 15 pints or more. Collection containers can be hooked up to a hose for continuous drainage through a floor drain or sump pump, pumped through water pipes or emptied manually. Many dehumidifiers are made to accommodate all of these options.

• An automatic shutoff option prevents overfilling when the storage tank is full. Most dehumidifiers have this feature.

• An adjustable humidistat controls the relative humidity level. If you choose a model that doesn't have this option, you can buy a separate hygrometer to hang next to the dehumidifier that indicates the relative humidity level in the air, but you still will have to turn the dehumidifier on and off. Mechanical hygrometers cost less than $10; electronic models cost $35 to $60.

• An automatic-reset feature is useful. Some say this is a nice convenience feature if your power supply is unreliable. It also comes in handy after you've emptied the collection tank.

• Consider a low-temperature dehumidifier. If there's any chance your dehumidifier will be working in temperatures less than 65 degrees Fahrenheit, it's best to get a model that's designed to operate in temperatures as low as 42 degrees or 44 degrees, with an auto-defrost feature to prevent ice formation. Some models come with anti-frost sensors that automatically shut off the unit when air temperatures drop below a certain point.

• Check for clearance around the unit. Dehumidifiers must have good air circulation all around them. For a unit with a top exhaust, you need eight to 10 inches on all sides -- so it shouldn't be placed flat against a wall. If you choose a dehumidifier that exhausts out the back, you'll need even more space.

• Casters allow the unit to be moved easily.

• Look for reusable, washable air filters. Many models have a filter to protect the coils from dust. Being able to wipe this clean periodically makes for easy maintenance without the need to buy replacement filters.

Determining dehumidifier sizing

When choosing a dehumidifier, you should first measure room size. Dehumidifier sizing is based on the amount of moisture extracted from the air in 24 hours, measured in pints. Experts recommend buying a larger model over a smaller one unless the room with the moisture problem is very small, such as a bathroom or closet. Large-capacity units can lower humidity in a shorter time. A small-capacity model in a large room might run continuously and still not lower the humidity to an acceptable level.

After considering size, you'll need to think about the room's current conditions (see the chart below), and consider issues that might also affect humidity, such as whether you use the room to dry clothing, or if you're running a dishwasher.

Dehumidifiers that meet energy-efficiency criteria set by the U.S. Environmental Protection Agency and Department of Energy earn Energy Star status. These models remove the same amount of moisture as standard models, but they use 10 percent to 23 percent less energy, meaning significant energy-cost savings. Most models that remove 40 pints or more of water per day are Energy Star-certified. You can view the entire product list on the Energy Star website.