Dehumidifiers are essential tools for regulating humidity levels. While humidifiers raise the
moisture content of air that is too dry, dehumidifiers remove moisture from air that is too wet.
They are useful in areas such as basements, laundry areas, crawl spaces, and any location
where excess moisture is a problem. While some moisture problems may be seasonal, many
households will want to use a dehumidifier year-round.
How can you tell if an area is too humid? There may be a musty smell or clammy feel to the
air, or there may be visible moisture condensing on walls, windows and floors. Sometimes,
walls or floors will appear to "weep" as moisture accumulates. Excess moisture can stimulate
the growth of mold and mildew, causing an unsightly mess that can also be a health problem,
especially for allergy sufferers. Wood frames of doors and windows can absorb water from
excessively moist air, causing warping and sticking. To get an exact numerical value, there are
hygrometers, which measure the moisture level and display it either digitally or on a dial.
While personal preferences will vary, it is generally recommended to keep the relative humidity
level of household air below 50 or 60 percent. Keep in mind that air that is too dry can also
cause problems, so try not to lower the humidity level below 30 or 40 percent.
Capacity
Dehumidifiers are most easily identified and categorized by their capacity to remove moisture
from the air. Capacity is generally measured in pints removed per day, although smaller units
may use ounces. Actual performance will vary based on how humid the air is and how
continuously the dehumidifier runs. To be sure of sufficient power, it is good to get the largest
dehumidifier that is likely to be needed without breaking the budget. Too powerful is often less
of a problem than too weak, since the unit can always be turned down or off on drier days.
Larger dehumidifiers are generally more expensive, however, both for the initial purchase and for
operating expenses.
In smaller spaces, such as closets or crawl spaces, a chemical dehumidifier can be very
effective, absorbing around 6 to 20 ounces of water over the course of a few weeks. Mini
dehumidifiers are also good for smaller spaces, and may remove 5 to 10 ounces per day. A 25
pint dehumidifier is generally sufficient for rooms and spaces that are mildly damp or only damp
in very humid weather, while consistently damp or very damp areas may need a 40 or even 50
pint dehumidifier. For extremely wet conditions, there are even larger capacity dehumidifiers,
and some designed for industrial conditions can remove several gallons of water from the air in
a day! Higher capacity dehumidifiers tend to be larger, noisier and more expensive.
How do they work?
Most dehumidifiers operate on the principle that cooler air has less ability to hold moisture. By
cooling the air, the moisture in the air is forced to condense and the humidity level of the air is
lowered. That's why the air outside is typically very dry in cold weather, and why air
conditioners drip water outside when they are running.
Compressors
The most common technology used for dehumidifiers is very similar to that used in air
conditioners. In fact, air conditioners also have a dehumidifying effect and a room that is air
conditioned will generally not need a seperate dehumidifier. The dehumidifier has coils through
which compressed air or refrigerants move, cooling one set of coils by moving the heat to the
other set of coils.
A fan pulls moist air from the room over the cool coils, cooling the air and
condensing water from the air into droplets which fall into the water bucket. The dehumidified
air then passes over the hot coils, warming back up to room temperature as it passes back
into the room. This type of dehumidifier can be loud, but is very effective. The larger the
dehumidifier, the louder it will be and the more electricity it will use.
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Thermo-Electric Cooling
Another method of cooling draws on Peltier or themo-electric cooling technology. The Peltier
effect was observed in 1834, and shows that passing an electric current between two dissimilar
metals transfers heat across the junction. This causes one side to heat up and the other side
to cool down, which is exactly the effect used in compression dehumidifiers.
This technology
produces the cooling effect needed to condense water out of the air using no moving parts, so
the only noise from these units is produced by the fan that circulates the air. Thermo-electric
cooling is less energy efficient than using a compressor, but it is very effective for smaller
capacity dehumidifiers and offers nearly silent operation.
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Chemical Dehumidifiers
Some chemicals act as dessicants - they naturally absorb water from their surroundings, which
is why dessicant packets are packaged with many medicines and vitamins or other materials
that need to stay dry. This same principle has been applied to dehumidifier technology to
provide silent, reusable units for smaller areas such as closets and crawl spaces.
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Eva Dry brand dehumidifiers use a specially engineered crystallized silica gel that efficiently
absorbs moisture and is odorless and non-toxic. They have a smaller capacity, generally 6 to
20 ounces over the course of a few weeks, but they are ideal for small enclosed spaces like
closets and don't need to be plugged in or use batteries while in use. Once the unit is at full
capacity, it will stop absorbing water, but it will not leak or spill.
The absorption capacity can
be renewed by plugging the unit in to an outlet. The silica gel then releases the absorbed
moisture and is ready to be used again. Damp Rid and Damp Gone are other chemical
dehumidifiers that operate by absorbing moisture from the air.
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Features
Dehumidifiers are available with a variety of features that can improve convenience and optimum
performance. Consider the space that the dehumidifier will be used in and the user's personal
preferences in order to decide which features are must-haves, which would be nice, and which
are impractical or unimportant.
Humidistat
A humidistat is extremely useful for dehumidifiers that need to operate continuously without
much supervision. Like a thermostat for heaters, the humidistat allows the user to specify the
desired humidity level. The dehumidifier will then operate while the humidity is above the set
level, and will automatically shut off once the humidity drops below the set level. The sensor
continues to operate and turns the unit back on when the humidity level rises again. This
feature has two main advantages. First, the dehumidifier won't wind up making the air too dry,
because it shuts off once it reaches the desired humidity level. Second, it saves energy by
shutting off the dehumidifier when it's not needed.
Water Collection
Dehumidifiers typically have a water bucket positioned to collect the water that condenses
within the unit as the air is processed. Bucket size can be an important factor when choosing a
dehumidifier. Look for a bucket that is large enough not to need frequent emptying, but small
enough to be easily managed and reduce the chance of spills. It's important to choose a
dehumidifier that makes it easy to check the water level and see if it needs emptying. Some
models use a clear bucket where the water level is easily visible, while others include a water
level indicator in the control panel. Auto shut-off is a common and very useful feature that
prevents the dehumidifier from overflowing the water bucket.
Another option for dealing with the water removed from dehumidified air is continuous drainage.
Many units offer a hose attachment that can run from the collection bucket to a convenient
drain. This removes the need to empty the bucket manually, since the water simply runs out as
it condenses. While this is the most convenient option, it is not always practical, so be sure to
consider where the dehumidifier will be placed and whether there are any drains nearby that
can be used.
Temperature Range
Since dehumidifiers are often needed to control the moisture levels in damp basements,
temperature can be a consideration. If the basement or other area is likely to be cold when the
dehumidifier is operating, it's important to make sure the dehumidifier can function at that
temperature. Dehumidifiers are generally designed to operate at room temperature, and can run
into problems if the temperature goes below about 60 degrees. Many dehumidifiers will list their
functional temperature range, and some are designed specifically to operate at lower
temperatures. Some can even continue functioning at temperatures as low as 38 degrees!
For lower temperature operation, frost can also be an issue. Some units offer a frost detector
that will shut off the unit if frost starts to build up on the compressor, while others incorporate
an auto-defrosting system to keep the dehumidifier operating and remove the frost.
Portability
In some cases, a dehumidifier may be kept in the same place year-round. For moisture
problems that are seasonal, however, it is useful to have an easy-to-move dehumidifier that can
be moved into storage or to operate in a new location. Size and weight are a consideration in
portability, but many models also come with rollers or casters, and handles to make it easier
to grip and move the unit. Even if the unit will not be moved frequently, this sort of feature can
be an advantage on the occasions that it does need to change location, such as when buying a
new house.
Air Filtration
Some dehumidifiers incorporate air filters to remove impurities from the air before it passes
through the machine. This improves air quality, but also contributes to lengthening the life of
the dehumidifier. Look for washable filters for easy maintenance.
Adjustable Fan Speeds
Fan speed can be a factor in how fast the dehumidifier works and how noisy the unit is during
operation. A unit with adjustable fan speed can be useful in an area where noise levels need to
be moderated at times, or where the humidity problem is variable.
Control Panel
Dehumidifiers are available with either digital or mechanical controls. The decision of whether to
push buttons or turn dials is largely a matter of personal preference, although in some cases
digital displays will offer more information or control options.
Hybrid Units
Because a dehumidifier treats all the air in a room, there are a variety of hybrid options
available. While dehumidifying, a
hybrid unit may also heat, cool, or purify the air
as it passes through the unit. Because of the extra functions, these models tend to be much
more expensive than a standard dehumidifier.
Energy Costs
While chemical dehumidifiers don't use electricity while operating, they are typically only
effective in small, enclosed spaces. For rooms or larger areas with moisture problems, a
dehumidifier using thermo-electric cooling or a compressor will be necessary. Because
dehumidifiers are typically run continually, operating costs can be an important consideration.
In general, smaller dehumidifiers will use less energy, but there may also be variations between
dehumidifiers of the same size. While it's important to get a dehumidifier that will be affordable
long term, it is also important to get one powerful enough to take care of the problem areas.
Typically, dehumidifiers draw between 50 and 200 watts. As a basis for comparison, a standard
lamp draws 60 watts. Because many dehumidifiers are run continually, a small difference in
power level can have a definite impact on the electric bill. Check for EnergyStar certification
when comparing features to help get the most energy efficient option for long-term operation.
Prices
Prices for dehumidifiers vary widely, from around $10 to over $500. In general, the larger the
dehumidifier's capacity, the more expensive it will be. Extra features such as low temperature
operation can also increase the price significantly.
Chemical dehumidifiers have the lowest capacity but also use no electricity to operate. They
tend to cost between $10 and $60 depending on the size of the unit and how the chemicals are
housed.
Small dehumidifiers up to about 30 pints generally cost between $50 and $150, while larger
capacity units can range from $150 to $400 or more. Most hybrid dehumidifiers cost more than
$400. Be sure to compare energy ratings in order to take into account long term operating
expenses.
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