How Does a Dehumidifier Work?

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Portable dehumidifier
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During the dampest times of the year, a dehumidifier is a must-have appliance in many regions. Unless it is removed, excessive indoor humidity can, over time, cause mold or mildew, exacerbate asthma and allergy symptoms or even cause structural problems within a home. A dehumidifier that treats the air in the entire home or in specific problem areas can greatly cut down on such threats, making the humidity level inside feel much more comfortable in the process.


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How Dehumidifiers Work

Dehumidifying means removing excess humidity, or moisture, from the air. A dehumidifier does this through the simple physics by which water vapor condenses into liquid form when it contacts a cooler surface. In the case of a dehumidifier, this cool surface is created by a system of metal coils inside the machine. The coils themselves are cooled through the magic of a gaseous refrigerant circulating inside the coils, which is repeatedly compressed and decompressed as it circulates through the coils, driven by the compressor. As liquid refrigerant transforms from a liquid back to a gas, it quickly drops in temperature, cooling the coils to a temperature that can cause moisture in the surrounding air to condense into water.

An intake fan brings moist, damp air into the dehumidifier. As this air flows across cooled coils within the appliance, that airborne moisture turns into condensation, which is heavy enough to drip into a collection chamber. For whole-home systems and in portable units outfitted with a drain hose, the process is streamlined so the water automatically pours out into a drain. In other types of humidifiers, the condensed water is captured in a collection vessel, which must be periodically emptied. In any type of dehumidifier, fans blow the dried air back out of the unit and into the room, resulting in a lower indoor humidity.

Dehumidifier diagram from Mitsibishi Electric.

A dehumidifier, like an air conditioner, uses a coolant circulating within metal coils to change the conditions of indoor air. Moist air enters the dehumidifier and travels over coolant-chilled evaporator coils. As this cooled air drops below its _dew point—_the temperature at which air can't hold any more water—the air's moisture turns into its liquid form, water. That condensed water drips and collects into some form of collection unit. In a portable unit, the collector is typically a removable bucket-style device. More deluxe units and whole-house units have either a hose or pump-and-hose system to empty the water directly into a drain.


The dehumidifier does more than just collect water. A compressor regulates the cooling of the coils by pushing a refrigerant through the system. A separate set of coils warms the chilled, less humid air before a fan blows this dry air back out into the room. The air emitted from the dehumidifier is both warmer and drier than the air entering the unit.

Dehumidifiers are controlled via a humidistat that controls the desired humidity level in the room. This control panel works like much like a thermostat, regulating moisture instead of air temperature. Setting the humidistat to a level of around 50 percent can help save energy, as the dehumidifier will turn off once the room's air reaches this relative humidity level. Setting it to a lower humidity level such as 35 percent will keep it running longer. Some dehumidifiers do not mention the relative humidity level at all; instead the controls are marked with "more dry" or" more humid" settings.

Different than Air Conditioning

Dehumidifiers and air conditioners are functionally similar, in that they both operate by means of liquid refrigerant that circulates through coils. But there is one major difference, especially when it comes to portable units. The fan system that draws moist air in over the dehumidifier's condenser coils also blows dry air back into the room, but the case of a dehumidifier, it is dry, warm air. An air conditioner, on the other hand, expels that hot air outdoors, whether through vents in a window unit or a hose in a portable unit. It also has fans that blow cooled air back into the room—or through ductwork, in the case of a whole-house unit. A dehumidifier doesn't cool the air, although the air may feel cooler once a comfortable humidity level is reached. A portable dehumidifier used in a small area may even make the room feel hotter due to the heated air released by the unit.

Whole-House Dehumidifiers

Aprilaire 1870 Whole House Dehumidifier

A whole-house dehumidifier works in much the same way as a portable unit, with less maintenance required. This type of dehumidifier is an add-on option for a home heating and cooling (HVAC) system that relies on ductwork to deliver heated or cooled air. The dehumidifier works along with the air conditioner to make the cooling process more efficient, or on a cool day, it can kick in automatically when indoor humidity levels go beyond the desired level. A whole-home dehumidifier removes excess moisture in the air that's drawn in through the home's return-air ductwork. As the dehumidifier's cooling coils chill the damp air, the resulting condensation is collected and emptied directly into a drain, assisted by a pump.


Assisting the Air Conditioner

A dehumidifier can reduce cooling costs.
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A dehumidifier, especially an entire-home unit, makes the air conditioner's job easier, potentially saving on cooling costs. Dehumidified air feels more comfortable than excessively humid air at the same temperature. For instance, on a hot, humid day with outdoor humidity levels of 70 percent, an air conditioner set to 72 degrees F. may cool the indoor air to a comfortable temperature, but the air may still feel somewhat clammy. Not only that, but this humid, clammy air is more likely to trigger allergy and asthma symptoms. A system that includes a dehumidifier removes more water from the air than air conditioning alone. Unlike the air conditioner, the dehumidifier allows you to choose an optimal relative-humidity level, generally 35 to 50 percent, which is much drier than the humid conditions outdoors.