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Typical "portable" dehumidifier

A dehumidifier is generally a household appliance which reduces the level of humidity in the air, usually for health or comfort reasons, or to eliminate musty odour. Excessively humid air can cause mold and mildew to grow inside homes, both of which pose numerous health risks. Humid climates, or humid air within buildings, make some people extremely uncomfortable, causing excessive body perspiration that can't evaporate in the moisture-saturated air. It can also cause body moisture precipitation that can disrupt sleeping, create a situation where the cold pipes in this area begin to drip (from the condensation), and can prevent laundry from drying thoroughly enough to prevent mustiness. Lower humidity is also preferred because it limits the population of most pests, including clothes moths, fleas, cockroaches, woodlice and dust mites. Relative humidity in dwellings is preferably 30 to 50 percent.[1]

By their operation, dehumidifiers extract water from the conditioned air. This collected water (usually called condensate) cannot be used for drinking, so it must be discarded.[2] Some designs, such as the ionic membrane dehumidifier, dispose of excess water in a vapor rather than liquid form. The energy efficiency of dehumidifiers can vary widely.

Dehumidifiers are also used in industrial climatic chambers, to reduce relative humidity to levels conducive to processing of certain humidity-sensitive products.


Thermal condensation[edit]

These methods rely on drawing air across a thermocline. Since the saturation vapor pressure of water decreases with decreasing temperature, the water in the air condenses on the cold surface, separating the water from the air.


Mechanical/refrigeration dehumidifiers, the most common type, usually work by drawing moist air over a refrigerated coil with a small fan. The cold coil of the refrigeration device condenses the water, which is removed, and then the air is reheated by the hot coil. This process works most effectively with higher ambient temperatures with a high dew point temperature. In cold climates, the process is less effective. They are most effective at over 45 percent relative humidity; higher if the air is cold[citation needed].

Reverse Cycle Air conditioners[edit]

Reverse Cycle Air conditioners inherently act as dehumidifiers when they chill the air,[3] and thus there is also a need to handle the accumulated condensate. Newer high-efficiency window units use the condensed water to help cool the condensing coils (warm side) by evaporating the water into the outdoor air, while older units simply allowed the water to drip outside. Central air conditioning units typically need to be connected to a drain. This drain water (condensate water) needs to go outside and should not be put into the sewer system or a septic system of a house unless suitably trapped ('S' bend etc.) otherwise back pressure can allow smells or sewer gases to enter the house, and large central air-conditioning systems will increase the water burden put on septic systems. The quantity of condensate water generated by central systems can be quite large over time. Condensate water should therefore be piped (directed) into the (external) rain downspout system of a house. If the height of the air-handler (which is the half of the central air conditioner unit that is inside the house), is above the ground level or in the attic of a house, such condensate lines (pipes or tubes) can often be routed outside into drain spouts or leaders/gutters via gravity. Air handlers located in the basement of a house require condensate pumps[4] to pump the water up to ground level and outside the house, or directed away from the foundation. A condensate pump is a small automatic pump that turns on when filled with (condensate) water and turns off when the water level becomes minimal. A conventional air conditioner is very similar to a mechanical/refrigeration dehumidifier. Air in a dehumidifier passes over a series of cooling coils (the evaporator) and then over a set of heating coils (the condenser). It then goes back into the room as drier air with its temperature elevated. The water which condenses on the evaporator in a dehumidifier is disposed of in the drain pan or drain hose.

In an air conditioner, however, air passes over the cooling coils (the evaporator) and then directly into the room. Spent refrigerant then is pumped by the compressor through a tube to outside the space being cooled, to where the heating coils (the condenser) are located. The waste heat is transferred to the outside air, which passes over the condenser coils and remains outside. The water that condenses on the evaporator in an air conditioner is usually routed through a drain channel to the outside of the window, thus removing extracted water from the conditioned space.


Electronic dehumidifiers use a Peltier heat pump to generate a cool surface for condensing the water vapor from the air. The design is simpler as there are no moving parts, and has the benefit of being very quiet compared to a dehumidifier with a mechanical compressor. However, because of its relatively poor Coefficient of Performance (energy efficiency), this design is mainly used for small dehumidifiers.

Makeshift dehumidifiers[edit]

Because window air conditioner units have condensers and expansion units, some of them can be used as makeshift dehumidifiers by sending their heat exhaust back into the same room as the cooled air, instead of the outside environment. If the condensate from the cooling coils is drained away from the room as it drips off the cooling coils, the net result will be room air that is drier but slightly warmer. However many window air conditioners are designed to dispose of condensate water by re-evaporating it into the exhaust air stream, which cancels out the net air humidity decrease caused by the condensation of moisture on the cooling coils. To be effective as a dehumidifier, an air conditioner must be designed or modified so that most or all of the water that condenses is drained away in liquid form, rather than re-evaporated. Even if condensate is drained, an air conditioner that drains is still less efficient than a single-purpose dehumidifier with a design optimized for dehumidification. Dehumidifiers are designed to pass air directly over the cooling coils and then the heating coils in a single efficient pass through the device, and air conditioners are not.

In addition, most air conditioners are controlled by a thermostat which senses temperature, rather than a humidistat that controls humidity and is typically used to control a dehumidifier. A thermostat is not designed for the control of humidity, and controls it poorly if at all.


Mobile Adsorption dehumidifier
Mobile Adsorption dehumidifier

This basic dehumidifying process uses a special humidity-absorbing material called a desiccant, which is exposed to the air to be conditioned. The humidity-saturated material is then moved to a different location, where it is "recharged" to drive off the humidity, typically by heating it. The desiccant is usually mounted on a belt or other means of transporting it during a cycle of operation. Dehumidifiers which work according to the adsorption principle are especially suited for high humidity levels at low temperatures. They are often used in various sectors in industry because humidity levels below 35% can be achieved.

Ionic membrane dehumidifier[edit]

Ionic membranes are used in many industrial areas such as fuel cell technology, chemical engineering, and for water improvement. A specialized type of membrane can be used as an "ionic pump" to move humidity into or out of a sealed enclosure, operating at a molecular level without involving visible liquid water.

The solid polymer electrolyte (SPE) membrane is a low power, steady state dehumidifier for enclosed areas where maintenance is difficult. The electrolytic process delivers dehumidifying capacities up to 0.2 grams/day from a 0.2m³ (7 cu ft) space to 58 grams/day from an 8m³ (280 cu ft). SPE systems generally do not have high dehydration capacities, but because the water vapor is removed through electrolysis, the process is maintenance free. The process also requires very little electrical energy to operate, using no moving parts, making the ionic membranes silent in operation and very reliable over long periods of time. SPE dehumidifiers are typically used to protect sensitive electrical components, medical equipment, museum specimens, or scientific apparatus from humid environments.

The SPE consists of a proton-conductive solid polymer electrolyte and porous electrodes with a catalytic layer composed of noble metal particles.[5] When a voltage is applied to the porous electrode attached to the membrane, the moisture on the anode side (dehumidifying side) dissociates into hydrogen ions (H+) and oxygen: the hydrogen ions migrate through membrane to be discharged on the cathode (moisture discharging) side where they react with oxygen in the air, resulting in water molecules (gas), being discharged.[6]


Partially disassembled dehumidifier. Condensate bucket with white-colored float sensor is visible mid-picture


Most portable dehumidifiers are equipped with a condensate collection receptacle, typically with a float sensor that detects when the collection vessel is full, to shut off the dehumidifier and prevent an overflow of collected water. In humid environments, these buckets will generally fill with water in 8–12 hours, and may need to be manually emptied and replaced several times per day to ensure continued operation.

Many portable dehumidifiers can also be adapted to connect the condensate drip output directly to a drain via an ordinary garden hose. Some dehumidifier models can tie into plumbing drains or use a built-in water pump to empty themselves as they collect moisture.[citation needed] Alternatively, a separate "condensate pump" may be used to move collected water to a disposal location when gravity drainage is not possible.


Generally, dehumidifier water is considered a rather clean kind of greywater: not suitable for drinking, but acceptable for watering plants, though not garden vegetables.[7] The health concerns are:[7]

  • The water may contain trace metals from solder and other metallic parts, most significantly lead (which is quite dangerous), but also copper, aluminum, and zinc. The trace metals pose a danger if used on edible plants, as they can bioaccumulate; however, the water is usable for irrigation of non-edible plants.
  • various pathogens, including fungal spores, may accumulate in the water particularly due to its stagnancy; unlike in distilled water production, the water is not boiled, which would kill pathogens (including bacteria);
  • as with distilled water, minerals are largely absent, hence it is somewhat flat-tasting.

Food-grade dehumidifiers, also called atmospheric water generators, are designed to avoid toxic metal contamination and to keep all water contact surfaces scrupulously clean. The devices are primarily intended to produce pure water, and the dehumidifying effect is viewed as secondary to their operation.

Ice buildup[edit]

Under certain conditions of temperature and humidity, ice can form on the dehumidifier cooling coils. The ice buildup can impede airflow and eventually form a solid block encasing the cooling coils. This buildup prevents the dehumidifier from operating effectively, and can cause water damage if condensed water drips off the accumulated ice and not into the collection tray. In extreme cases, the ice can deform or distort mechanical elements, causing permanent damage.

Many better quality dehumidifiers have a so-called frost or ice sensor. These will turn off the machine and allow the ice covered coils to warm and defrost. Once defrosted, the machine will automatically restart. Most ice sensors are simple thermal switches and do not directly sense the presence or absence of ice buildup. An alternative design senses the impeded airflow and shuts off the cooling coils in a similar manner.

See also[edit]


  1. ^ "Dehumidifier Basics. U.S. EnergyStar Program. Retrieved 2011-07-15.)
  2. ^ Frigidaire (2009). All about the Use & Care of your Dehumidifier. Electrolux Home Products, Inc. p. 7. 
  3. ^ KMC Controls. "How Hot Do You Feel?". Retrieved 3 July 2013. 
  4. ^ Condensate pump
  5. ^ "How Rosahl works". Rosahl (website). Ryosai Technica Company (Japan). Retrieved 2011-08-11. 
  6. ^ Sakuma, Shuichi; Yamauchi, Shiro; Takai, Osamu. "Estimation of dehumidifying performance of solid polymer electrolytic dehumidifier for practical application". Journal of Applied Electro chemistry (Springerlink) 40 (12): 2153–2160. doi:10.1007/s10800-010-0197-4. 
  7. ^ a b Laumer, John (July 29, 2005). "Can I Water My Plants With It?". Retrieved 2011-07-15. 

Further reading[edit]

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