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A humidifier is a household appliance that increases humidity (moisture) in a single room or in the entire house. There are point-of-use humidifiers, which are commonly used to humidify a single room, and whole-house or furnace humidifiers, which connect to a home's HVAC system to provide humidity to the entire house. Medical ventilators also often include humidifiers for increased patient comfort.
Excessively low humidity may occur in hot, dry desert climates, or indoors in artificially heated spaces. In winter, when cold outside air is heated indoors, the humidity may drop as low as 10-20%. This low humidity can cause adverse health effects, by drying out mucous membranes such as the lining of the nose and throat, and can cause respiratory distress. The low humidity also can affect wooden furniture, causing shrinkage and loose joints or cracking of pieces. Books, papers, and artworks may shrink or warp and become brittle in very low humidity.
In addition, static electricity may become a problem in conditions of low humidity, causing annoying static cling of textiles, and causing dust and small particles to stick stubbornly to electrically charged surfaces.
A "portable" humidifier may range in size from a small tabletop appliance to a large floor-mounted unit. The water is usually supplied by manually filling the unit daily or on some periodic basis.
The most common portable humidifier, an "evaporative", "cool moisture", or "wick humidifier", consists of just a few basic parts: a reservoir, wick and fan.
The wick is made of a porous material that absorbs water from the reservoir and provides a larger surface area for it to evaporate from. The fan is adjacent to the wick and blows air onto the wick to aid in the evaporation of the water. Evaporation from the wick is dependent on relative humidity. A room with low humidity will have a higher evaporation rate compared to a room with high humidity. Therefore, this type of humidifier is partially self-regulating; as the humidity of the room increases, the water vapor output naturally decreases.
These wicks become moldy if they are not dried out completely between fillings, and become saturated with mineral deposits over time. They regularly need rinsing or replacement; if this does not happen, air cannot pass through them, and the humidifier stops humidifying the area it is in and the water in the tank remains at the same level.
One type of evaporative humidifier makes use of just a reservoir and wick. Sometimes called a "natural humidifier", these are usually non-commercial devices that can be assembled at little or no cost. One version of a natural humidifier uses a stainless steel bowl, partially filled with water, covered by a towel. A waterproof weight is used to sink the towel in the center of the bowl. There is no need for a fan, because the water spreads through the towel by capillary action and the towel surface area is large enough to provide for rapid evaporation. The stainless steel bowl is much easier to clean than typical humidifier water tanks. This, in combination with daily or every other day replacement of the towel and periodic laundering, can control the problem of mold and bacteria.
Houseplants may also be used as natural humidifiers, since they evaporate water into the air through transpiration. Care must still be taken to prevent bacteria or mold in the soil from growing to excessive levels, or from dispersing into the air.
A vaporizer (steam humidifier, warm mist humidifier) heats or boils water, releasing steam and moisture into the air. A medicated inhalant can also be added to the steam vapor to help reduce coughs. Vaporizers may be more healthful than cool mist types of humidifiers because steam is less likely to convey mineral impurities or microorganisms from the standing water in the reservoir. However, boiling water requires significantly more energy than other techniques. The heat source in poorly designed humidifiers can overheat, causing the product to melt, leak, and start fires.
An impeller humidifier (cool mist humidifier) uses a rotating disc to fling water at a diffuser, which breaks the water into fine droplets that float into the air. The water supply must be kept scrupulously clean, or there is a risk of spreading bacteria or mold into the air.
An ultrasonic humidifier uses a metal diaphragm vibrating at an ultrasonic frequency to create water droplets that silently exit the humidifier in the form of a cool fog. Ultrasonic humidifiers use a piezoelectric transducer to create a high frequency mechanical oscillation in a film of water. This forms an extremely fine mist of droplets about one micron in diameter, that is quickly evaporated into the air flow. Unlike the humidifiers that boil water, these water droplets will contain any impurities that are in the reservoir, including minerals from hard water (which then forms a difficult-to-remove sticky white dust on nearby objects and furniture). Any pathogens growing in the stagnant tank will also be dispersed in the air. Ultrasonic humidifiers should be cleaned regularly to prevent bacterial contamination from being spread throughout the air.
The amount of minerals and other materials can be greatly reduced by using distilled water, though no water is absolutely pure. Special disposable demineralization cartridges may also reduce the amount of airborne material, but the EPA warns, "the ability of these devices to remove minerals may vary widely." The mineral dust may have negative health effects. Wick humidifiers trap the mineral deposits in the wick; vaporizer types tend to collect minerals on or around the heating element and require regular cleaning with vinegar or citric acid to control buildup.
For buildings with a forced-air furnace, a humidifier may be installed into the furnace. They can also protect wooden objects, antiques and other furnishings which may be sensitive to damage from overly dry air. In colder months, they may provide modest energy savings, since as humidity increases, occupants may feel warm at a lower temperature. Bypass humidifiers are connected between the heated and cold air return ducts, using the pressure difference between these ducts to cause some heated air to make a bypass through the humidifier and return to the furnace.
The humidifier should usually be disabled during the summer months if air conditioning is used; air conditioners partially function by reducing indoor humidity.
Drum style (bypass) uses a pipe to bring water directly to a reservoir (a pan) attached to the furnace. The water level in the pan is controlled by a float valve, similar to a small toilet tank float. The wick is typically a foam pad mounted on a drum and attached to a small motor; hot air enters the drum at one end and is forced to leave through the sides of the drum. When the hygrostat calls for humidity, the motor is turned on causing the drum to rotate slowly through the pan of water and preventing the foam pad from drying out.
For the latter reason especially, drum-style humidifiers should always be turned off at the water supply during summer (air conditioning) months, and should always be used with high quality furnace air filters (MERV ratings as high as possible to ensure small numbers of mold spores reaching the humidifier pan) when the water supply is turned on.
A disc wheel style (bypass) is very similar in design to the drum style humidifiers; this type of furnace humidifier replaces the foam drum with a number of plastic discs with small grooves on both sides. This allows for a very large evaporative surface area, without requiring a great deal of space. Unlike the drum style humidifiers, the disc wheel does not need regular replacement.
Bypass flow-through style (bypass – also known as "biscuit style" or many other, similar variant names) uses a pipe to bring water directly to an electrically controlled valve at the top of the humidifier. Air passes through an aluminum "biscuit" (often called a pad; the term "biscuit" emphasizes the solid rather than foamy form) which is similar to a piece of extremely coarse steel wool. The biscuit has a coating of a matte ceramic, resulting in an extremely large surface area within a small space. When the hygrostat calls for humidity, the valve is opened and causes a spray of water onto the biscuit. Hot air is passed through the biscuit, causing the water to evaporate from the pad and be carried into the building.
Spray mist type uses a pipe, usually a small plastic tube, to bring water directly to an electrically controlled valve in the humidifier. Water mist is sprayed directly into the supply air, and the mist is carried into the premises by the air flow.
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Additional types include non-bypass flow-through (fan augmented), steam, impeller or centrifugal atomizer, and under duct designs.
Overuse of a humidifier can raise the relative humidity to high levels, promoting the growth of dust mites and mold, and can also cause hypersensitivity pneumonitis (humidifier lung). A relative humidity of 30% to 50% is recommended for most homes.
A properly installed and located hydrostat should be used to monitor and control humidity levels automatically, or a well-informed and conscientious human operator must constantly check for correct humidity levels.
The USEPA provides detailed information about health risks as well as recommended maintenance procedures. If the tap water contains a lot of minerals, also known as hard water, then the ultrasonic or impeller humidifiers will produce a "white dust" (calcium is the most common mineral in tap water), which usually spreads over furniture, and is attracted to static electricity generating devices such as CRT monitors. The white dust can be prevented by using distilled water or a demineralization cartridge in ultrasonic humidifiers.
In addition, a stuck or malfunctioning water supply valve can deliver large amounts of water, causing extensive water damage if undetected for any period of time. A water alarm, possibly with an automatic water shutoff, can help prevent this malfunction from causing problems.
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