AskDefine | Define humidistat

User Contributed Dictionary



  1. a device that measures, or controls, the relative humidity of a gas


Extensive Definition

A humidifier is a household appliance that increases humidity (moisture) in a single room or in the entire home. 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.

Evaporative humidifiers

The most common humidifier, an "evaporative" or "wick humidifier", consists of just a few basic parts: a reservoir, wick and fan.


Reservoirs are common to most humidifiers and come in different shapes and sizes. The reservoir is a containing tank of water filled prior to operation, and provides the water for the moisture output. Other types have a continuous feed from a water line.


The wick is a filter that absorbs water from the reservoir. Evaporation of water 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 self-regulating: As the humidity of the room increases, the water vapor output naturally decreases. These wicks regularly need cleaning and replacement — if this does not happen, the humidifier stops humidifying the area it is in and the water in the tank remains at the same level.


The fan is adjacent to the wick and blows air onto the wick, thus aiding in the evaporation of the water within the wick.

Other types of humidifiers

Other types of humidifiers include:
  • Vaporizer (Steam Humidifier) (Warm Mist Humidifier) — 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 are more healthful than cool mist types of humidifiers because steam is sterile and free from minerals. Vaporizers use more electricity to boil the water.
  • Impeller Humidifier (Cool Mist Humidifier) — A rotating disc flings water at a diffuser, which breaks the water into fine droplets that float into the air.
  • Ultrasonic Humidifier — A metal diaphragm vibrating at an ultrasonic frequency creates water droplets that exit the humidifier in the form of a cool fog silently. Ultrasonic Humidifiers should be cleaned regularly to avoid bacterial contamination which may be projected into the air.
Impeller and ultrasonic humidifiers do not selectively put water in the air, they also add any suspended material in the water to the air such as microorganisms and minerals. The amount of minerals and other materials can be greatly reduced by using distilled water, though no water is absolutely pure. Filters may also reduce the amount of material but the EPA warns, "the ability of these devices to remove minerals may vary widely." Depending on the volume, this dust may have negative health effects. Wick humidifiers trap the mineral deposits in the wick.

Forced-Air Humidifiers

For buildings with a forced-air furnace, a humidifier may be built right 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 substantial energy savings, since as humidity increases, occupants feel warm at a lower temperature. These units are typically called bypass humidifiers (because they 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. There are three basic styles:
  • Drum style: A pipe brings 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.
Advantages include:
  • Low cost
  • Inexpensive maintenance (drum-style pads are cheap and readily available)
Disadvantages include:
  • Requirement for frequent (approximately monthly) inspections of cleanliness and pad condition
  • Water evaporation even when humidification is not required (due to the pan of water which remains exposed to a high velocity air stream)
  • Mold growth in the pan full of water (this problem is exacerbated by the large quantity of air, inevitably carrying mold spores, passing through the humidifier whether in use or not).
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's pan) when the water supply is turned on.
  • Disc Wheel Style: 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 replacing.
Advantages include:
  • Very low maintenance (basin of humidifier should be cleaned out periodically, unless automatic flushing device is installed)
  • No regular replacement of parts necessary
  • Higher output due to large evaporative surface area
  • Can be installed in hard water situations
  • Maintains efficiency throughout lifespan
Disadvantages include:
  • Higher price
  • Water evaporation even when humidification is not required (due to the pan of water which remains exposed to a high velocity air stream)
  • Flow-Through Style (also known as "biscuit style" or many other, similar variant names): A pipe brings water directly to an electrically-controlled valve at the top of the humidifier. Air passes through an aluminum "biscuit" (often called a pad; using the term "biscuit" to emphasize 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.
Advantages include:
  • Reduced maintenance (new biscuit only when clogged with dust or mineral deposits, typically once per year)
  • Lack of a pan of potentially stagnant water to serve as a breeding ground for mold as with a drum-style humidifier
  • No incidental humidification caused by a constantly-replenished pan of water in a high velocity air stream
  • Reduced requirement for expensive air filters
  • Uses no electricity
Disadvantages include:
  • A somewhat higher purchase price
  • Manufacturer and model-specific replacement biscuits versus the relatively generic drum-style pads
  • A portion of the water supplied to the unit is not evaporated. This can generate a considerable amount of waste water, and does require connection to a drain.

Disadvantages and risks

The use of a humidifier can allow the reproduction of dust mites or the growth of harmful mold, which can be especially harmful for children and the elderly. The relative humidity should be kept between 30% and 50%. . Can also cause Hypersensitivity pneumonitis (humidifier lung)
Some humidifiers now use Microban technology to reduce mold and bacteria growth within the humidifier.
The EPA provides detailed information of the risks as well as recommended maintenance procedures.
Some humidifiers have no silent operation.


A controller that measures and controls relative humidity. A humidistat may be used to control either humidifying or dehumidifying equipment by the regulation of electric or pneumatic switches, valves, or dampers.
An electronic humidistat includes a sensing element and a relay amplifier. The sensing element consists of alternate metal conductors on a small flat plate. An increase or decrease of the relative humidity causes a decrease or increase in the electrical resistance between the two sets of conductors and the change in resistance is measured by the relay amplifier.


humidistat in German: Luftbefeuchter
humidistat in Spanish: Humidificador
humidistat in Japanese: 加湿器
humidistat in Korean: 가습기
humidistat in Russian: Увлажнитель
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