Humidifier
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.
Evaporative humidifiers
The most common humidifier, an "evaporative", "cool mist", or "wick humidifier", consists of just a few basic parts: a reservoir, wick and fan.
The wick is a filter 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 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 filled 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 weight is used to sink the towel in the center of the bowl. There is no need for a fan as 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, can eliminate the problem with mold and bacteria.
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 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.[1] 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.[2]
- 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 silently exit the humidifier in the form of a cool fog. Ultrasonic humidifiers use a piezo-electric transducer to create a high frequency mechanical oscillation in a body of water. The water tries to follow the high frequency oscillation but cannot because of its comparative weight and mass inertia. Thus, a momentary vacuum is created on the negative oscillation, causing the water to cavitate into vapor. The transducer follows this with a positive oscillation that creates high pressure compression waves on the water’s surface, releasing tiny vapor molecules of water into the air. This is an extremely fine mist, about one micrometre in diameter, that is quickly absorbed into the air flow. Unlike the humidifiers that boil water, these water droplets contain any impurities that are in the reservoir, including minerals from hard water (which then forms a difficult to remove white dust on nearby objects and furniture), and pathogens growing in the stagnant tank. 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."[1] Depending on the volume, this 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.
Forced-Air Humidifiers (whole-house)
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 substantial energy savings, since as humidity increases, occupants 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. Common styles:
- Drum style (bypass): 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 (bypass): 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)
- Bypass Flow-Through Style (bypass - 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 little 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.
- Spray Mist Type: A pipe, usually a small plastic tube, brings 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.
- Advantages include:
- Low cost.
- Simpler than bypass types to install - single cut hole for installation, no additional ducting.
- Uses little electricity.
- Small, compact unit - fits where other types cannot. (Approx 6 inches square.)
- Because it does not require bypass ducting it does not undermine the pressure separation (and therefore, blower efficiency) of the return and supply ducts.
- Does not require use of moisture pads (on-going expense).
- Highly efficient usage of water. Does not generate waste water and does not require separate connection to a drain.
- Requires little maintenance. Periodic cleaning of nozzle may be required in hard water environments.
- Lack of a pan of potentially stagnant water to serve as a breeding ground for mold as with a drum-style humidifier.
- Disadvantages include:
- Spray nozzle can become clogged in hard water situations, necessitating the use of water filter, periodic cleaning of nozzle, or nozzle replacement.
- Additional types: 'Non-Bypass Flow-Through' (fan augmented), 'Steam', 'Impeller or Centrifugal Atomizer' and 'Under Duct'.
Disadvantages and risks
Unnecessary or overuse of a humidifier can raise the relative humidity to high levels, promoting the growth of dust mites and mold. A relative humidity of 30% to 50% is recommended for most homes.[3] Can also cause Hypersensitivity pneumonitis (humidifier lung).[4]
The EPA provides detailed information of the risks as well as recommended maintenance procedures[1]. 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 usually be prevented by using distilled water and a demineralization cartridge in ultrasonic humidifiers.
Deeming the design to be unsafe, the Supreme Court of Minnesota held the manufacturer of a humidifier strictly liable in products liability in the 1960s when a three-year-old knocked it over, severely burning herself.
See also
References