Which humidifiers are the quietest?
Ultrasonic humidifiers are the quietest humidifiers available, so if noise level is your number one concern, then you should consider an ultrasonic humidifier. These are ideal if you are a light sleeper and require as little noise as possible. In general, a warm moisture humidifier will be much quieter than a cool moisture evaporative humidifier. Warm moisture models release steam vapor into the air without the use of loud, powerful fans - they do make an occasional gurgling sound as the water empties from the water tank and is heated into steam, and there is also a boiling noise that is a result of the heating element, however these sounds are much quieter than cool moisture evaporative humidifiers which use loud fans to circulate evaporated humidity. Cool moisture evaporative humidifiers use a fan to blow air through an evaporative wick filter. These types of humidifiers make more noise than a warm moisture humidifier due to the fan. The level of noise varies greatly depending on the fan speed, therefore the more fan speeds your humidifier has, the greater flexibility you will have in choosing the best combination of humidity output and noise level. In general, when running on the lowest fan speed, cool moisture evaporative humidifiers produce a quiet background "white noise", which many people find beneficial when sleeping.

Do I need to replace filters, and if so, how often?
Most evaporative cool moisture humidifiers use replaceable wick filters, which need to be replaced every 2-3 months on average (wick filter life varies depending on your specific humidifier model, frequency of use, and your water's mineral content). Most wick filters are coated with a special antimicrobial coating designed to slow the growth of mold on the filter. There are some evaporative cool moisture humidifiers which use permanent reusable filters, so these will not need to be replaced, just cleaned regularly. Warm moisture humidifiers do not use wick filters at all. Some models do use an optional mineral absorption pad, which collects the mineral deposits left behind by the heated water. These pads typically last about 2-3 weeks and are very inexpensive to replace. Some ultrasonic humidifiers do not use any filters at all, but the higher end models use demineralization cartridges, which helps to reduce mineral deposits, which exit the humidifier in the form of white dust.

What is white dust, and should I be concerned about it?
White dust is caused by minerals in your water being released into the air during the humidification process, and potentially settling on furniture and other horizontal surfaces nearby the humidifier. White dust, while it can be annoying, is not harmful. Warm moisture (steam vaporizers) and evaporative cool moisture humidifiers do not release any white dust into the air. The only types of humidifiers that can potentially produce white dust are ultrasonic humidifiers and cool mist impeller humidifiers. Higher end ultrasonic humidifiers typically come with a demineralization cartridge specifically designed to trap mineral deposits before they can become airborne. These demineralization cartridges are not always 100% effective, and the amount of white dust you may experience will vary depending on the mineral content of your water supply. If white dust is a concern, you may wish to use distilled water in your humidifier instead of regular tap water. Distilled water, although more expensive than tap water, has very low mineral content, and will not produce white dust. For other types of humidifiers, using distilled water will also reduce the amount of time you will need to clean your humidifier, and will help prolong the life of any filters.

How often do I need to clean my humidifier?
All humidifiers require regular cleaning to prevent the growth of mold, mildew, fungi, and bacteria. These organisms can thrive in the standing water that is left inside your humidifier for more than one day. Therefore, you should make sure to empty and refill your humidifier on a daily basis, or keep your humidifier running throughout the day, to avoid having standing water sitting in your humidifier. Regular cleaning is recommended at least once every 1-2 weeks (more often depending on how often your humidifier is in use, and the level of your water's mineral content). The product manual that comes with your humidifier will have specific instructions as to the best way to clean and maintain your humidifier. Please follow your specific humidifier's cleaning instructions carefully to ensure the most healthy and efficient operation of your humidifier.

What does humidifier room size mean, and which size humidifier is best for me?
Humidifier "room size" refers to the recommended room size (usually stated in square feet) that many humidifier manufacturers publish for each model humidifier they produce, in order to help you select the best sized humidifier for your room or area. A recommended room size is not a guarantee that the humidifier will be able to raise the relative humidity (RH) level of any particular room size to any specific RH level. Room size recommendations should be used only as rough guidelines when shopping for a humidifier, as these recommendations are subjective, are based on ideal environmental conditions, and can vary from manufacturer to manufacturer. Each manufacturer bases their recommended room sizes on the maximum amount of moisture the humidifier is capable of producing per day, assuming the humidifier is operating in perfect environmental conditions, with the humidifier constantly running on its maximum speed setting.

No two room environments are exactly the same, and the RH level even in a single room can often vary greatly throughout the course of a single day. A humidifier is only responsible for the moisture it adds to a room. Once that added moisture exits the humidifier and enters the room air, each room has many factors working to remove that added moisture and dry out the air again. This is a constant struggle between room conditions and humidifier output that changes from room to room, day to day, even hour to hour, depending on a countless number of factors, including the outdoor temperature and RH level, the indoor temperature, how often your heat is running, the type of heat being used (forced air heating, baseboard heating, wood stoves, etc.), the location of forced air ducts and return air ducts, the number and location of windows and doors, how well each window is insulated, the height of the ceilings, how often the door to the room is open or closed, etc.

As a result of these various factors and room environments, which cannot possibly be encompassed by a single room size recommendation, the actual RH level that you will be able to achieve with any given humidifier will depend on your unique room environment. We choose to publish manufacturer's recommended room sizes because that is how most humidifiers are sold today. However, after years of our own testing of every humidifier we sell in real world conditions, we have found that you will get the best performance by using the highest output humidifier possible that will fit in your room and budget. If achieving the highest possible RH level for your room is of the utmost important to you (even more important than other factors such as noise level or filter replacements), then it is our recommendation that you select as large a humidifier as possible, to ensure that you get as much additional moisture into your room as possible.

What is humidifier output?
Because room size recommendations are subjective and do not accurately reflect real-world environmental conditions, a much more accurate way to compare different humidifiers is with "moisture output per day" (also known simply as "output"). Humidifier output is often confused with water tank capacity (see below for an explanation of tank capacity). The output of any given humidifier refers to the maximum number of gallons of water the humidifier is capable of producing per 24 hour period. The output of a humidifier can easily be tested and verified, which makes it a very easy way to compare the effectiveness of various humidifiers, regardless of the manufacturer or even the type of humidifier.

For example, a humidifier that has an output of 2.0 gallons per day will be capable of adding a maximum of 2.0 gallons of water to the room per 24 hours (assuming the humidifier is running constantly throughout the day, and is set to the highest fan speed or mist level). Humidifiers vary greatly in output, ranging from as low as 1.0 gallons per day all the way up to 14.0 gallons per day, and anywhere in between. The higher the output, the more moisture the humidifier will be able to produce in any given day. A humidifier with an output of 10.0 gallons per day will be capable of producing 5 X more moisture than a humidifier with a 2.0 gallon output. And two different humidifiers, from two different manufacturers, that both have an output of 4.0 gallons per day, will both be just as effective, regardless of the room sizes that each manufacturer recommends.

So, while you can look at recommended room sizes to give you a general idea of which humidifiers to consider, keep in mind that room sizes are not guarantees of performance, and a much more useful and accurate piece of information is the humidifier output, which is an easily testable and verifiable figure that does not vary from manufacturer to manufacturer.

What is humidifier tank capacity, and how often will I need to refill my humidifier?
Humidifier tank capacity refers to the maximum amount of water the humidifier can hold at any one time. Tank capacity varies from as low as 0.5 gallons of water all the way up to 6 gallons of water. The higher the capacity of the humidifier, the more water it will hold, and the longer you may be able to go between refills. Most humidifiers have a removable water tank which is filled at a sink or bathtub and then placed onto the humidifier base. As the water in the base of the humidifier is used, more water from the tank will be released into the base of the humidifier. Some humidifiers have an automatic shut-off feature which will turn off the humidifier once the water needs to be refilled. There are also some humidifiers with a "top-fill" design, which eliminates the need for the removable water tank. With these humidifiers, you pour water directly into the top of the humidifier using a pitcher, cup, or any other water container, bypassing the water tank entirely. The humidifier itself holds all of the water. This can be helpful for those who may have trouble lifting and carrying a water tank. Whichever humidifier you choose, if the humidifier is running constantly on maximum speed, you will most likely need to refill the humidifier at least once every 24 hours. For example, if your humidifier has a 5.0 gallon output per day, and it has a tank capacity of 2.5 gallons, you will need to refill the humidifier once per 24 hours in order to obtain the full possible 5.0 gallon maximum output that the humidifier is capable of producing. In theory, if a humidifier had an output of 5.0 gallons per day and a tank capacity of 5.0 gallons, then you could run the humidifier on high for 24 hours without a refill. In practice, most humidifiers hold less water than their maximum output, but the closer the tank capacity is to the output of the humidifier, the less often you will need to refill the humidifier. In addition, if you run your humidifier on a lower speed, or if you do not run it constantly throughout the day, you will not need to refill it as often.