The two main types of portable cool-mist humidifiers - ultrasonic and evaporative. And while either can be efficient in terms of humidifying a room, they have significant differences in how they produce that moisture. You may have to accept some trade-offs if you want to get the unit with your preferred characteristics.
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There are basically three major differences between an ultrasonic and evaporative humidifier: Price, operation noise and how each handles mineral scale deposits (white/grey dust). But first, you need to understand how the two types differ from one another:
An ultrasonic humidifier is one that uses high-frequency sound vibrations to produce an extra fine water mist that is then expelled to add moisture to the room. It is often considered the quieter of both types of humidifiers. The ultrasonic generally has no filter factored into its design. That does save operation costs, but it does come with a trade-off.
You'll get more greyish dust around the room when operating an ultrasonic humidifier than with an evaporative because there's usually no filter to absorb mineral scale deposits. Some models, however, come with ceramic-type cartridges that can be refreshed, and these are effective to some degree.
Another way to reduce the scale dust is to use demineralized water in the reservoir, though that can be pricey.
Some prefer this type of humidifier because it is considered safer in that there is no hot water present in the unit and therefore no risk of scalding since the unit does not heat the water in any way.
As with any humidifier that does not boil the water to produce a mist, there is a risk of bacteria being present in it and being discharged to the room. Disinfecting the humidifier becomes even more important than with a warm mist humidifier that does boil the water.
These humidifiers have been in use for decades and are considered the most common or garden-variety type of humidifying appliance. The most basic (traditional) way to humidify a room is to place a pot or jar of water in the room and allow the moisture to naturally evaporate into the air to relieve dryness.
An evaporative humidifier works on basically the same concept but is more efficient in getting the humidity out to the room. Evaporative humidifiers can vary by design. There are a few non-filter units that simply churn out a water mist or vapor, but most have filters which require routine cleaning or replacing. So generally, there is an ongoing operation cost in way of replacement filters for evaporative humidifiers.
With the help of a wick filter to hold and draw the water up to a fan, it expels it from the unit as a water mist or spray. Though the word 'wick' sounds very small, a wick filter can be quite large and takes up most of the room inside of a humidifier's casing.
Some models can combine some humidification features along with a filter design.
This is a popular filter design that because of the filter's function, traps mineral scale and reduces the risk of fine white/ grey scale dust filtering to the room. Some models are also designed for aroma therapy during the humidification process. Though you would expect the type of humidifier to be clearly identifiable on the packaging, that isn't always easy. A key thing to remember is that if a cool mist humidifier has a filter, you will need to clean or replace it, but you shouldn't have to worry about scale dust.
1. Price Considerations
With two totally different types of humidification systems, there's bound to be some noticeable differences in operation.
An evaporative humidifier is cheaper to buy, but most have filters, usually wick style, that must be either washed or changed periodically. So a lower cost up front is followed by more maintenance costs later, not to mention having to find the correct replacement filters. Replacement filter costs tend to vary depending on size and brand, but most are low and reasonable cost.
On the other hand, an ultrasonic humidifier usually does not use filters and generally costs more to buy than an evaporative model. I say usually because there are some models that have an ultrasonic style of misting, yet also some type of filter pack. An ultrasonic humidifier still should be maintained properly to avoid mold and bacteria, as with any water-borne appliance.
2. Quiet Operation
The most common place for a portable humidifier is in the bedroom; secondary is the main living space, so it's no wonder that the majority of consumers want a humidifier with quiet operation. If humidifier noise is your main concern, an ultrasonic humidifier is your best choice.
While there are some with optimum quiet operation, most ultrasonic humidifiers tend to operate quieter than other models. That's because they do not have a fan, as evaporative humidifiers usually do. This keeps noise levels down.
If noise is not a concern but the budget is, choose a more affordable evaporative humidifier. You should understand though that even those humidifiers that tout a quiet operation will be noisier when used on anything but low (moisture volume); it's impossible to totally eliminate the operation noise of humidifiers.
3. Annoying Humidifier White Dust
Ultrasonic humidifiers tend to discharge more white/grey dust than evaporative humidifiers especially if they have no onboard filter pack. That white/grey dust is actually water mineral deposits, which will be more extreme the harder your water is. Hard water is caused by the presence of mineral deposits in your water source and varies depending on your particular source for drinking water.
The reason an ultrasonic humidifier produces more dust is that it usually has no filters to trap or contain those mineral deposits and they are returned to the room along with the moisture, only to land on furniture and electronics as dust - basically on anything close to the humidifier.
Remedies to control the humidifier white dust are: Use distilled or reverse osmosis water; a humidifier decalcification filter if there is one available for your humidifier model; or run the humidifier moisture on low to reduce the amount of dust you need to clean.
It's impossible to eliminate this white dust. The mineral deposits in the water have to go somewhere. It's a little trade-off when you choose an ultrasonic humidifier. The larger the output of moisture, the more dust to clean. Also keep in mind that there is some compact, especially personal size evaporative humidifiers that have no filters in their design. Because the output is a smaller volume of moisture, mineral scale dust is barely noticeable, if at all.
Which Type is Best for You?
If you loathe humidifier filter change/cleaning or simply want a quieter humidifier - chose an ultrasonic filter-free model. You'll pay more upfront and be prepared to clean around the humidifier more often.
For budget pricing, choose an evaporative humidifier, but understand that filters do need periodic changing or cleaning to keep them running efficiently. Locate filter replacements before buying a humidifier and always have a spare filter on hand.
Read More About Humidifiers:
Humidifier Facts: what you need to know about humidifiers
Portable vs Console Humidifiers
What's the Difference Between a Hygrometer and Humidistat?
What's a Hygrometer and Do You Need One?