Humidifiers: Types, Tank Sizes and What to Check

What a humidifier does, and who needs one

A humidifier puts water into the air as fine mist or invisible vapor. That raises the relative humidity, which is how much moisture the air holds compared with the most it could hold at that temperature.

Winter is when most houses need one. Cold outdoor air holds little water, and when the furnace warms it up, the relative humidity drops. That's why the static shocks start in December and the wooden floors tick at night. A humidifier for dry air puts some of that moisture back.

Not every house needs one. If a gauge in your living room reads 40 percent or higher in January, leave it alone. MedlinePlus, from the U.S. National Library of Medicine, recommends 40 to 50 percent. The EPA says not to go above 50 percent, because more moisture than that can encourage biological growth. Water on the inside of the windows on a cold morning is the sign you've gone too far.

The table above lists every humidifier in our data, from a few dozen brands. It sorts them smallest stated tank first. A dash means neither the maker nor the listing states that spec.

What to look at on a humidifier

Five specs decide most choices. I'll take them in the order I'd read them off the box.

Type: ultrasonic, evaporative or steam

This is the choice that matters most, because it changes what comes out of the machine.

• Ultrasonic: a plate vibrates and throws a fine, cool mist you can see; the most common type in our data
• Evaporative: a fan blows air through a wet wick filter; no visible mist; the second most common
• Steam (vaporizer): a heater boils the water; far fewer listings

The EPA says ultrasonic and impeller units appear to disperse the most microorganisms and minerals into the air. It says evaporative and steam units aren't expected to disperse substantial amounts of minerals. It also warns that steam and boiling water may cause burns and says to keep steam vaporizers out of children's reach.

The mist type field is related but not the same. Cool mist is room temperature, warm mist is heated, and some units offer both. Our guide to cool mist vs warm mist covers the trade in detail.

Tank capacity and runtime

The tank decides how often you refill. Stated tanks go from a fraction of a gallon on travel units to several gallons on the largest floor units. Runtime is how long the maker says one tank lasts, usually on the lowest setting. Stated runtimes go from an overnight to several days.

A bigger tank isn't always better. It's heavier to carry full, and in a small room it holds more water than the room needs. Size the tank to the room and to how often you're willing to walk to the sink.

Top fill helps more than people expect. It means you pour water in from a pitcher without lifting the tank out. A lot of listings state it now.

Coverage, as the maker states it

Coverage is the room size in square feet the maker says a unit can serve. It's a good way to compare two units, and that's about all. The maker's number assumes a room. Your room has a ceiling height, a stairwell, drafty windows and a door that opens to let the dog out.

Stated coverage goes from a nightstand's worth of room on small personal units to a whole house for steam units mounted on the furnace. If you're between two sizes for an open floor plan, go up a size.

Humidistat and auto shut-off

These two features are easy to confuse, and they do different jobs.

• Humidistat: a sensor that turns the unit off when the room reaches the humidity you set, and back on when it drops
• Auto shut-off: turns the unit off when the tank runs dry

A humidifier with humidistat control keeps the room inside a range instead of running until the tank is empty. Plenty of listings state one or both, and plenty state neither. If you're looking for the best humidifier with humidistat control for a bedroom, the bedroom list sorts quiet units, and most of them list one.

A humidifier with auto shut-off is the minimum I'd want in any room someone sleeps in. A humidistat is what keeps the windows dry.

Filters, cartridges and hard water

There are three kinds of parts you might have to replace:

• Wick filters in evaporative units, which hold the water the fan blows through
• Demineralization cartridges in some ultrasonic units, which trap minerals before they become mist
• Pre-filters or sponges in a few designs

Many ultrasonic and steam units list themselves as filter-free. That doesn't mean there's nothing to look after. It means the minerals stay in the tank as scale, or go into the room as dust.

If you're looking for the best humidifier for hard water, start with the type. Evaporative and steam units don't spread the minerals the way ultrasonic units do, according to the EPA. With any type, the EPA calls distillation the most effective way to remove minerals from water. It says to use demineralization cartridges only if the maker supplies or recommends them, and to watch for white dust, which means minerals are getting through.

We're on a well, and our water leaves a ring on everything it touches. That's why this section is longer than the others.

Sub-types and when to choose which

Most people shopping for a winter humidifier want one of five kinds. Here's how they differ.

Personal and travel units. Tanks under half a gallon, coverage around 160 to 250 square feet. For a desk, a nightstand or a hotel room. They need refilling daily or more.

Tabletop and bedroom units. Tanks of about 0.5 to 2 gallons. This is the biggest group. For one room, usually a bedroom or a home office. The bedroom list and the baby and nursery list pull from this group.

Large-room and floor units. Tanks of 2 to 6.8 gallons, with stated coverage into the thousands of square feet. For an open floor plan or a basement family room. The best humidifiers list sorts these largest tank first.

Warm-mist and steam units. They boil water, so the vapor coming out is warm. They don't spread minerals the way ultrasonic units do, per the EPA, but they carry a burn risk, so keep them out of children's reach. See warm mist humidifiers.

Whole-house units. These are a different animal. The AprilAire and Honeywell Home whole-house units in our data are sold as furnace humidifiers. They mount on the ductwork and humidify the air the furnace moves through the house. Installing one means connecting it to the furnace, the ductwork, a water supply and often a drain. That's a call for a pro, not a Saturday project.

I spent 34 years around furnaces, so I'll say this much. A whole-house unit is only as good as the person who maintains it. The water panel or wick needs changing on the maker's schedule, and the water line needs a shutoff you can find. If you buy a house with one already on the furnace, find the manual before the first cold snap.

Cleaning: the part the box leaves out

Here's my one opinion on this page: buy the humidifier you'll actually clean. The cleaning schedule, not the price or the tank, is the real cost of owning one. The reason is in the EPA's own guidance.

For portable units, the EPA says:

• Every day: unplug it, empty the tank, wipe it dry and refill with fresh water
• Every third day: clean it and remove scale and film
• After a cleaner or disinfectant: rinse the tank thoroughly
• For console and central units: follow the maker's directions and don't let water sit for long periods

A unit with a wide tank opening you can get a hand and a brush into will get cleaned. A unit with a narrow neck and six small parts will get cleaned less. Look at the photos of the tank before you buy. The easy-to-clean humidifiers page groups the listings that say so.

Ana's ruling, after looking into one tank too many: a humidifier you can't get your hand into is a vase with a cord. She has turned down two at the store on that rule alone. Neither of them is on our hall shelf, and the one that is gets cleaned.

Noise, if you sleep near it

Only a minority of humidifier listings in our data state a noise level. For scale, the CDC's NIOSH noise chart puts a whisper at 30 dB and normal conversation at 60 dB. Makers don't always say how far away or at which setting they measured, so treat the number as a way to compare two units. If noise decides it, the bedroom list only takes units that state one.

Brands that make humidifiers

Every brand in our data is listed the same way, alphabetically. The table shows which models each one has right now.

• AEOCKY, Afloia, AIRCARE, BLUEAIR, BONECO, BREEZOME
• Canopy, Carepod, Crane, DREO, Everlasting Comfort
• Germ Guardian, GoveeLife, Homedics, Honeywell, Hunter
• Lacidoll, LEVOIT, Pure Enrichment, Safety 1st, Turbro
• Vicks, Vornado

Other brands in the table, AprilAire among them, appear in the brand catalog.

Before you buy

Get a humidity gauge first. They're small, they run on a battery, and they tell you whether you need a humidifier at all. If the number is under 30 percent in winter, you'll know. If it's over 40, you've saved yourself a machine. Our guide on ideal indoor humidity explains what to do with the reading.

Read the gauge, pick the type, then read the plate.