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How Humidifiers Work

As discussed in the Humidifiers Buying Guide, humidifiers come as both room-size table-top and consoles and as whole-house units that attach to the home’s heating and cooling forced-air system.  [GARD align=”left”]

All humidifiers are basically appliances that create moisture and transfer that moisture to the air. They may use any of several technologies to accomplish this feat.

Central evaporative humidifiers are made to be attached to the home heating system. They can output up to 25 gallons per day. High-capacity models are fan powered; other types don’t require a motor. All are hooked up to your house’s plumbing so manual filling isn’t needed.

Whole-house humidifier attaches to the home’s furnace or air handler.  ©


Evaporative Humidifiers

Humidifiers may employ a belt or drum to transfer moisture to air.  ©

Evaporative humidifiers, the most popular technology on the market today, capture virtually all minerals and pollutants from even the hardest water, eliminating the white dust that ultrasonic humidifiers can create. They work by passing an air stream through a wet medium such as a sponge or dampened grille.

Some evaporative humidifiers have fiber honeycomb-type panels that wick water upward from a reservoir; others employ a foam or cloth drum that spins through a water-filled trough as air passes by. Still others blow air through a woven aluminum pad that is saturated by a constant stream of water. In some cases, filters are treated with a special compound that retards bacterial growth.

With tabletop and console models, a quiet fan blows air through the damp surfaces. Some whole-house models use the heating system’s blower to do this job. A filter collects the dissolved solids; with most models, this filter is disposable.

One newer product, which Emerson offers, utilizes no moving parts, electrical cords, hot water, or even a motor. Ideal for a child’s room, this unit sits directly over a forced-air floor register and can be easily moved from one place to another. It retails for about $20.


cool mist impeller humidifier Sunbeam

Cool Mist Impeller Humidifier    Photo: Sunbeam

Cool Mist Impeller Humidifiers

A cool-mist impeller humidifier spins tiny droplets of moisture into the air. Available primarily as tabletop units, these hold from 2 to 3 gallons of water, are filled manually, and are easy to move around.

Cool-mist impeller humidifiers don’t have the problem with white dust that ultrasonic units have because the droplets produced are larger. And unlike steam humidifiers, cool-mist units use electricity only to propel the air, not to provide heat. This means they consume far less power.


Steam Humidifiers

Steam Humidifier   Photo: Vicks

Steam Humidifier         Photo: Vicks

Some humidifiers disperse water vapor into the air the same way a tea kettle does: by boiling water. Steam humidifiers are manufactured as tabletop or wall-attached models that spray steam into a living space or forced-air heating ductwork through a dispersion tube.

With all models except for tabletop varieties, tap water is piped directly into a steam unit so that it doesn’t require manual filling. Because steam is evaporated water, it doesn’t contain bacteria or mineral deposits that might contaminate the air you breathe.

Ductwork systems, which are manufactured for both commercial and residential applications, can be custom designed to output any amount of steam a house might need—though custom units may be prohibitively expensive for most home applications. And they can be costly to operate.

Steam units employ an electrical element to heat the water; when this is on for extended periods, it can rack up the energy dollars.


Ultrasonic Humidifiers

Though ultrasonic humidifiers used to be popular, they have fallen into disfavor because of the suspicious “white dust” they create.[GARD align=”right”]

With ultrasonic humidifiers, a thin sheet of water is passed over a transducer that vibrates and excites the water, breaking it into millions of tiny droplets. The problem is that calcium and other minerals in the water become airborne with the moisture, particularly in areas that have hard water. This ends up as a dust that falls on objects and pollutes the air you breathe. The Environmental Protection Agency has warned that the tiny particles of minerals and microorganisms that may be propelled into the air can cause illness or allergies. If you use an ultrasonic humidifier, fill it only with distilled water and clean it properly every day. Because of the questions surrounding these devices, we suggest you avoid using an ultrasonic humidifier in a child’s room or an unventilated enclosed space.

How to Fix a Humidifier

Like all appliances, sometimes humidifiers do not function properly and require simple troubleshooting and repairs.Typical problems are that they don’t work at all or that they work but fail to humidify a room properly.

Always check your manufacturer’s instructions first, and understand that some repairs you perform yourself may void your warranty.

repair humidifier multimeter

Multimeter       Photo: Innova

Tabletop and console humidifiers are controlled by a humidistat that turns the unit off and on when the humidity levels stray from a set range. If you don’t believe your humidifier is maintaining the proper humidity for your room or house, check its humidistat.[GARD align=”right”]

Though a humidistat allows for more or less “automatic” operation, you will need to dial it up and down as the temperature changes in order to maintain fairly constant indoor relative humidity levels.

Keep in mind that higher humidity levels in cold air will make the air feel colder. If you have a humidifier and your house feels cold in the winter, even when your heating system is working correctly, then your humidistat may be set improperly or may be working incorrectly.

You can test a humidistat with a volt-ohm meter.

To establish whether power is reaching the humidistat when you know there are no problems with the plug or switches, unplug the humidifier, remove the control panel, and lift the lid. Test the humidistat with a multimeter or volt-ohm meter set to the RX1 scale (or to K-Ω or Ω resistance on a digital meter). Clip the meter to the humidistat terminals. The meter should jump if you turn the humidistat from high to low. If it doesn’t, you’ll need to replace the humidistat or return the humidifier to the manufacturer.

Humidifier Does Not Work

When a humidifier doesn’t run, it generally means it isn’t receiving electrical power or the controls have automatically shut it off. Do the following to source the problem:

1Be sure the humidifier is plugged into an outlet that works and that the unit is turned on. Check the electrical receptacle for power using a working lamp, appliance, or voltage tester. If the receptacle seems to be dead, check the circuit breaker or fuse that serves the humidifier’s circuit.

2Check the setting on the humidistat. If it’s set lower than the room’s relative humidity, the humidifier won’t go on; in fact, it may take several hours for the humidistat to respond to the room’s changing humidity.

3Be sure the humidifier’s reservoir is full of water.


4Unplug the humidifier unit. Check the power cord and, if need be, repair or replace it.


5Remove the humidifier’s cover panel. Remove the humidistat and check it as described above. Replace it if necessary.

6If the humidifier still doesn’t work, call an appliance repair person, take the unit into an appliance repair shop, or replace the unit.

Humidifier Works But Not Well

If your humidifier doesn’t do a good job or runs constantly, it may be undersized for the space or humidified air may be escaping from the room. Perform these diagnostics:

1Check the unit’s specifications to be sure it can handle the room size.


2Be sure your home’s doors and windows are closed; also check the fireplace damper.[GARD align=”right”]

3Be sure the reservoir has plenty of water.


4Be sure walls or curtains are not blocking the unit.


5Disconnect the power cord and clean the unit according to the owner’s manual.


6Lubricate the fan motor bearings with a couple of drops of light oil if the motor has oil holes (some units are oil-less).

7If the humidifier still operates poorly, call an appliance repair person or take the unit into an appliance repair shop.

Humidifier Makes Noise, Leaks or Smells Bad

Though humidifiers can work reliably for several years, they can take on certain problems with time—most of these are issues that have to do with wear and tear or maintenance. Here’s a look at how to diagnose and treat some of the more common issues.

Working Parts of a Humidifier  ©

Working Parts of a Humidifier ©

Humidifier Makes Noise

A noisy humidifier may be the result of a fan motor or a drive mechanism that is not lubricated or is clogged with debris. Periodically clean the water reservoir, fan, and nozzle to ensure they are working properly.

Lubricate the fan motor bearings with SAE #20 oil. Be sure to consult your owner’s manual for other proper care and maintenance procedures to keep your humidifier functioning smoothly and efficiently.

Another cause of noise can be vibrating parts. To check for this:

1Unplug the unit, remove the cover, and look for any loose parts. Tighten any loose screws.

2Wiggle the fan to see if it is loose on its shaft. If it is, tighten the mounting fasteners.

3If the humidifier continues to makes noise, call an appliance repair person or take the unit into an appliance repair shop.

Humidifier Leaks

If water drips or pools at the base of your humidifier:

1Disconnect the power. Check the pan or reservoir, and empty it if necessary.


2Be sure the wick, hose, or other water-delivery system isn’t kinked or fouled.


3Be sure there are no cracks or broken seals in the water reservoir.

Humidifier Smells Bad

Odors are common because water stagnates in the tank. Over time, this grows mildew, mold, and bacteria that can stink. Cleaning a humidifier usually eliminates odors.

Don’t use chemical household cleaners or bleach to clean a humidifier—they can leave a residue and be caustic to parts. Instead, use vinegar (its odor will dissipate quickly) and hydrogen peroxide to clean the reservoir and filter. Use these two non-toxic cleansers in sequence, first applying and removing the vinegar, and then repeating with the hydrogen peroxide (this helps remove the vinegar’s smell). Work with two spray bottles—one filled with full-strength white vinegar and the other filled with full-strength hydrogen peroxide.

1 Unplug the unit, empty the reservoir, and mop up any damp areas.


2Clean the reservoir. Spray the inside with vinegar, allow to sit for 10 minutes, thoroughly rinse with clear water, and wipe with a clean rag or new sponge. Repeat the same process with the hydrogen peroxide.

3Clean the evaporator belt or pads. If you have manufacturer’s recommendations for cleaning the inside of the humidifier, follow them. If you can’t find directions, remove the cover to access the evaporator belt or pads, spray these surfaces with vinegar, and brush with a soft brush. Allow this to sit for 10 minutes, and then rinse thoroughly. Then repeat with hydrogen peroxide, let it sit for 10 minutes, rinse again, and wipe clean with a clean rag.

4Wipe or vacuum the fan blades clean. If the fan has a cover, remove it. Then use a damp cloth or vacuum to clean the fan blades and the area around the motor.


Home Humidifiers Buying Guide

Steam-mist humidifier emits a fine, moist mist into room air.

[/media-credit] Steam-mist humidifier emits a fine, moist mist into room air.

When your home’s interior humidity level—the amount of water vapor in the air—drops too low, interior air can become uncomfortable, damaging, and downright unhealthful.

Dry air draws moisture out of everything—from your eyes, lips, and skin to the carpets and furniture. The respiratory system, which relies on moisture to fight off viruses and bacteria, dries out, making you more susceptible to colds, coughs, and asthma. And bothersome static electricity increases.

When the dry winter heating season sets in and drops relative humidity to skin-chafing lows, boosting the humidity usually calls for a humidifier.

A humidifier is simply a device that puts water vapor into a home’s air. Depending upon its water-output capacity, it may serve a single room or the entire house.

Room-size humidifiers are referred to as either consoles or “tabletop” units. Consoles can humidify large areas; tabletop models are for single rooms.[GARD align=”left”]

To humidify a whole house, console humidifiers will work if the house is very open inside. Otherwise, you need a central humidifier that ties into the home’s forced-air heating system and delivers humidified air through the home heating and cooling system’s ductwork.

Buying a Central Humidifier

Central evaporative humidifiers are hooked up to the heating equipment, and water is piped directly to them, so they’re out of sight and out of mind most of the time.

Whole-house humidifier attaches to the home’s furnace or air handler. Photo: Aprilaire

Because they deliver humidified air directly to rooms throughout the house, they’re particularly efficient. The only drawbacks are that you need a forced-air system to operate a central humidifier and humidification takes place only when the forced-air system is running (this isn’t a problem in most homes because it is generally the heating process that dries out the air).

Central whole-house humidifier delivers conditioned air through the home

Equipment prices range from $140 to $200; the cost of installation depends upon the complexity of the work, but it is likely to run about $100.

If you’re looking into a central humidifier, be sure it can be installed relatively easily. In most cases, maintenance is limited to changing an evaporator pad about once a year; this component should be easy to inspect and access.

Central units are sold at home improvement centers or by heating equipment dealers. If you’re an accomplished do-it-yourselfer, a dealer can help you select the proper size, make sure you have all the necessary components, and give you installation advice. If you haven’t done this type of work before, have the unit professionally installed.

Buying a Room Humidifier

Tabletop and console room humidifiers are relatively inexpensive, easy to move from one room to another, and easy to hook up. They may any of use several different technologies to humidify the air, as discussed on the related pages that follow. Regardless of the method they use to humidify air, tabletop models cost from about $20 to $70, and consoles run from $75 to $150.

Tabletop units output from 2 to 4 gallons per 24 hours. Small consoles output 8 gallons per 24-hour period. The latter can handle up to a 2,000-square-foot area. A large console may output 14 gallons, enough to humidify a 3,250-square-foot house.  Both types must be filled manually—usually on a daily basis.[GARD align=”right”]

The Association of Home Appliance Manufacturers (AHAM) tests room humidifiers using uniform methods to verify water-output ratings and to allow consumers to compare models accurately. (For AHAM’s Directory of Certified Humidifiers, visit

Tabletop humidifier serves a single room and requires daily refilling. Photo: Honeywell

When buying a tabletop humidifier, check to see whether or not it has a humidistat that will turn the unit off and on as required. Also evaluate the number of speeds or settings it has—some have two or three speeds or a quieter night setting, for example.

Console humidifier is designed for serving large areas or an entire house. Photo: Bemis

It’s important to examine the types of controls on a console humidifier, which should have a low-water light, automatic humidistat, and air-flow controls. Multi-speed and variable air-flow settings and an automatic shut-off are also helpful. Some models even have a light that goes on when the filter is dirty.

For any humidifier that must be filled manually, find out how often it must be refilled when running at full speed.

To ensure the lasting and safe performance of a humidifier, be sure to follow the manufacturer’s directions for use and care.

NEXT SEE:  The four main types of humidifiers



Is the humidity inside your home too low? Dry air can be a real problem when it comes to your family’s comfort and health.

Whole-house humdifier attaches to the home’s furnace or air handler. Photo: Aprilaire

Low levels of water vapor in the air can dry out and irritate skin. The respiratory system, which relies on moisture to fight off viruses and bacteria, dries out, making you more susceptible to colds, coughs, and asthma.

Ideal levels of humidity in rooms should range from 30 percent to 50 percent in winter and 40 percent to 50 percent in summer. In dry-air climates, maintaining humidity at these levels demands mechanical assistance. That’s where humidifiers come in. These appliances are designed to add water vapor to the air on an as-needed basis.[GARD align=”right”]

This section of HomeTips explains how humidifiers work and will help you make informed buying decisions as well as guide you through step-by-step care and repair of your humidifier.


• Home Humidifiers Buying Guide
• How Humidifiers Work
How to Repair a Humidifier

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