Mold in a Dehumidifier, AC or HVAC System: What to Do

Mold in a dehumidifier usually grows in the water bucket, on the filter or on the cold coils, because those parts stay damp. Unplug the unit, empty and wash the bucket with mild detergent and water, clean or replace the filter, and let everything dry. Mold inside an AC unit, air handler or ductwork is different: the EPA says not to run an HVAC system you know or suspect is contaminated with mold, because it can spread spores through the house.

Why mold grows in dehumidifiers, AC units and ducts

Mold needs moisture, a surface to feed on (dust is enough), and time. Dehumidifiers and air conditioners pull water out of the air on purpose, so they create exactly those conditions:

• Dehumidifier bucket: standing water sits for days in a dark container, often with dust and debris in it.
• Coils: the evaporator coil is cold and wet whenever the unit runs.
• Filters: they trap dust and organic particles, and they stay damp in humid rooms.
• AC drain pans and condensate lines: water that does not drain fully sits under the coil. The EPA advises keeping air conditioning drip pans clean and drain lines unobstructed and flowing properly.
• Ductwork: ducts that sweat in humid attics or crawl spaces, or that sit near a leaking coil, can get damp enough to support growth. Dust inside the duct gives mold something to eat.

The fix always starts with the moisture. As the EPA's air duct guidance puts it, if the conditions that caused the mold are not corrected, mold growth will recur.

Signs of mold in your dehumidifier or HVAC system

Early stage signs of black mold in air vents often look like small dark specks or fuzz around the register, or a smell that appears only when the blower runs. Keep in mind that dark dust on a vent cover is common and is not always mold. The EPA notes that only an expert can make a positive determination of whether something is mold, and lab analysis may be needed. Our guide to mold testing and inspection explains when testing is worth it.

How to clean mold out of a dehumidifier

Most portable dehumidifiers can be cleaned at home. Always follow your model's manual first. A typical routine, based on manufacturer instructions such as this GE dehumidifier owner's manual:

1. Turn it off and unplug it. Move it somewhere you can work.
2. Empty the bucket. Pour the water down a drain, not onto carpet or soil near the house.
3. Wash the bucket. Partly fill it with clean water and a little mild detergent, swish it around, empty and rinse. The GE manual says to clean the bucket every few weeks to prevent mold, mildew and bacteria, and not to put it in a dishwasher.
4. Clean the filter. Most filters can be washed in warm, soapy water. Let the filter dry completely before you put it back. Never run the unit without its filter.
5. Wipe the case and grille. Use water and mild detergent. The same manual warns against bleach and abrasives on the case.
6. Look at the coils. If you see growth on the coils deep inside the unit, check the manual or contact the manufacturer. Many coils are hard to reach without taking the unit apart.
7. Dry everything, then reassemble. The bucket must be fully seated or the unit will not run.

How to clean a dehumidifier bucket in short: empty it often, wash it with mild soap every few weeks, and dry it before reinserting. If your unit has a continuous drain hose option, using it means no standing water sits in the bucket at all. Check that the hose drains downhill and is not kinked.

A note on mold clean up products and bleach

It is tempting to reach for a strong mold removal product. The EPA says the use of a chemical or biocide that kills mold, such as chlorine bleach, is not recommended as a routine practice during mold cleanup, partly because dead mold can still cause allergic reactions. Removing the growth and fixing the moisture matters more than killing it. If you do use bleach on a hard surface, the CDC says to use no more than 1 cup of bleach in 1 gallon of water and never mix bleach with ammonia or any other cleanser.

Mold in an AC unit, air handler or ductwork

HVAC mold is a bigger job because the system moves air through the whole house.

Stop running the system. The EPA says do not run the HVAC system if you know or suspect it is contaminated with mold.

Find the moisture source. Common causes are a clogged condensate line, a drain pan that does not slope toward the drain, an oversized system that short-cycles and leaves air humid, leaky ducts pulling in damp crawl space or attic air, or a past water leak near the air handler.

Know what can and cannot be cleaned. The EPA's guidance on air duct cleaning makes several points homeowners often miss:

• Cleaning may be worth considering when there is substantial visible mold growth inside hard surface (sheet metal) ducts or on other parts of the system.
• Wet or moldy duct insulation cannot be effectively cleaned and should be removed and replaced.
• EPA has not registered any biocides for use on fiberglass duct board or fiberglass-lined ducts.
• Sealants should never be used on wet duct liner, to cover actively growing mold, or to cover debris in the ducts.
• Moisture-based methods such as steam cleaning should not be used on ductwork.

What a professional HVAC mold removal job usually involves: inspection of the coil, pan, blower and ducts; fixing the drainage or humidity problem; removing growth from hard surfaces; replacing moldy insulation or flexible duct; and confirming the system is clean before it runs again. Because duct mold removal touches both HVAC equipment and mold remediation, you may need an HVAC contractor and a mold remediation company working together. Ask any company exactly which parts they will clean, which they will replace, and how they will fix the moisture cause.

If mold has spread beyond the system into walls, ceilings or a basement, that is a mold remediation project, not just a duct cleaning.

Does a dehumidifier kill mold?

No. A dehumidifier does not kill existing mold. It lowers humidity, which helps keep new mold from growing and can slow existing growth. You still have to clean up or remove the mold that is already there.

Using a dehumidifier for mold prevention works well when you aim for the EPA's target: keep indoor humidity below 60 percent, ideally between 30 and 50 percent. A cheap hygrometer (humidity meter) tells you whether the unit is doing its job.

Tips that keep the dehumidifier from becoming the problem:

• Empty the bucket daily in humid seasons, or use a drain hose.
• Clean the bucket and filter on the schedule in your manual.
• Close windows and doors to the room so the unit is not trying to dry the outdoors.
• Leave space around the unit for air to flow.
• Fix the water source. A dehumidifier in a basement with a leaking foundation is treating a symptom. See our guide to basement mold for the bigger picture.

What are the downsides of using a dehumidifier? It uses electricity, adds some heat and noise, needs regular emptying and cleaning, and can make a room uncomfortably dry if set too low.

Signs your dehumidifier is going bad include the bucket filling much more slowly than usual in the same conditions, the room's humidity reading staying high, frost on the coils, the compressor not running, unusual noises, or a musty smell that comes back right after cleaning. Your owner's manual's troubleshooting section is the first place to check.

Health: who should be careful

Mold affects people differently. The CDC says exposure to damp and moldy environments may cause health effects such as stuffy nose, wheezing, and red or itchy eyes or skin, or none at all. People with asthma or mold allergies may have severe reactions, and people with weakened immune systems or chronic lung disease can be at higher risk. The CDC's cleanup guidance also says people with asthma, COPD or weakened immune systems should not stay in a moldy home or be present while it is cleaned.

This guide is informational only. If you or anyone in your home has symptoms you think are related to mold, see a doctor. Our guide to mold exposure symptoms covers what the health agencies say.

When to call a professional

Call a professional if:

• Mold is inside the air handler, on the coil, or in the ductwork.
• The ducts are insulated or made of fiberglass duct board and show growth.
• Mold keeps coming back after you clean it.
• The moldy area totals more than about 10 square feet. The EPA says most people can handle smaller patches themselves but larger areas call for its professional remediation guidance.
• Someone in the home has asthma, a mold allergy or a weakened immune system.
• The mold followed a leak, flood or sewage backup.

In those cases, a company that does professional mold remediation can contain the area so spores do not spread while the growth is removed.

Costs depend heavily on how far the mold has spread; our guide to mold remediation cost explains the main drivers. If you need to find a mold remediation or HVAC mold cleaning service, you can start with our locations page.

Flood Rescue Pros is a free service that connects homeowners with independent local mold and water damage restoration companies. If you want help finding one near you, Get help now.

Frequently asked questions

Is it safe to run a dehumidifier with mold in it?

It is better not to. Unplug it, clean the bucket, filter and case, and let it dry. If mold is on the internal coils and you cannot reach them, check the manual or contact the manufacturer before using it again.

Can mold in AC ducts make you sick?

Mold can cause symptoms such as stuffy nose, wheezing and itchy eyes in some people, according to the CDC, and people with asthma or allergies can react more strongly. The EPA says not to run an HVAC system you suspect is contaminated with mold. See a doctor if you have symptoms.

How do I know if it is mold or just dust in my air vents?

You often cannot tell by looking. Dust wipes away and does not return quickly, while mold tends to come back and may smell musty. The EPA says a positive determination can only be made by an expert, sometimes with lab analysis.

Does a dehumidifier get rid of black mold?

No. A dehumidifier lowers humidity so mold is less likely to grow, but it does not remove growth that is already there. Black mold needs to be cleaned or removed and the moisture source fixed. Our black mold guide explains more.

Is it safe to live in a house with mold in the basement?

That depends on how much mold there is, whether the moisture is ongoing, and who lives in the home. The CDC says people with asthma, allergies or weakened immune systems are more sensitive. Fix the water source and clean up the mold, and call a professional for areas larger than about 10 square feet.

What humidity should I set my dehumidifier to?

The EPA recommends keeping indoor humidity below 60 percent, ideally between 30 and 50 percent. Use a hygrometer to check the real reading in the room.

Key facts

• Dehumidifier mold usually grows in the bucket, filter and coils; wash the bucket with mild detergent every few weeks and follow your manual.
• The EPA says not to run an HVAC system you know or suspect is contaminated with mold.
• Moldy duct insulation and fiberglass duct board cannot be effectively cleaned and should be replaced, according to the EPA.
• A dehumidifier does not kill mold; it prevents new growth by keeping humidity below 60 percent, ideally 30 to 50 percent.
• The EPA does not recommend bleach or other biocides as a routine part of mold cleanup; removing the growth and fixing the moisture matters most.
• Call a professional for mold in the air handler or ducts, recurring mold, or areas larger than about 10 square feet, and see a doctor for any symptoms.