Black Stuff In Your Dehumidifier – Diagnosis, Solution And Prevention


The black substance in or around the water tank of a dehumidifier is black mold, mineral build-up, or dirt. Black mold, the most common cause of black residue in dehumidifiers, can lead to respiratory problems and allergenic reactions in hypersensitive people.

Dehumidifiers are great devices for maintaining healthy humidity levels in your home. However, sometimes black stuff appears on the sides, top, or just above the dehumidifier’s water collection tank.

This article explains the causes of the black stuff in your dehumidifier and ways to diagnose, fix and prevent these problems. It also discusses some frequently asked questions about black mold

Black Stuff in your Dehumidifier- What is it?

A black substance can easily develop inside the water collection tanks of dehumidifiers. This is because water tanks are predominantly wet environments.

The black residue inside your dehumidifier can be:

  1. Black mold
  2. Mineral build-up
  3. Dirt

If it looks like your dehumidifier is also not collecting water properly, Plentiful Air has a troubleshooting guide that can help you available here.

1. Black Mold

Black mold is the most common reason for the black residue in your dehumidifier. This is because this fungus thrives in dark and moist environments.

The term black mold refers to several mold species having a blackish appearance. These fungi are non-toxic.

The water extracted from the air is stored in the dehumidifier’s water tank, and any water that sits for long durations can cause black mold growth. If the water from the dehumidifier’s water bucket is not drained regularly, black mold will grow on the surface, top, and sides of the water tank. It can also appear if the dehumidifier is not cleaned properly.

The black mold can be invisible initially. However, you will likely notice a musty odor from the dehumidifier.

Black Mold spores can be transmitted into the air, allowing this fungus to spread throughout your home where it finds suitable environments and continues to grow. It’s important to take quick preventive and treatment measures to prevent the growth of black mold in your dehumidifier.

How to Diagnose Black Mold

The texture of black mold is usually slimy. Although, some varieties of black mold start off as a powdery substance. The most common sign of mold build-up is a musty odor.

You can determine the texture of black mold by gently rubbing it with some paper. Before identifying the black mold, don’t forget to put on gloves and a face mask. If you notice any unusual symptoms or allergic responses, stop using the dehumidifier immediately and make an appointment with a professional to inspect for black mold in your dehumidifier’s water tank.

Symptoms of Black Mold Exposure

Black mold is not a toxic mold. According to EPA, generally, mold exposure symptoms are:

  • Sneezing
  • Skin irritation
  • Nasal congestion
  • Irritation in eyes or nose
  • Coughing
  • Sore throat
  • Lung problems

2. Mineral Build-up

The black substance in a dehumidifier can appear due to mineral build-up (such as iron, manganese, and calcium) inside the water tank. Calcium mineral deposits are usually white or grey.

However, as the dehumidifiers collect water from the air, it passes through the different components, combines with any dust or dirt, and turns black. This is clearly visible as mineral build-up inside the water tank.

This could be caused by various minerals mixed with water collected from the air and deposited over time. This can result in black specks in the tank’s bottom.

Generally, this black residue has no harmful impacts on human health. However, you should still handle it carefully.

How to Detect Mineral Build-up

Look for visible black stains to detect the mineral build-up in a dehumidifier’s water tank. They’ll look more like patches stuck to walls or surfaces instead of any particular texture.

They range from soft to hard depending on how long they’ve been accumulating.

3. Dirt

A polluted and dusty environment also causes a black residue in a dehumidifier. If you live in a dusty environment, dirt can easily enter your water tank, and collect on the sides or bottom of the dehumidifier as a black substance.

How to Detect Dirt

To determine if dirt is causing the black residue in the water tank of your dehumidifier, firstly, examine the bottom of the water tank.

The dirt tends to sink into the bottom rather than stay suspended. In most cases, you’ll see black stuff only at the bottom of the water tank if dirt is the culprit.

How to generally clean your Dehumidifier

Regular cleaning can prevent mineral build-up, dirt, and the growth of black mold. Here’s a step-by-step guide to clean your dehumidifier.

  • Please turn off the dehumidifier and unplug it.
  • Remove the dust from the surface using a vacuum or wiping with a soft cloth.
  • You can use a damp cloth to wash the surface. Avoid using any harsh chemicals.
  • Please take out the air filter and wash it with tap water.
  • The moisture collects in the water bucket and needs to be cleaned regularly. Please take out the water bucket and wash it using a mild soap. Gently scrub the surface of the water bucket to get rid of any accumulated dust and mineral deposits.
  • Air dry all parts and assemble them into the unit.

How to Remove Black Mold from A Dehumidifier

To get rid of black mold from the dehumidifier, you can use:

  1. Vinegar solution
  2. Distilled white vinegar spray
  3. Bleach
  4. Commercial Products

Always wear personal protective equipment such as a face mask (EPA recommends N-95 respirator), goggles, and gloves before initiating mold clean-up.

Prior to Cleaning

  • Hold the sides of the water tank and remove it by pulling it out.
  • Open your windows or doors for proper ventilation.

1. Using Diluted Vinegar Solution

  • Add diluted vinegar (1vinegar:1water) into the water tank. Let this solution stay inside the water bucket for at least 1 hour. For proper sanitization, let it stand for 6-8 hours.
  • Discard the vinegar solution.
  • Rinse the water tank with clean water.

2. Using Concentrated Vinegar Spray

  • Pour concentrated white vinegar into a spray bottle and spray it directly onto the water tank’s surface, sides, and bottom. Make sure you’re using undiluted and distilled white vinegar.
  • Allow at least an hour for the vinegar to work. Then scrub the water tank thoroughly with a soft brush and rinse it with water.

3. Using Bleach

You can use bleach to kill black mold, if necessary. To use bleach for killing mold:

  • Add 1 cup of bleach into 1 gallon of water and pour it into a spray bottle.
  • Spray the bleach mixture all over the water tank.
  • Let this solution sit there for at least one hour.
  • Rinse the water tank with clean water. You can also use a soft brush to remove any residues in the water tank and wash them away.

4. Using Commercial Products

You can also use mold-killing sprays (such as RMR-141 Disinfectant spray or Concrobium 25326 Mold Control Spray) to get rid of black mold.

Once complete rinse the water tank thoroughly with clean water and allow it to air dry before placing back into the dehumidifier unit.

Repeat this process at least every three weeks to avoid mold growth.

Mold Sampling

Mold sampling and identifying mold species are usually not required. However, you can opt for surface sampling to ensure that your dehumidifier is free of mold or mold spores.

Mold sampling should only be conducted by highly skilled professionals.

Frequently Asked Questions

Can Dehumidifiers Spread Mold?

Dehumidifiers don’t spread the mold directly. However, the mold can grow inside the water tank or nearby damp parts. The mold matures and produces spores over time. These spores can be dispersed through the air and spread throughout the house.

How to Prevent Mold from Growing in My Dehumidifier?

To prevent the growth of mold in a dehumidifier:

  • Empty the water bucket on a regular basis.
  • Clean your dehumidifier regularly.
  • EPA recommends using efficient air filters to prevent dust and mold spores from entering the system.
  • Use a hose directly connected to the dehumidifier for continuous water drainage.

Is Black Mold Harmful?

Healthy persons are not at risk from black mold according to a research report. In hypersensitive or immunocompromised people, it might trigger allergic reactions, asthma, and other respiratory ailments.

Can I Use Bleach to Kill Mold?

Bleach can be used to kill mold. However, according to the Environmental Protection Agency, bleach should not be used as a regular practice to get rid of mold. Although, some professionals might suggest using bleach for mold clean-up only if it is highly needed. If you plan on using chlorine bleach, make sure to ventilate the area well.

Don’t mix chlorine bleach solution with detergents or other ammonia-containing solutions as it can release toxic fumes.

How Often Should I Empty a Dehumidifier’s Water Bucket?

A dehumidifier’s water tank should be emptied at regular intervals. The frequency will depend on the capacity of the dehumidifier, the humidity settings, and the length of operating time. For example, a dehumidifier with a small capacity, and low humidity setting that operates all day will need its water bucket emptied at least twice a day.  

Draining the water once to twice a day is recommended for highly humid environments.

Plentiful Air has more information on dehumidifiers that don’t need to be manually emptied available here.

Why is my Dehumidifier’s Water Dirty?

The water in your dehumidifier is dirty if it stays there for a long time. This is because stagnant water harbors different microbes such as algae, mold, and mildew. It can also contain various chemicals or metal residues contaminating the dehumidifier’s water.

Theresa Orr

Theresa Orr holds a PhD in Earth Science and specializes in determining past climates from rocks using geochemistry. Her passion for clean water, soil and air drives her to provide easy to understand information for everyone to read.

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