Signs of Black Mold in Air Vents? (An Inspector’s Guide)

What exactly does black mold look like in air vents?

During my home inspections, I have seen many air vents and HVAC systems that had signs of mold.

In this article, I will go over...

  • Why mold grows in air vents
  • How to clean mold from vents
  • Common types of mold species on air vents

Let's get started with this guide!

signs of black mold in air vents

What Does Mold On Air Vents Look Like?

If you see a dark-colored substance on an air vent that isn't paint, rust, or something explainable... it is likely a species of mold or mildew. Of course, as home inspectors, we are not "allowed" to actually confirm that a substance is mold until a swab is taken and sent to the lab.

Here are some pictures of mold on air vents that I have come across during inspections...

mold on vent
mold air vent
mold on hvac vent

Should The Mold On Air Vents Be Cleaned Or Left Alone?

Mold grows on air vents because moisture forms on the metal vents when the HVAC is on, and dust accumulates on the surface --- which is a food source.

Cleaning the visible mold in vents is strongly recommended because the mold may release spores into the air, possibly causing harmful health issues.

Sometimes when we do a DIY job, such as mold remediation, we spend a lot of time and money — and then end up hiring a pro to fix our mistakes. That's why I created my Contractor Search Tool, so my readers can get free quotes from local mold remediators who are licensed and pre-vetted.

I invite you to at least see the pricing for any home project from a few trusted mold remediators—there is no obligation—and then you can decide whether to go ahead with a DIY job. Get your free quotes with my contractor search tool right here.

How To Remove Air Vent

HVAC air vents are very easy to remove, it's usually just two screws that hold it in place. I would remove the vent, and thoroughly wash it with a brush and dish detergent. After cleaning the air vent and letting it dry --- I highly recommend coating the air vent with a sealing spray paint such as Kilz which will help prevent new mold growth.

If the air vent is very old and rusted, it is a good idea to scrape away all rust and rough patches with a wire brush.

If the air vent is very old and deteriorated, dust is more likely to build up on the air vent. And remember... dust is a food source for mold.

HVAC air vents are pretty cheap. I think it is a good idea to consider just buying new vent covers if your vents are very old.

dust is a food source for mold

What Is Black Mold?

Black mold technically refers to a certain type of mold species called stachybotrys chartarum. Many people may not understand that the term black mold refers to this particular species.

And  since many other molds can take on a dark appearance—things can get confusing.

Stachybotrys is actually rarely found in the wild, and it will only grow with a specific environment such as very high moisture, low nitrogen, low competition (from other molds), and no sunlight.

Read Also: What Are The Best UV Lights For HVAC To Kill Mold?

Most Mold Found in Homes Is Not Stachybotrys

In my experience as a home inspector (who has done laboratory mold testing) most of the mold I find on air vents is actually not stachybotrys chartarum but different species of mold. 

Stachybotrys is really well-known for growing on wet drywall rather than metal air vents.

Many times when I have inspected homes that had basement flooding or water damage, the drywall gets saturated with water and mold grows on the drywall. This type of mold is usually the classical black or toxic mold stachybotrys chartarum.

Of course, a home inspector cannot confirm just by looking at a mold on an air vent as to what species. 

Common Species From My Swab Tests

However, in my experience mold found on air vents would more likely be either...

  1. Cladosporium
  2. Chaetomium
  3. Penicillium/Aspergillis
  4. Ulocladium

What Are The Different Type of Mold Tests?

There are two main mold tests in the industry—swab tests for visible mold and air tests.

Air tests is when home inspectors use controlled volume air pumps to sample the air for mold spores. We usually take an air test at minimum of one per home level and one outside as the control sample.

After we take the air tests, we seal the tests and ship them to a laboratory for testing. The test results will tell you the estimated number of mold spores per cubic meter of air, as well as the different species of mold discovered.

Swab Or Surface Tests

The second way to test mold is by using a swab test which is only performed for visible mold. 

My favorite mold test for homeowners is the DIY Mold Test.

The DIY Mold Test provides three surface tests, an inspection booklet, and includes the lab and report (shipping to lab not included). You can view the price of the DIY Mold Test on Amazon  here.

When I take a swab test, I basically take a cotton swab or clear tape and take a physical sample of the suspected mold.

This sample is also sent to a certified laboratory which will tell my client if it is mold, and what species. It will not tell you the amount of spores in the air however.

Is Mold In Air Vents A Deeper Problem?

There is a possibility that the mold problem goes further than just the air vents. If I lived in a home with molded air vents, the first thing I would do is take off the cover of my indoor air handler or furnace.

The indoor furnace, air handler, and evaporator coil is commonly a breeding ground for mold because it is...

1. Dark: Mold needs darkness to survive and thrive.

2. Food Source: The furnace is the central spot where dust builds up. (The filter doesn't catch everything).

3. Moisture: According to the CDC, mold will grow in places with a lot of moisture. The indoor air handling unit can produce a large amount of water --- especially when in air conditioning mode.

Inspecting The Furnace For Mold

Many furnaces (or heat pumps) have very simple front covers to remove. Usually there are just a few metal latches that have to be undone.

With other units, you will have to remove a few screws.  Also, don't forget to remove the cover to the evaporator coil (part of the exterior air conditioner) which is usually located above the furnace.

After you have removed the cover, look for signs of mold. You may have to discriminate between brown "normal" dust, and black or white mold.

Make sure you check the fan blades of the blower fan. Sometimes mold can build up on the blower fan giving it a "bubbly appearance".

signs of mold in air vents

Sources of Excess Moisture On Air Vents

A Properly Draining A/C

It's important to make sure that your air conditioner is draining moisture (condensate) to the drain and away from the unit. If there is standing water in your air handler because the drain pan or condensate piping isn't sloped correctly, then this will feed mold growth in the unit.

I can't even tell you how many times I have seen A/C drain pans that have had issues. Below is a picture of a backup drain pan (there is another drain pan inside) in an attic that is heavily rusted because of standing water.

Uninsulated Air Ducts

If you have an older home that has uninsulated air ducts in rooms without air conditioned air such as in the attic or in a crawlspace, then due to the temperature differences, these sections of air ducts may be contributing a significant amount of moisture into the air.

It would be a good idea to insulate these air ducts to prevent excessive moisture buildup which may be contributing to mold growth.

A Defective Whole House Humidifier

Many homes have whole house humidifiers, which are great products to add moisture into the air during the cold weather months. 

However, if you have never cleaned the evaporator panel, or if the unit isn't working properly, then whole house humidifiers can contribute a large amount of moisture into your furnace or air duct system --- which can lead to mold growth.

Always change the evaporator panel every year before usage and make sure that it is draining properly and not adding too much moisture into the air. 

Plumbing Leaks Or Exterior Rainwater

It's also important to look for leaks from plumbing pipes or from the outside that may be adding moisture to your HVAC system.

During my home inspections, the most frequent cause of water or moisture into the home is from poor exterior grading --- which means that the ground slopes towards the home rather than away. For the first ten feet from the home, the ground should gradually slope away from the home at least seven inches.

Besides poor grading, it's important to make sure that the downspouts drain water away at least several feet away from the home --- the easiest way to fix this problem is to install downspout extensions.

If you suspect that your basement walls has excessive moisture, or that there may be  a plumbing leak near the air vent, a very handy tool to use is called a moisture meter. A moisture meter will detect very small amounts of moisture and you can check a large area very quickly.

Read Also: The Best Moisture Meters For Home Inspectors (Review)

rusted drain pan

What Should I Do If Mold Is Inside My Furnace?

If you see mold inside your furnace, it will be a much more difficult job than just cleaning your air vents.

Cleaning air vent covers are an easy DIY job that pretty much anyone can do.

But with furnaces, you may have to remove the blower fan or evaporator coil to properly clean the unit. Also, you will likely want to clean your air ducts as well.

Air Duct Cleaning

It's time to contact an air duct cleaning company. Air duct cleaning companies specialize in cleaning HVAC air ducts and deal with mold in vents and inside air handlers on a daily basis.

HVAC UV Lights

I usually recommend installing a UV light on my clients HVAC system. A UV light is a light bulb that is on usually 24-hours with a special light frequency called UV-C.

This special frequency has been proven to kill mold, mildew, and even viruses. These light bulb systems are installed on the furnace duct, either on the supply or return side.

Some UV light systems only come on when the HVAC turns on (saving energy and extending the life of the bulb) such as the OdorStop UV Light

OdorStop UV Lights come with 6-ft power cords, and are easily installed because it comes with an install template and a rubber gasket (for a tight seal).

You can view the price of OdorStop UV Lights here on Amazon.

Filter Changes To Reduce Dust

Besides installing an ultraviolet light, it's also important to change your filter on a regular basis --- I usually recommend replacing the HVAC filter monthly.

I recently did a home inspection where there was mold growing on some of the air vents, and I noticed that the filter hasn't been changed in over a year!

People don't understand that dust is a food source for mold. Dust is mainly composed of dead skin cells which mold loves to eat. As the dust builds up on your air vent, and combined with the natural moisture from the air conditioned air --- sometimes mold takes hold and grows.

Read Also: Top 5 Best Rated Washable AC Filters

UV Lights To Kill Mold

It's also a very beneficial thing to install a UV light on your furnace or air handler after the air ducts were cleaned --- to prevent mold from re-growing in the system.

UV lights use a special light frequency that disrupts microbial DNA and prevents mold growth. 

Hospitals have been using ultraviolet light for years to disinfect operating rooms and other infected areas.

Sometimes when we do a DIY job, such as mold remediation, we spend a lot of time and money — and then end up hiring a pro to fix our mistakes. That's why I created my Contractor Search Tool, so my readers can get free quotes from local mold remediators who are licensed and pre-vetted.

I invite you to at least see the pricing for any home project from a few trusted mold remediators—there is no obligation—and then you can decide whether to go ahead with a DIY job. Get your free quotes with my contractor search tool right here.

The Bottom Line On Black Mold In Your Vents

Hopefully if you see mold in AC vents, it isn't a pervasive mold issue in your HVAC system, but there is only one way to know for sure --- take off your furnace (or heat pump) cover.

If the mold is contained to just your air vents, then I would say you are lucky homeowner.

All you will have to do is remove the air vents, give them a good scrub, or even possibly paint or replace them.

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6 thoughts on “Signs of Black Mold in Air Vents? (An Inspector’s Guide)”

    • Hi Susie,

      Mold on air vents is pretty common in my experience as an inspector, but it should be immediately cleaned. Most vents are easily removed and you can simply brush it with soap and water. Besides cleaning air vents, I would definitely check the furnace (air handler) and evaporator coil for signs of mold growth because the mold on air vents may just be signs of a deeper problem. The furnace is a central area for mold growth and I frequently discover mold growing there.

      Some furnace covers are easily removed, others are a bit more difficult. There is usually a second cover for the evaporator coil area which I would definitely inspect also. After cleaning the air vents of mold, and thoroughly inspecting the furnace — and there is no visible mold, then I would stop worrying about it. (Everybody responds differently to mold — some are highly allergic.)

      You may also want to get a consult from a trustworthy air duct cleaning company… many of them commonly clean mold from air ducts and furnaces.

      Good luck!

  1. Hello, I have been unexplainably sick since a leak in laundry room directly next to furnace and water heater went unresolved for 1.5 yrs of complaints to landlord(public housing). Recently as my health has not improved but worsened.. we looked at the back outside brick wall about 1 ft away from bedroom window, found a large patch of green mold) also on panel of outside furnace vent on back wall above green patch is a black slimy looking and rust colored substance. The people on the maintenance crew are telling us that it is not mold even though the vent was tilted unproperly and running condensation water back into the furnace not to mention the leak that went on for a year and a half in the laundry room I do not believe was ever tested for toxic mold. I requested a mold test be done they said since it does not look like mold it does not require when we should be back in the house with no concernAfter leaving the home a week ago. Now my animals have been sick and my children as well. Looking for advice or information on what that substance on the furnace vent could be and also if the green substance below that on the brick wall and running onto the cement could be actually mold of some sort that would explain all of us being so ill

    • Hello Desiree,

      I am a little uncertain about the situation you are describing. But if you are seeing mold growing on the exterior wall, I wouldn’t be concerned about it — I would still have it cleaned of course.

      However, if there is mold growing on an air vent, such as a through-wall A/C condenser, then mold spores could be spreading throughout the home — it is difficult to understand the situation without seeing it.

      If you believe that the substance is mold, and that it is making you sick, you can either call a home inspector who does mold testing for help, or you can buy a DIY mold test kit here. They also sell them at Home Depot, they are very common.

      If the test confirms that it is mold, then it will give you some leverage to ask the housing manager to call in a professional mold remediation company.

      As a home inspector, I am not allowed to officially call anything “mold” when I inspect a home. I also have to take a swab and send it to a lab for testing.

      Good luck!

  2. Thank you for writing an article that didn’t scream about how all mold is deadly and also for the tip about the DIY test. That helps a lot. We just had the furnace and outside unit replaced, so hopefully they are okay, but we have what looks like mold around vents in two rooms. Should I call someone to clean the ducts, or is it enough to just clean the vents? Thanks again.

    • Hi Jen,

      Just cleaning the vent covers should be fine. I find it an extremely rare occurrence that mold is growing inside the air ducts (not the furnace) such as somewhere in a ceiling duct. If it is growing inside of an air duct, it is probably from excessive condensation such as if you have an air duct going through an unconditioned space like a garage. I would still remove the furnace cover to inspect it for mold even if its just a few months old. It’s a good practice to visually inspect the furnace 1-2 a year just to be sure there isn’t any mold. You can also install an HVAC UV light to help prevent future mold growth inside the furnace. Also cleaning the air ducts every few years is a good way to remove dust which is a food source, and it will also make your HVAC system run better. Just make sure they also clean the actual furnace and not just the ducts.

      Good luck,



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Arie Van Tuijl

Arie Van Tuijl

I am a licensed home inspector in two U.S. states and the founder of Home Inspector Secrets. After performing hundreds of inspections, and seeing thousands of house defects, I realized people would love to have an online resource dedicated to home maintenance. I write detailed home guides and product reviews sprinkled with inspection tips. You can read my bio here.

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About Home Inspector Secrets

Home Inspector Secrets is an online resource for owners, buyers, and sellers to understand all aspects of home maintenance. We have detailed home guides, product reviews, inspection advice, and much more.