7 Pictures Of Mold In Air Ducts (What It Looks Like?)

Do you want to see some pictures of mold in air ducts? 

Mold growing inside of your HVAC can cause allergies, chronic cough, soar throat, and even skin rashes. It's important to know what it looks like and how to prevent it --- that's why I created this guide.

In this article I will go over...

  • several pictures of mold in air ducts
  • why mold grows in ducts
  • how to prevent mold growth in the HVAC
  • And more...

Let's get started with this guide!

pictures of mold in air ducts

Why Does Mold Grow in Air Ducts?

As a home inspector, I have looked into many air ducts and furnaces over the years. One thing that never surprises me is finding mold growing on air vents, inside air ducts, and inside air conditioners.

There are three main reasons that mold grows in HVAC systems.

1. Food Source

Mold needs a food source to grow and thrive. There are natural mold spores in the air that are totally harmless. But when these spores find a mold source and start to actually grow --- then it becomes a problem.

So, if you haven't cleaned your air ducts in a while, or maybe you don't change the filter as often as you should (such as monthly) --- then you may have an excessive amount of dust in your system. Dust is the primary food source for mold inside of air ducts. Dust is largely composed of dead human skin cells.

2. Moisture Source

Mold also needs a moisture source in order to multiply. With air conditioners (and even high efficiency furnaces) water is produced as a by-product of the cooling or heating process --- this water is called condensate by home pros.  Excess water in the system can also happen if there is a faulty whole house humidifier installed.

One of the most common issues that I find when inspecting an HVAC is that there are signs of water leaks. Usually it is the result of a condensate drain or pan that isn't draining properly.

Regardless of the cause, if there is excessive moisture in the HVAC system, and combined with a food source (dust), then the conditions are ripe for mold to grow.

3. Darkness

In addition to a food source and moisture --- mold also needs darkness to grow. Most people know that sunlight is a natural anti-septic. In fact, you can even sanitize water in a clear bag or bottle if you are in a survival situation (just leave it out in direct sunlight for a few hours).

Well, hardly a surprise, the HVAC system is very dark --- and mold loves it.

Read Also: How To Install An HVAC UV Light (5 Step Guide)

How Can I Tell If There Is Mold in My Ductwork?

Air Vent Covers

During my home inspections, the first and most obvious signs that there is mold in the air ducts is at the vent covers.

Since every room has at least one or two vent covers, I always shine my flashlight on them to see if there is any dark colored substance growing on them --- which is usually mold. Just make sure that it really is an organic mold growth, and not just dust. Mold is almost always a black color on air vent covers. You also don't want to confuse it with rust.

The Interior Air Handler

Besides the air vent covers, the usual place I check for mold is at the indoor air handler or furnace. There is no point in checking the exterior condenser or outside A/C unit because even if there is mold on it, it isn't connected to the interior duct system.

For most furnace covers, you will simply have to remove a few screws or a few latches and it simply comes off. It is a good idea to turn off the furnace or A/C at the thermostat (or shut off switch) prior to taking off the cover.

Once the cover is off, use a flashlight and look carefully around the blower fan, the evaporator coil, the wiring, and the air ducts. Look for a black substance which is almost always mold.

Non-Visible Air Ducts

Lastly, there is the problem of mold growing in the air ducts that isn't easily visible. What if mold is growing in an air duct within a ceiling where it isn't easily seen? Well, you can of course find a contractor who has an inspection camera snake, however, this would be very expensive.

You have to understand that it is very rare for mold to grow outside of the interior air handler (or vent covers).  This is because all of the moisture happens at the furnace or vent covers. Moisture accrues on the vent cover because when the air hits the cold metal cover, condensation forms.

The only logical way in my opinion to look for mold growth that is hidden is to use a moisture meter.

If you have a suspicion that there is a water leak or moisture problem in a particular room, such as a ceiling or wall --- then you can use a moisture meter to check for moisture problems.

If the moisture meter reads high moisture, and there is an air vent in that area, then there is a possibility that mold is growing in the air duct due to the excessive moisture.

I commonly see this problem in garages if there is a air duct trunk going through the ceiling. Since the garage is usually the coldest or warmest part of the home, if there is a a dissimilar temperature air flow through the duct, then condensation can form, leading to mold problems.

Read Also: Does UV Light Kill Mold? (How Effective?)

How Do I Get Rid of Mold in My HVAC Air Ducts?

Air Vent Covers

For air vent covers, the only way to get rid of mold is to unscrew the cover, and to physically clean it in a sink using a brush. It isn't that hard. However, if the vent cover is very old and rusted, it may be simpler and more efficient just to replace it with a new cover, they are pretty cheap.

Indoor Air Handler

If you see mold inside your furnace, then you can also physically wipe away the mold. However, if there is mold in the blower fan, evaporator coil, or electrical wiring—then it can be significantly more difficult to clean.

If you also suspect that there is mold growing somewhere inside your air ducts that isn't visible, then there really is only one option, and that is to call a professional air duct cleaning company.

Air duct cleaning companies specialize in cleaning air ducts, and many companies also have extensive experience in cleaning mold and sanitizing the air ducts.

UV Light For HVAC

If you do have your air ducts cleaned, then I highly recommend installing a UV light after the ducts have been cleaned to prevent mold from re-growing inside. UV Lights emit a special frequency of UV-C light waves that have been proven to kill mold, mildew, viruses, and other microorganisms.

My favorite HVAC UV Light is made by a company called OdorStop. These UV lights only turn on when the HVAC system turns on, which means it will save energy and extend the life of the bulb. 

They can also be installed on the return duct side of the furnace which helps prevent UV damage to any plastic furnace parts such as wiring or the drip pan.

mold in air ducts

7 of My Best Pictures of Mold in Air Conditioner Ducts

1. This is a picture of a HVAC cover that was removed in an attic. You can notice the reflective surface which is a thin layer of fiberglass insulation on the cover. The mold is growing extensively all over the insulation. The air handler is usually the source of mold growth.

mold growing inside hvac

2. This is image of the air duct right next to the air conditioner evaporator coil. If you look closely at the black rim, mold is growing all over it.

mold in a/c

3.  Mold on an air vent cover.

mold on hvac vent

4.  Black mold growing on insulation inside air duct and by blower fan.

mold inside air duct plenum

5.  Mold growing in corner.

picture of mold in air duct

6.  A mold-like substance clearly growing on blower fan exterior inside the air ducts.

picture of mold on air duct blower

7.  Toxic mold growing on another air vent cover.

picture of mold on air duct vent cover

Final Thoughts

So there you have it. Clear pictures of mold inside of air ducts and on vent covers. If there is mold further inside your ducts, then you will have to use a camera snake to check it out, which in my opinion, probably isn't worth the cost.

It would be better to use that money and hire a professional air duct cleaning company to clean your air duct system.

And after the ducts are cleaned, you can install a UV light that shines 24 hours a day inside of your A/C so mold doesn't regrow.

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6 thoughts on “7 Pictures Of Mold In Air Ducts (What It Looks Like?)”

  1. If you have mold in behind the vents is it okay to have air on? I mean I live at the beach it’s hot I’m having it fixed ASAP but I’m scared to run air conditioner. Please would it hurt ?

    • Hi Rena,

      Everybody reacts differently to mold. Some people have extensive mold contamination in their air ducts without ever realizing it. However, if anyone in your household has severe allergic reactions such as itching, sneezing, or watery eyes — I would hightail out of there immediately.


  2. I have had mold issues since acquiring a new whole-house HVAC (and new contractor) in the 90s and then again in 2004. With the earlier replacement we literally had silver-dollar size mold spores sitting on our first-floor vent covers when we returned from a week-long summer trip. I have read that if HVAC is too large for home it will not dehumidify properly. The basement carpet feels damp when you go downstairs in warm weather, even on the stairs. There is a black mold on a basement ceiling air vent which also then grows onto the ceiling around the vent (drywall ceilings). Basement gets very cold in summer, too. We had the issue you mentioned for the duct above our garage- the drywall garage ceiling had mold growing on it exactly where the duct was located. This never occurred before new unit. Is all of this an insulation issue, is the unit too strong or both?

    • Hi Beth,

      I will probably include this in an article in the future, but yes, if your HVAC is oversized, it will not dehumidify properly. One of the first things I would do is buy a cheap humidistat device for ~$10 and monitor the indoor humidity. The EPA recommends that it should be between 30%-50%. In the meantime, you can buy a dehumidifier for the basement and connect a tube to a sink/drain so it can run continuously without having to manually empty the tank.

      The easiest test to know if the HVAC is oversized is to wait for a hot day, and time how long the A/C turns on for. The dehumidification of the A/C really only occurs after around 15-20 minutes. If your A/C only runs for 10-minutes, and then shuts off — this is a sign that it is oversized. Your house may be cool after running for several minutes, but it did not run long enough for humidification.

      For the garage ceiling, even with a properly sized A/C, it would still be a good idea to insulate the duct. The garage door is usually poorly sealed, and you will get that temperature difference with the outside air and the conditioned air inside the duct which will create condensation.

      For the rest of the house — it really sounds like an oversized system as you suspected, I don’t think it is an insulation issue. Many older homes have very poor insulation and don’t have these issues.

      Good luck!


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