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

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.

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.

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.

3.  Mold on an air vent cover.

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

5.  Mold growing in corner.

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

7.  Toxic mold growing on another air 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.