Why Is There Black Dust/Mold On My AC Vents?

Worried about the black dust coming out of your vents?

You’re right to be concerned—whatever the black dust is, it’s probably not healthy to breathe.

Black dust coming out of AC vents is usually caused by:

  • Candle or fireplace soot
  • Leaky ducts
  • Mold growth

In this blog, we’ll explain each of these causes in more detail, and how you can determine which cause you may have.

Want a professional to fix your black dust issue? Our technicians are ready to help with any AC repairs you may need.

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Cause #1: Candle or fireplace soot

The black dust coming out of your vents could be soot, which is the byproduct of burning a candle or using the fireplace. Because your AC is constantly sucking in air from your home to be cooled, it can pull in soot and blow it back into your home.

One way to check if soot is the culprit is to check your air filter. Does it look black, like the one pictured below? If so, soot could be the problem.

An air filter clogged with black dust/soot

An air filter clogged with black dust/soot

Note: If your filter looks clogged, replace it with a new one. Below we’ll share some tips on how to choose a good filter to prevent this soot problem in the future.

Solution: If you use candles or your fireplace frequently, below are some tips on how to prevent soot from coming out of your vents.

Tip #1: Try not to use scented candles

Don’t use scented candles or candles made from petroleum jelly/vegetable oil, as these types of candles produce more soot.

Tip #2: Keep your candles away from vents and fans

Removing your candles away from fans and vents will reduce the amount of soot your HVAC system picks up.

Tip #3: Upgrade your air filter to a higher MERV rating

MERV (minimum efficiency reporting value) measures the effectiveness of an air filter. The higher the MERV rating, the better the filter is at catching small pollutants.

To catch small soot particles and other pollutants such as mold spores, we recommend using an air filter with a MERV rating of 8 or higher.

If you’re interested in a higher-MERV filter, consult with an HVAC professional before you purchase one. Since higher-MERV filters are thicker, an HVAC professional may need to make modifications to your ductwork to accommodate the filter.

Want a recommendation on a high-MERV filter? We can help! Learn more about our air filter services.

Cause #2: Leaky ducts

Leaks in the ductwork could be another cause of black dust coming out of the vents.

We’ll explain: Your ducts are located in unconditioned spaces, such as an attic or ceiling. These spaces naturally have lots of dust and dirty insulation which can enter the ductwork if the ducts are leaky or disconnected. Over time, ducts can become disconnected from the air handler or other ducts, especially if they were not installed correctly to begin with.

baby-bird feeding you an opportunity to contribute.

Ductwork in the ceiling

The truth is, most homes have leaky ducts. In fact, in a typical house, EnergyStar estimates that 20 to 30 percent of the air that moves through the duct system is lost to leaks, holes or disconnected ducts.

Signs of leaky ducts include:

  • Higher-than-normal energy bills
  • A dusty home
  • Hot and cold spots in certain areas

Solution: You’ll need to have an HVAC professional inspect your ductwork to check for leaks. If the professional finds leaks, they will likely recommend a solution called duct sealing.

Duct sealing involves using a sealant to cover any holes in the ductwork. If a duct is leaky beyond repair, the HVAC professional may recommend replacing the duct altogether.

Want a certified HVAC professional to inspect your ductwork? Schedule an appointment today.

Cause #3: Mold growth

The black dust on your AC vents could be mold. Mold is commonly found in AC systems, particularly in the indoor AC unit. The indoor unit is naturally a damp, dark place, which makes it easy for mold to grow there.

If mold is present, the air that passes through the indoor unit can carry mold spores to the ductwork, where it can grow near your vents. Mold spores can also enter your ductwork if you have leaky ducts, as we discussed above.

Besides the visible black spots, the following signs also indicate you could have a mold problem:

  • You or someone in your home has unexplained allergy symptoms
  • You notice a persistent musty odor (similar to the smell of dirty socks)
  • You see signs of water damage on the ceiling, near the vent with the black dust

Solution: If you think you the black spots could be mold, we recommend the following tips:

Tip #1: Try to clean the moldy area yourself

If you only notice the black spots in one small area, you can try to clean the mold off yourself. Otherwise, if the spots have become widespread, leave it to the professionals.

To clean the moldy area, you can scrub the black spots on the vent or surrounding wall with a bleach solution (1 cup of laundry bleach in 1 gallon of water). If the black spots come back, or you start noticing black spots in other places, try tips 2 and 3 below.

Tip #2: Contact a mold remediation specialist

If the mold is widespread, you should consider contacting a mold specialist. A mold inspector or a remediation specialist will have the right tools to quickly test and verify if the black spots are mold. They can also help you find what’s causing the problem and safely remove the mold.

Tip #3: Have your AC maintained by an HVAC professional

An HVAC professional can inspect your AC system and see if mold is growing inside the indoor unit. They can safely disinfect the indoor unit and also inspect your ductwork to make sure there are no leaks.

Want a trained AC pro to get rid of the black dust?

Schedule appointment today

Our certified HVAC professionals can thoroughly inspect your ductwork and AC system to determine what is causing the black dust problem.

When you hire Four Seasons, you can always count on our technicians to give honest recommendations and exceptional customer service, during every step of your AC repair.

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