Mold in HVAC or Air Ducts

If you smell a musty odor coming out of your air ducts or see strange discoloration on your air vents the origin is likely active mold growth.

There are multiple reasons why we often find mold in air conditioning units and air ducts.

We start with hot, humid summertime air. As we cool this air we are also removing the moisture from the air. Ideally we collect the moisture at the AC coil in the drain pan and allow it to drain to the outdoors.

Old or Inefficient HVAC Units

Extreme temperatures and high humidity render older or inefficient HVAC units often inadequate in removing enough humidity from the air. They simply cannot keep up with the demand. This allows the humidity levels in the home or business to stay high enough to promote mold growth.

AC Coil

When recommended maintenance is not performed to the HVAC unit, the AC coil can begin to collect dust and debris. This is a food source for mold. When the cool water drips onto the dust and debris, mold starts to grow and spread. Cladosporium is the most common type of mold identified within the HVAC unit. Cladosporium grows and spreads very quickly and produces musty odors.

Cladosporium has been associated with allergies and asthma symptoms as well as infections of the skin, eyes, sinuses and in rare cases within the brain.

Drain Line

If a condensation drain line is not cleaned out yearly, it can become clogged. When this happens, water may back up in the HVAC unit. Most HVAC units have insulation at the base. When internal insulation becomes wet, it can take months to dry out. It only takes a week for mold to start growing and spreading. Typically the mold grows on the wet insulation first, spreading first to the AC coil, then the fan unit.


Humidifiers can be a source of great relief in the winter months when the air is very dry. The dry air can cause discomfort to the sinus cavities, and humidifiers relieve this problem. The problem occurs when the summer months roll around and they are forgotten about. Many people simply forget to turn them off, so they keep on adding moisture to already humid air. This causes cases of extreme mold growth in both the HVAC unit and also the duct where the humidifier was installed.


The location of the HVAC unit and/or ductwork may sometimes be the problem.If either are located in a wet, damp crawl space or a wet, damp basement mold may start to grow on the exterior or the interior of the HVAC and ductwork.

Mold Inspection and Remediation

If you are seeing mold growth on an HVAC register it most likely means there’s a larger hidden moisture problem. Mold grows and spreads quickly, so the incident where mold is only a factor within the HVAC unit itself is exceedingly rare. The best course of action in the event that you have a problem with mold growth is a Mold Inspection. The inspector should do a thorough inspection of the HVAC unit, the drain pan, the AC coil, the fan, and the area surrounding the HVAC unit, such as the crawlspace, basement, or attic. Since it is rare that mold is on an air register and there’s not a larger problem, a full inspection of the home should be done. It could be discovered that a vent is located too close to a shower. The steam from the shower combined with the cold air condensation from the air vent could be the source of moisture creating mold growth.

In most cases, however, seeing mold on an air vent means there’s a bigger moisture problem and more mold growth is deeper inside the HVAC unit and/or Air Ducts.

If your air ducts have internal insulation or are made out of fiberboard (compressed fiberglass), this ductwork will likely need to be removed and replaced, if contaminated with mold. It is very difficult to remove mold and apply a mold preventative sealant to this type of ducting. The only times this is acceptable practice is:

  • If the ductwork is not accessible to be removed and replaced or ;
  • As a short-term stopgap measure until the funds are available to install new duct work.

If the problem is that the HVAC unit and/or air ducts are located in a damp area then the water and moisture problem in this area must be resolved first, before mold cleanup should begin.

As with any mold problem, you must correct the water and moisture problems before cleaning the mold to prevent any future mold recurrences.

If you suspect you have a mold problem within your HVAC unit or duct work, call Carolina Air Care / Advanced Environmental Services today.


John has over 23 years of Indoor Air Quality experience. Some of the certifications he holds are: C.I.E Certified Indoor Environmentalist #00666 C.M.R Certified Mold Remediator #04124 Construction Engineering Technology Degree NADCA (National Air Duct Cleaning Association) Certifications: A.S.C.S Air System Cleaning Specialist # 9011357 C.V.I Certified Ventilation System Inspector # C203080526 V.S.M.R Ventilation System Mold Remediator # V303060706 Active Member of IAQA #1070 EPA Radon Certified
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