Testing for Mold in HVAC Systems

With continuous use, heating and cooling systems often breed moisture in their ductwork, especially if HVAC ducts are not sealed properly. The air inside the ducts varies from the air outside, leading to condensation, one of the favorable conditions for mold growth. Unlike the basement and other wet parts of your home, HVAC units remain the most overlooked source of mycotoxins, specifically mold. They are also very dangerous as they distribute mold spores throughout, predisposing your family to serious health conditions.

Outlined below are some signs that your HVAC system may be mold-infested and tips on confirming its presence.

Signs of Mold in HVAC Units

Even if you diligently maintain your HVAC systems, you can easily overlook moldy air ducts. Some signs of mold infestation in your home include;

  • Musty Odor

Large amounts of mold in your air ducts often leave a distinctive earthy or musty smell. While this smell is unpleasant, it makes it easy to detect mold growth in your systems. The presence of mold in your ducts often presents a strong smell, especially when the HVAC systems are running.

  • Allergic and Respiratory Distress Symptoms

Health effects are probably the most serious issue of mold growth in your HVAC systems. Inhaling mold spores leads to allergic symptoms and can exacerbate respiratory problems. Allergic symptoms include coughing, sneezing, itchy/watery eyes, and breathing difficulties.

  • Visible Molds in the Units

Visible black and white spots in your HVAC vents and ducts are by far the most obvious sign of mold growth. Mold settling on vents appears either as black, white, or green spots.

Mold Inspection Tips

If your home has any of the signs mentioned above, you should consider doing a thorough inspection of your HVAC systems. Below are tips and advice to leverage when inspecting your heating and cooling systems for mold.

  1. Walk Around and Do a Visible Mold Inspection

As the phrase suggests, simply walk around and search for visible signs of the presence of mold, such as discoloration in vents. For a thorough inspection, carry along a screwdriver and a step stool. Remove some accessible supply vents for close inspection. Make sure to wear a face mask, gloves, and other protective equipment when doing this to avoid exposure.

While at it, inspect all the exterior parts of the HVAC system, including the joint, overflow pan, filters, fiberglass, and connections. As you inspect for the presence of mold, ensure that there is no condensation forming outside the unit when turned on. Condensation outside the unit means that the parts are not sealed tightly and thus have a high chance of mold growth.

  1. Inspect if HVAC Unit has Been Attached to a Humidification System

Some homeowners make the mistake of attaching a humidification system to their HVAC units to remedy indoor allergies purportedly caused by air dryness. According to them, humidifiers provide moisture that combines with heated air, especially during winter. Unknown to them, this excessive moisture promotes the growth of mold and other bacteria. If you have such a system, turn it off and remove it immediately.

Testing for Presence of Mold

Based on the findings of your inspection, you may want to do a confirmatory test for the presence of mold in your HVAC systems. To do this;

  • Buy a mold test kit locally or order one online. The test kit contains a petri dish and a culture medium.

  • Open the petri dish and empty the culture medium. Leave it for at least one hour for the culture medium to solidify.

  • Close all the HVAC vents except one, which is preferably the one that you spotted signs of mold growth or has a strong, musty odor.

  • Suspend the petri dish on the open vent using tape. Leave the dish in this position for at least 10 minutes.

  • After 10 minutes, close the dish, place it on a flat surface and allow it to sit for between 48 and 96 hours.

  • Open the dish after this time frame elapses. Suspect the presence of mold if there are dark spots or signs of discoloration at the bottom of the petri dish.

You can forward your findings to advanced labs for further tests. The lab can pick on the specific type of mold that has infested your HVAC units. Some types are dangerous and should be eliminated immediately.


Mold growth is a serious health risk that should be remediated immediately. Depending on the size of mold infestation, you may prefer handling the issue on your own, if you are a DIY expert, or hire expert mold removal service providers. Regardless of your choice, keep in mind that the EPA recommends professional mold removal services if the mold covers more than 10 sq ft. If mold growth in your HVAC units is mild, you can try removing them using various home remedies and other advanced techniques.