Microbial growth in air ducts: what it looks like, when it is dangerous, and who should handle it

Texas humidity, a cold coil and years of dust are all it takes. Microbial growth inside an HVAC system is common in Central Texas, and it is also the topic with the most fear-selling in the industry. Here is a straight version: what it looks like, what it does, and what the right fix is at each level.

What it looks like

• Dark spotting or a fuzzy film on the ceiling and wall around a supply register. Often mistaken for dust; dust wipes off and does not come back in days.
• Black or grey staining on the insulation lining inside the return plenum, visible with a flashlight through the return grille.
• Slime or growth on the evaporator coil and in the drain pan.
• A musty or earthy smell that is strongest when the system starts.

Why it forms

Moisture plus food plus dark. The coil and the plenum are wet all summer, the dust that settles on them is the food, and the inside of a duct is always dark. Add a clogged drain line, a roof leak over the attic ductwork, or an old plenum lined with porous insulation and the conditions are permanent. Homes near the lakes and homes with a history of water damage see it most.

Is it dangerous

Not every microbial growth is toxic, and most of what we find is the ordinary kind. But anything growing inside the air path gets pushed into every room on every cycle, and for people with asthma, allergies or sinus problems that is enough to matter. The homeowners who call us usually noticed the health side first: symptoms that ease when they leave the house.

What fixes it, from simple to serious

• Surface growth on the coil and pan: coil and blower cleaning, drain clearing, and an EPA-registered antimicrobial treatment of the accessible components.
• Growth inside a lined plenum: cleaning does not work on porous insulation. The liner or the whole plenum gets replaced. This is the most common real fix we do.
• Growth in flex ducts: the flex runs get replaced, they are not worth cleaning.
• Growth in building materials, drywall, framing, or an area larger than Texas rules allow for unlicensed work: this is licensed microbial growth remediation, and we refer you to a licensed company rather than pretending.

What we are and are not

We clean and repair the HVAC system: coil, blower, plenum, ducts, and we apply antimicrobial treatment where it makes sense. We do not perform licensed microbial growth remediation and we say so up front. The inspection is free and comes with photos, so you know which level you are dealing with before anyone quotes anything.