Short answer
HVAC mold develops because air conditioning cools air below its dew point and condenses water on the coil and in the drain pan. When a condensate line clogs, a pan overflows, a system short-cycles without dehumidifying, or ducts sweat, moisture lingers in a dark enclosed space and supports growth. Signs include a musty smell when the system starts and dark spots at vents. Correcting the moisture source is the fix.
Why air conditioners create moisture
An air conditioner does two jobs: it cools air and it removes humidity. It does the second by cooling air below its dew point, which condenses water onto the cold evaporator coil. That water drips into a drain pan and is carried away by a condensate line.
In other words, a working AC is a controlled moisture-generator. Everything is fine as long as that water drains away and the system actually dehumidifies. Problems begin when the water lingers.
How the system goes wrong
The most common failure is a clogged condensate line. Algae and debris block the drain, the pan fills, and water overflows into the air handler cabinet, insulation, and surrounding materials. A cracked or rusted drain pan does the same thing more slowly.
Oversized or short-cycling systems are another culprit. A unit that cools the air quickly but does not run long enough to wring out humidity leaves the home damp even while it feels cool. Ducts running through hot attics can also sweat, wetting insulation and nearby framing.
The signs to watch for
The classic sign is a musty, earthy odor that appears or strengthens when the system starts and distributes through the vents. Because the HVAC moves air through the whole home, an odor tied to the system cycling points to the equipment or ducts.
Visually, check the supply registers for dark speckling on and around the grille, and inspect the air handler area — the drain pan, near the coil, and any insulation — for dark growth or water staining. Not every dark mark is mold, and suspected growth should not be disturbed, but speckling at multiple vents plus an odor warrants a professional look.
What to do about it
If the system is actively leaking, turn it off and address the source — often clearing a clogged condensate line or repairing a failed component. Because AC leaks wet concealed materials, a moisture investigation helps confirm how far the water spread and whether hidden materials stayed wet long enough to support growth.
If mold is confirmed in the system, correcting the moisture source comes first, followed by proper cleaning or replacement of affected components. Cleaning a coil while a drain keeps overflowing only resets the clock.
Preventing it going forward
Routine HVAC maintenance is the best prevention: keep the condensate line clear, service the system so it runs efficiently, change filters, and make sure the unit is sized to actually dehumidify. Keeping indoor relative humidity in a reasonable range — generally below 60%, with many practitioners targeting roughly 45–55% — reduces the moisture load the system has to manage.