Mold in AC vents

Why does mold grow around AC vents?

Condensation. When cold air comes out of a supply register into a warm, humid room, the metal grille and the ceiling or wall around it can drop below the dew point and collect water. That damp ring is where spots appear.

Inside the system, the same thing happens on a larger scale. The EPA says condensation on or near the cooling coils is a major factor in moisture contamination of heating and cooling systems, and that condensation or high humidity is an important indicator of potential mold growth in any type of duct.¹

Los Angeles adds its own twist. Near the coast, the marine layer keeps indoor air humid in late spring and early summer, so registers in closed-up rooms sweat more. Inland, long cooling seasons mean the coil and drain pan handle water for months at a time.

Common causes we see:

• Registers in humid rooms such as bathrooms and laundry areas
• Supply ducts in the attic or crawl space that aren’t sealed or insulated, which the EPA warns can let condensation into the system¹
• A condensate drain pan that doesn’t drain, or a clogged drain line
• An oversized AC unit; the EPA notes a unit that’s too big cycles on and off frequently and removes moisture poorly¹

Is it mold or just dust?

Often you can’t tell by looking. Dark dust collects on supply grilles, especially along the edges where air moves fastest, and it looks a lot like mold. The EPA says that although a substance may look like mold, a positive determination can only be made by an expert and may require laboratory analysis.¹

One practical clue: dust tends to build up evenly where air moves, while growth from condensation tends to come back in the same spots after cleaning, where the grille and the surface around it stay cold and damp. If you want a second opinion, send a photo to our free photo review.

Is it a vent problem or a system problem?

That’s the question that decides what you do next.

What you see	Usually means
Spots on one or two grilles, nothing inside the duct	Local condensation at those registers
Spots on most grilles and a musty smell when the AC runs	Possible moisture in the air handler, coil or ducts
Water dripping from a register or ceiling stain near it	Sweating or leaking duct, or a drain problem
Standing water or debris in the drain pan	Drain problem; the EPA says it needs immediate attention1

If it looks like a system problem, the EPA advises not running the HVAC system if you know or suspect it’s contaminated with mold, because it could spread mold through the building.²

Should you have your air ducts cleaned?

Not by default. The EPA does not recommend routine duct cleaning, only cleaning as needed, and says duct cleaning has never been shown to prevent health problems.¹ It suggests considering it when there is substantial visible mold growth inside hard-surface ducts or on other parts of the system.¹

Two points from the same EPA guidance matter most:¹

• If insulated ducts get wet or moldy, the insulation can’t be effectively cleaned and should be removed and replaced.
• If the conditions that caused the mold aren’t corrected, it will grow back. Fix the cause before any cleaning.

The EPA also suggests asking any service provider to show you the mold they say exists, since much of a system can’t be seen directly.¹

What does an inspector check, and what does an HVAC tech check?

They answer different questions. An HVAC contractor services the equipment: the coil, the drain pan and line, refrigerant, airflow, and whether the unit is the right size. They’re the person to fix a sweating duct or a clogged drain.

An inspector documents the moisture and mold around it. On a visit we look at registers and the accessible air handler area, take humidity readings room by room, take moisture readings on ceilings and walls near registers, and check attic or crawl space ducts if that space is part of the scope. We don’t open sealed ducts or service equipment. The result is a written report showing where the moisture is, so you know whether you’re dealing with a humid room, a duct problem or something unrelated to the AC.

When do you need an inspector, and when don’t you?

You likely don’t need one if it’s a few spots on one grille that wipe off and stay away once the room’s humidity comes down. Clean the grille, run the bathroom fan, and keep indoor humidity below 60%, ideally 30% to 50%, as the EPA recommends.² Our guide to the best indoor humidity explains how to check it.

Book a moisture and humidity assessment or a mold inspection when spots keep returning, show up on many registers, come with a musty smell when the AC runs, or appear alongside ceiling stains. Both start at $495 for one dwelling up to 2,000 sq ft. If ducts run through the attic or crawl space, the crawlspace and attic inspection covers those spaces.