Mold in AC Vents: Causes, Cleaning and When to Call

An air conditioner removes moisture from the air. The EPA says one function of the building heating, ventilation and air conditioning (HVAC) system is to remove moisture. That water collects on the cooling coil and drains away through a pan and line. When any part of that goes wrong, mold has a place to grow.

Common causes:

• Condensation on vents and ducts. The EPA explains that when warm, humid air contacts a cold surface, condensation may form. Cold supply registers in a humid room, or uninsulated ducts in a hot attic, can sweat. The EPA shows condensation on uninsulated air conditioning duct as a classic moisture problem.
• Dirty, wet coils. University of Georgia Extension says dirty wet coils can result in mold in ductwork.
• Clogged drain pans and lines. The EPA says to keep air conditioning drip pans clean and the drain lines unobstructed and flowing properly.
• System flaws. UGA lists air conditioning system flaws (oversized unit, duct leaks, dirty coils, condensation, etc.) as sources of excess moisture. The EPA's duct guide adds that a replacement unit should be correctly sized, since an oversized unit removes moisture poorly.
• Fan setting. UGA recommends the "auto" fan setting, because using the constant fan setting will return moisture to the home.
• Filters and service. Fixr says ducts can grow mold from moisture and condensation when filters are not changed properly, or the HVAC system is not properly vented and serviced.
• Flooding. NDSU Extension says mold will not normally be found in furnace or air-conditioning ducts unless they were flooded, because the air moving through them is dry. Flooded ductwork is a different, bigger job. See mold after a flood in Texas.

Texas AC season makes it worse

Texas has hot summers and mild to cool winters, and NOAA reports that the number of very warm nights has been increasing since the 1970s. In the eastern half of the state, the Gulf serves as the primary source of moisture. Long cooling seasons and humid air mean more condensation, more water through the drain pan and more chances for something to back up. The EPA recommends keeping indoor humidity below 60 percent (ideally between 30 and 50 percent).

Black specks on a vent cover are often mold, and sometimes just dust or soot. Color doesn't settle it either way. The CDC says you do not need to know the type of mold, and that CDC does not recommend mold testing as a routine step. Treat any growth as a moisture problem to fix.

Mold affects people differently. The CDC says people with asthma or who are allergic to mold may have severe reactions. Only a doctor can diagnose a mold-related illness; a mold test can't. TDLR recommends that people consult a health care provider if they're concerned about mold and their health. Our black mold guide has more.

The grille or register you see on the ceiling or wall is a hard surface, and surface mold on it can usually be cleaned. This is the part a homeowner can do.

1. Turn the system off at the thermostat so air isn't blowing while you work.
2. Wear basic protection. For areas under 10 square feet, the EPA's minimum is gloves, eye protection, and an N-95 respirator.
3. Remove the cover and take it outside or to a sink.
4. Scrub with detergent and water. The EPA says to scrub mold off hard surfaces with detergent and water, and dry completely.
5. Skip the bleach near the unit. UGA says do not use bleach on metals or near the air conditioning system. If you use it on a removed plastic grille, the CDC limit is no more than 1 cup of household laundry bleach in 1 gallon of water, and never mix bleach with ammonia or other cleaners.
6. Dry it fully and reinstall.
7. Look inside the duct opening with a flashlight. If you see growth inside, stop here.

If the same vent keeps getting moldy, the problem is moisture: condensation at that register, a leak in the duct, or high indoor humidity. Cleaning the cover again won't fix it. A dehumidifier can help in a damp room, but an HVAC check is the real answer.

Don't spray products into the ducts. The EPA says no chemical biocides are currently registered by EPA for use in internally-insulated air duct systems, and that it is a violation of federal law to use a pesticide product in any manner inconsistent with the label directions.

The EPA's homeowner guide to duct cleaning is the best neutral source here. Key points:

• When cleaning makes sense. The EPA says to consider it if there is substantial visible mold growth inside hard surface (e.g., sheet metal) ducts or on other heating and cooling components.
• Ask to see it. Many sections of your heating and cooling system may not be accessible for a visible inspection, so ask the contractor to show you the mold. The EPA notes that confirming mold may require laboratory analysis for final confirmation.
• Insulated ducts are different. Wet or moldy duct insulation cannot be effectively cleaned and should be removed and replaced.
• No sealant over mold. Sealants should never be used on wet duct liner, to cover actively growing mold, or to cover debris in the ducts.
• Fix the cause. If the conditions causing the mold growth in the first place are not corrected, mold growth will recur.
• Clean the whole system. Failure to clean a component of a contaminated system can result in re-contamination of the entire system. That includes the coils, drain pan, blower and air handler, not just the ducts.
• No proven health benefit from routine cleaning. The EPA says duct cleaning has never been shown to actually prevent health problems.

After any mold cleanup, the EPA notes that filters in the air handling units serving the affected area may have to be replaced.

Prevention checklist

Table: How to prevent mold in air vents and the HVAC system.

Table: Mold in HVAC system costs, national estimates. No source gave Texas-only prices.

Fixr calls HVAC work one of the most expensive remediations. For more, see our mold remediation cost guide.

The right call depends on where the mold is and how far it has spread.

Table: Who to call for mold in air vents.

Under Texas rules, TDLR says that in most instances areas of visible mold less than 25 contiguous square feet in area may be cleaned or removed by people who are not licensed. Bigger jobs generally need a licensed remediation contractor. Texas law also says a license holder may not perform both mold assessment and mold remediation on the same project, so the company that tests should not be the one that removes. Our guide on why testing and removal are different companies explains why.

Find licensed mold testing companies in Houston or San Antonio, and mold removal companies in Houston, Dallas or Austin. Browse all of Texas, check a license with our license check, and read how we verify listings.