November 10, 2026
By Tri County All Restorations
Mold in Your AC Unit: Causes, Warning Signs and Prevention
Mold Damage
Quick answer: AC systems grow mold because cooling pulls water out of the air, and that water collects on the coil, drain pan and drain line. In South Florida the system runs almost all year, so a clogged drain or dirty coil stays wet. A musty smell when the AC starts, visible growth around vents and water near the air handler are the main warning signs. The EPA advises not running a system you suspect is contaminated with mold until it is checked.
Reviewed by Gary Ackel, owner of Tri-County All Restorations, a Florida licensed mold remediator (MRSR2307) based in Oakland Park. Updated November 2026.
Why does mold grow in air conditioners?
An air conditioner cools air by passing it over a cold coil. Moisture in the air condenses on that coil, drips into a pan and leaves through a drain line. When the coil is dirty, the pan holds standing water or the drain line clogs, those parts stay wet. Add dust as food and the dark, damp air handler becomes an easy place for mold.
What are the signs of mold in an AC system?
- A musty or earthy smell when the AC turns on
- Dark spots on supply vents, grilles or the ceiling around them
- Water in or around the air handler closet, or a stained ceiling under it
- Visible growth on the drain pan or coil when the panel is opened
- Allergy-like symptoms that get worse when the AC runs
What can I safely do myself?
- Replace the air filter on the schedule your system calls for
- Keep the area around the air handler dry and uncluttered
- Wipe dust off vent covers and grilles
- Ask your HVAC technician how to keep the drain line clear between visits
Do not spray bleach or other chemicals into the coil or ducts, and do not keep running a system you suspect is contaminated, as the EPA advises.
How do you prevent mold in an AC system?
- Have the system serviced once a year, including the coil, drain pan and drain line
- Keep indoor humidity below 60 percent, as the EPA recommends
- Set the thermostat fan to Auto instead of On, so the coil can drain between cycles
- Fix sweating ducts and missing insulation, which drip onto ceilings
- Keep the AC on at a higher setting when you travel, instead of off
Who should I call: an HVAC company or a mold company?
Call an HVAC technician to clean the coil, clear the drain and repair the equipment. Call a mold remediation company when mold has spread beyond the unit: onto the drywall and ceiling around the air handler, into the ductwork, or into materials soaked by an overflow. The two often work together. Tri-County handles the remediation side, arranges independent testing when needed and dries out any water damage from the overflow.
See our mold remediation in Fort Lauderdale and water damage restoration, or read how to know if you have mold. Call (954) 776-1717, 24/7.
Frequently asked questions
Can mold in the AC make me sick?
It can cause allergy-like symptoms such as a stuffy nose, sneezing, itchy eyes or wheezing, and the CDC notes that people with asthma, allergies or weakened immune systems can react more strongly. If symptoms improve when you are away from home, mention it to your doctor and have the system checked.
Should I have my air ducts cleaned?
Only when there is a clear reason. The EPA says duct cleaning is warranted when there is substantial visible mold growth inside hard-surface ducts or other system components, and that the moisture problem causing it must be fixed first, or the mold will return.
