Mold in an Air Conditioner: Window Units, Central Systems, and When It's a Pro Job (2026)

The System That Makes Its Own Water

An air conditioner cools air by running it over a coil cold enough to condense the moisture out of it — gallons a day in a humid climate. That water drips into a pan and out a drain line. Everything in that path is wet whenever the system runs, and if any of it stays wet between cycles, mold grows on it: the evaporator coil, the drain pan, the drain line, the blower housing, and — the expensive case — the ducts and their insulation.

The symptom is the musty blast when the system starts, sometimes called dirty-sock syndrome, and symptoms in the household that improve on vacation. The fix depends entirely on which kind of system you have.

Window and Portable Units: Yours to Clean

A window unit is a complete system in a box, and every wet part is reachable.

1. 1Unplug it and take it out of the window if you can — this is an outdoor job.
2. 2Filter: slide it out, wash with warm soapy water, dry fully.
3. 3Front panel off: the evaporator coil is the fin array behind the filter. Clean the fins and the drain pan below them with detergent and water and a soft brush, or a foaming coil cleaner made for the purpose. Do not bend the fins.
4. 4Drain hole: at the back of the base. Clear it with a pipe cleaner; a blocked drain is why the pan grew mold.
5. 5Dry everything before reassembly — a wet unit put back together starts the problem over.

Gloves and an N95, because brushing dry growth releases spores. A unit whose foam insulation or blower housing is moldy and will not come clean is at the end of its life; the parts are not worth the labour. Portable units follow the same steps, plus emptying and cleaning the internal tank.

Central Systems: What's Yours, What's the Technician's

The drain line is worth knowing: at the air handler, a PVC pipe with a T-fitting and a cap. Pour water through it; if it does not run freely out at the exterior or the drain, it is blocked, and a wet-dry vacuum on the outside end usually clears it. Slime in the line is the biofilm that grows in every condensate line eventually; a technician clears it properly during a service.

Ducts: EPA's Rule Is Narrower Than the Ads

Duct-cleaning advertising treats every musty smell as a duct problem. EPA's guidance is specific: duct cleaning has not been shown to prevent health problems, and it is not routinely necessary. EPA recommends it in three cases — visible mold growth inside hard-surface ducts or on other components of the system, a pest infestation, or ducts clogged with debris to the point that it is being released into the home. Two further points from the same guidance matter more than any cleaning: moldy insulated ducts and duct insulation cannot be cleaned and should be replaced, and the moisture problem causing the mold has to be corrected or the growth comes back.

So: before paying for duct cleaning, look. Remove a register and shine a light into the duct. Have a technician open the air handler and show you the coil and the plenum. If there is visible growth, a remediation contractor working to the IICRC S520 standard — containment, HEPA, source correction — is the right call, and the who-to-call guide explains how to keep the company that tests separate from the company that removes. If there is not, the smell is almost always the coil, the pan or the line.

The Moisture Underneath

A clean system in a humid house grows the colony back in a season. The causes worth checking:

• Indoor humidity over 60%. A hygrometer in the hallway. EPA's range for controlling mold is 30–50%. The humidity guide covers targets and dehumidifiers.
• An oversized unit cools the house fast and shuts off before it has dehumidified — short cycles, clammy air, wet coil. A technician can confirm sizing.
• Fan set to ON instead of AUTO keeps blowing air over a wet coil after the compressor stops, re-evaporating the water into the house.
• Bathroom and kitchen moisture with no exhaust fan ends up in the return air — the same fault that grows mold on a bathroom ceiling.

When It Is a Health Question

Symptoms tied to the AC running — congestion, coughing, headaches that clear outdoors — deserve the coil-and-line cleaning first, and if they persist, an air quality test and a look at mold's health effects. Anyone immunocompromised in the house should not be the one cleaning a moldy unit.

Bottom Line

Window unit: unplug, wash the filter, clean the coil and pan, clear the drain, dry it. Central: filter and drain line are yours, the coil and blower are a technician's, the ducts are a specialist's only when there is visible mold — and moldy duct insulation is replaced, not cleaned. Then find the moisture, because every AC mold problem is a wet part that never dries.