Mold in an air conditioner unit | South Florida Mold Help

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Why cooling systems get damp

Air conditioning systems produce condensate at their coils. Standing water or damp materials around the system can support mold growth.

Florida health officials say to check around air handling units for standing water and to routinely inspect coils, liner surfaces, drain pans and drain lines.

Close view of an air handler coil with dust and water droplets, lit by a flashlight.

When to turn the system off

A moldy HVAC system moves air through the home. If you know or suspect the system is contaminated, turn it off and do not use it until it is remediated.

Wet or moldy duct lining cannot be cleaned effectively and should be removed and replaced.

Check around the unit for standing water.
Inspect coils, liner surfaces, drain pans and drain lines.
Leave HVAC mold work to professionals experienced with these systems.

Common situations

Air handler in a hallway closet with a little water in the drain pan below it.

Water sitting in the drain pan

A pan that does not drain leaves standing water under the unit.

White drain pipe dripping water onto the soil beside an outdoor air conditioning unit.

A blocked or slow drain line

Drain lines carry condensate away; when they stop, water stays behind.

Ceiling air vent with light gray dust spots on the louvers.

A damp duct liner

Hidden growth can sit in porous thermal or acoustic duct liners.

Do not run a system you know or suspect is contaminated. It could spread mold through the building.

Guides in this section

Mold in an AC unit

Common questions

Should I have my air ducts cleaned for mold?

EPA does not recommend routine air duct cleaning, only as needed. It says duct cleaning has never been shown to actually prevent health problems, and EPA neither sets duct cleaning standards nor certifies or endorses duct cleaning companies. If insulated ducts or duct lining are wet or moldy, they cannot be effectively cleaned and should be removed and replaced. HVAC mold work should be done by professionals experienced with these systems, with the system off until it is remediated.

Should I turn on the AC after a flood?

Only if you know the system is not contaminated. EPA says do not run an HVAC system if you know or suspect it is contaminated with mold, because it could spread mold through the building, and a mold-contaminated system should be turned off and not used until remediated. Dry the rest of the home with dehumidifiers and airflow instead. If water reached the air handler, coils, drain pan or ducts, have the system inspected before you run it.

Does the AC remove humidity?

Yes, that is part of how it works. Warm humid air passes over cold evaporator coils, moisture condenses and drains away before cooler drier air is recirculated. Two habits matter: keep the fan on AUTO rather than ON, because FSEC experiments measured lower indoor humidity in AUTO, and avoid an oversized unit, because moisture removal does not reach full capacity for roughly the first three minutes of a cooling cycle, so shorter cycles remove less water.