An air conditioning system pulls moisture out of the air, and that water has to go somewhere. When it collects in a drain pan that does not drain, soaks insulation inside the air handler, or condenses on ducts, mold can grow inside the system. Because the system moves air through the whole house, mold there can spread odor and spores to every room.
This guide covers where mold grows in an AC system, the warning signs, what the EPA says about duct cleaning, and what removal involves.
Where mold grows in an air conditioning system
- Evaporator coil. It is cold and wet whenever the system runs, and dust on the coil gives mold something to feed on.
- Drain pan and condensate line. Standing water in a pan that does not drain, or a line clogged with sludge, is one of the most common sources.
- Air handler cabinet. Many cabinets are lined with insulation that can hold moisture.
- Blower. Dust and moisture on the fan wheel can grow mold and spread it with every cycle.
- Ducts. Especially lined ducts, duct board, and flexible ducts running through hot, humid attics, where condensation can form.
- Supply vents and return grilles. Cold air meeting humid room air can leave moisture on grilles and the ceiling around them.
Signs of mold in the AC or ducts
- A musty smell when the system kicks on, which fades or changes as it runs
- Visible growth on vent grilles, the ceiling around vents, or inside the air handler
- Water in or around the air handler, or a drain pan that holds water
- A drain line that keeps clogging, or a float switch that keeps shutting the system off
- Wet or stained ceilings below the air handler or ducts
- Symptoms such as a stuffy nose or coughing that seem worse when the system runs; these have many possible causes, so check the system and talk to a doctor
Black dots on grilles are sometimes mold and sometimes dust or soot. The EPA points out that although a substance may look like mold, only an expert can make a positive determination, and lab analysis may be needed.
Why mold grows in the system
The EPA says controlling moisture is the most effective way to prevent biological growth in ducts. Common moisture problems include:
- A drain pan that does not slope toward the drain, or a clogged condensate line
- Dirty coils and filters that let dust build up on wet surfaces
- Leaky or poorly insulated ducts in a hot attic, where humid air condenses on cold duct surfaces
- A system that is too large for the house and cycles on and off without removing enough humidity
- Long periods with the system off in a humid climate, followed by restart
What the EPA says about duct cleaning
The EPA's guide on duct cleaning is useful before you hire anyone:
- Consider duct cleaning if there is substantial visible mold growth inside hard-surface ducts, such as sheet metal, or on other components of the system.
- Many parts of a system cannot be seen, so ask the service provider to show you any mold they say exists.
- Once fiberglass duct liner is contaminated with mold, cleaning is not enough to prevent regrowth, and no EPA-registered biocides exist for porous duct materials. EPA, NADCA, and NAIMA all recommend replacing wet or moldy fiberglass duct material.
- Mold on bare sheet metal is much easier to treat; cleaning and treatment with an EPA-registered biocide are possible.
- Understand the pros and cons before letting anyone apply chemical biocides or sealants inside ducts.
- Steam cleaning and other methods that add moisture should not be used on ductwork.
And while you are deciding: the EPA advises not running the system if you know or suspect it is contaminated with mold.
What professional removal involves
A proper job usually combines HVAC work and mold remediation:
- Shut the system down and protect the house from spores during the work.
- Find and fix the moisture. Clear or replace the drain line, correct the pan slope, seal and insulate leaky ducts.
- Clean the air handler. Coil, drain pan, blower, and cabinet surfaces.
- Replace what cannot be cleaned. Moldy cabinet insulation, duct liner, duct board, and flexible duct are usually replaced rather than cleaned.
- Clean hard ducts. Sheet metal ducts are cleaned with brushes and HEPA vacuum equipment.
- Clean the house if needed. If the system spread spores, surfaces and vents in the living space may need cleaning too.
- Verify. Visual checks, and sometimes clearance testing, before the system goes back into normal use.
What you can safely do yourself
- Change the air filter on schedule
- Keep the condensate drain line clear, following the manufacturer's or technician's guidance
- Wipe down and dry vent grilles with light surface spotting
- Keep indoor humidity below 60 percent
Opening the air handler, cleaning coils, and removing duct material are jobs for an HVAC or remediation professional.
What it costs
HomeGuide puts mold remediation in an HVAC system at about $2,000 to $10,000, depending on the type of system and whether the mold has spread into the ducts. Basic air duct cleaning costs about $200 to $1,000, and sanitizing adds about $70 to $150. Replacing lined ducts, duct board, or major components adds to the total. See our mold remediation cost guide.
If the mold followed a sudden event, such as an overflowing drain pan that soaked the ceiling, check does homeowners insurance cover mold.
AC mold in Florida homes
In Florida, air conditioning runs most of the year, air handlers often sit in closets, garages, or hot attics, and condensate lines clog easily. Homes left closed with the system off during a trip, or without power after a storm, can develop mold throughout the system. See local pages for Orlando, Tampa, Lakeland, and the rest of Florida.
Next step
If you own your home and notice a musty smell from the vents, visible growth, or water around the air handler, call the number on this page to be connected with an independent local mold inspection or mold remediation company.