A musty smell from an air conditioner (AC) usually means moisture is sitting somewhere in the heating, ventilation, and air conditioning (HVAC) system. Check the filter and the condensate drain first.
At The Indoor Report, we explain what to measure and which fix a result calls for. The sections below rank the likely sources by how easy they are to check, starting with the ones you can look at yourself.
The U.S. Environmental Protection Agency (EPA) states that moisture control is the key to mold control. The checks below focus on where water can collect, drain, or stay damp.
Where a Musty AC Smell Usually Starts
A musty AC smell usually traces to one of four sources: the filter, the condensate drain pan and line, the evaporator coil, or the ducts. Check them in that order, because the filter and the drain are the easiest to look at.
- Filter: pull out and inspect the filter, since it is among the easiest checks.
- Condensate drain pan and line: standing water in the pan points to a blocked line.
- Evaporator coil: residue on the coil is the source tied to the dirty sock smell on heat pumps.
- Ducts: check for damp ducts. EPA advises against running a system with suspected mold.
Check the Filter First
A dirty filter is one of the quickest sources to check. Turn the system off, then pull the filter and look at it before you open any other part of the unit.
If the filter is visibly dirty, replace it, run the system, and see whether the smell changes. If the smell is still there, move to the drain. For filter basics, see our furnace filter guide.
The Condensate Drain Pan and Line
Cooling coils condense water out of the air. That water drains to a pan and then through a line. If the line is blocked, the pan and line can hold standing water.
Look in the drain pan for standing water. Standing water can mean the line is blocked. EPA names moisture control as the key to mold control.
If the pan holds no standing water and the smell persists, move on to the coil.
Coil Residue and the Dirty Sock Smell
On heat pumps, a musty smell at startup is sometimes called dirty sock syndrome. The HVAC industry uses the term for a smell tied to microbial residue on the coil. A startup smell on a heat pump points at the coil, so check the filter first and then have the coil looked at.
If you can see growth on the coil, call an HVAC technician.
Start with the filter, drain, and humidity checks.
The condensate description above is a general description of cooling coils.
Matching the Smell Pattern to Its Source
| Pattern | Source It Points To | First Step |
|---|---|---|
| Musty at startup on a heat pump | Coil residue (dirty sock smell) | Check the filter, then have a technician look at the coil if the smell persists |
| Standing water in the drain pan | Blocked condensate line | Check the line, then call a technician if the smell persists |
| Rotten-egg odor | Not a musty smell (natural gas odor additive) | Leave the building and call the gas utility |
| Musty air from supply vents | Damp ducts | Do not run the system if you suspect mold, and have it assessed first |
| Constant musty smell with no standing water | Check with your HVAC technician | Check the filter and indoor humidity |
Ducts, Mold, and Whether to Run the System
EPA's guidance on duct cleaning states that duct cleaning has not been shown to prevent health problems. If mold is found in the ducts, the moisture source must also be fixed.
EPA also advises that if you suspect the HVAC system has mold, you should not run it until the situation is assessed, because it could spread spores through the building. Turn the system off and arrange an assessment before you run it again. For what an assessment involves, see our mold inspection guide.
For vent-specific questions, see mold in air vents. For the cleaning question, see duct cleaning.
EPA's Indoor Humidity Reference
EPA advises keeping indoor relative humidity (RH) below 60 percent, ideally 30 to 50 percent. Use those numbers as the reference when you read an indoor humidity level.
If the smell comes back after a clean filter and a dry drain pan, check indoor humidity against the 60 percent ceiling. A reading above 60 percent falls outside EPA's advice.
For how to read a humidity level and what to do about it, see our house humidity level guide.
When the Smell Is Not Mold
Not every smell from the AC is a musty smell. A rotten-egg smell is a separate problem: natural gas is given a rotten-egg odor additive.
Leave the building.
Call the gas utility. This is separate from a musty smell.
What to Do Yourself and When to Call a Technician
Do these checks yourself: look at the filter, look for standing water in the drain pan, and compare indoor humidity with EPA's ranges. Call an HVAC technician if the smell persists after those checks, or if you can see growth on the coil.
If you suspect mold in the ducts or the system, turn it off and get an assessment before running it again. Anyone who smells rotten eggs should skip these checks and leave the building.
What would change this answer: a clean filter, a dry pan, and a humidity reading within EPA's range, with the smell still present, means the checks above are done. A technician visit is the next step at that point.
If anyone in the home has health symptoms, the checks above do not assess them. Our toxic mold symptoms page is the place to start, and a clinician makes any diagnosis.
Common questions
Why does my AC smell musty only when it starts up?
On a heat pump, a musty smell at startup is sometimes called dirty sock syndrome, a term tied to microbial residue on the coil. Check the filter first, then have a technician look at the coil if the smell persists.
Can a deodorizer or air freshener fix a musty AC smell?
No. Find and fix the source first.
Should I keep running the AC if I think the ducts have mold?
No. EPA advises that if you suspect mold in the HVAC system, you should not run it until the situation is assessed, because it could spread spores through the building. Turn it off and arrange an assessment. For the assessment, see our mold inspection guide.
Does duct cleaning fix a musty smell?
Not by itself. EPA's guidance says duct cleaning has not been shown to prevent health problems, and if mold is found in the ducts, the moisture source must also be fixed. Read our duct cleaning guide before you book one.
Can a musty AC smell make me sick?
The checks here cannot tell whether the smell is affecting anyone's health, because they do not diagnose. A clinician makes that call. Our toxic mold symptoms page is the starting point for the symptoms themselves.