A musty, sour, or “dirty sock” smell from an air conditioner is almost always mould or mildew growing on a damp surface inside the system. The evaporator coil and the condensate drain pan are both naturally damp during operation, and if airflow, drainage, or a clogged filter lets moisture linger, mould finds exactly the environment it needs.
Why AC systems grow mould
Cooling warm, humid air causes moisture to condense on the evaporator coil, exactly like condensation on a cold glass. That moisture normally drains away and the coil dries between cooling cycles. When that drying process is disrupted — by restricted airflow, a clogged drain, or a dirty filter holding moisture against the coil — mould and mildew get the damp, dark environment they need to establish themselves.
Common causes
| Cause | How likely | What to look for |
|---|---|---|
| Dirty air filter trapping moisture | Very common | Filter overdue for a change, visible dust and dampness |
| Mould on the evaporator coil | Very common | Smell strongest right when the system first turns on |
| Clogged condensate drain line | Common | Standing water in the drain pan, or water damage near the indoor unit |
| Restricted airflow allowing prolonged dampness | Common | Blocked vents or a dirty filter compounding the moisture problem |
| Mould growing in the ductwork itself | Less common | Smell persists throughout the house, not just near the indoor unit |
How to fix it
- Replace the air filter
A dirty filter is both a direct contributor to the smell and a cause of the airflow restriction that lets moisture linger elsewhere in the system.
- Check the condensate drain line and pan
Look for standing water or visible buildup. A wet-dry vacuum can often clear a clogged drain line from the outside termination point.
- Run the fan-only mode after cooling cycles
Running the blower fan alone for a while after the compressor shuts off helps dry out the coil rather than leaving it damp until the next cycle.
- Have the coil inspected if the smell is strong
Visible mould on the evaporator coil needs proper cleaning, and coils are not easily accessible or safely cleaned without the right equipment and cleaning products.
Setting the thermostat fan to “Auto” rather than “On” lets the coil dry out between cooling cycles instead of staying constantly damp — a small setting change that can meaningfully reduce mould-friendly conditions.
When to call an HVAC technician
If the filter is fresh and the drain line is clear but the smell persists, or if you suspect mould growth on the evaporator coil or inside the ductwork itself, have a technician inspect and professionally clean the coil and assess the ductwork.
Stop and call a qualified appliance technician or electrician if you find scorched or melted wiring, smell burning, see water that has reached electrical components, or if the fault returns immediately after a repair. Never bypass a safety device — thermal fuses, door interlocks and flood switches fail for a reason, and defeating one converts a broken appliance into a fire or flood risk.
