Cold air does not necessarily mean an air conditioner is problem-free. If the unit starts giving off a musty odor, that can be a sign that something inside needs attention, as Southern Living reported.
An HVAC expert who spoke with the outlet said such odors often trace back to systemic issues.
What's happening?
Even when an AC is cooling properly, a damp or stale smell suggests that moisture is getting trapped somewhere in the unit.
"A musty smell is most commonly caused by moisture that allows mold, mildew, or bacteria to grow inside the air conditioning system," JG van Graan, general manager at Boer Brothers Heating and Cooling, explained to Southern Living.
He added the spots most likely to be involved are the parts that regularly gather condensation, including the evaporator coil, condensate drain pan and line, and blower compartment.
That risk increases in humid climates, where dampness can stick around longer. Van Graan described the conditions to the magazine as "the perfect environment for microbial growth."
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Window units tend to run into this issue more often, Southern Living reported, because they are more exposed to outdoor humidity and often are not maintained as consistently as other units.
Van Graan also noted that the same kind of odor can develop in central air systems, heat pumps, and ductless mini-splits if moisture is not draining as it should or if upkeep has been neglected.
A dirty air filter is usually not the root cause, he clarified to Southern Living, but it can worsen the problem by reducing airflow and keeping the system wetter for longer.
Why does it matter?
A lasting musty smell can indicate microbial growth, a drainage blockage or other drainage problem, or too much humidity indoors. Any of those issues can affect indoor air quality and may lead to costly repairs.
When airflow drops because of a dirty filter, moisture buildup becomes more likely.
Van Graan told Southern Living that the issue is often manageable when caught early, but leaving it alone can allow a minor problem to grow into a much larger one.
What can I do?
A good first step is to inspect the air filter and clean it or replace it if needed.
Van Graan told the outlet that most homeowners should check filters monthly and replace them every one to three months, depending on the filter type, whether they have pets, allergy concerns, and how heavily the system is used.
It also helps to confirm that vents are not blocked, the area around the indoor unit remains clean and dry, and the condensate drain is flowing properly. Those measures can support better airflow and cut down on excess moisture, per the outlet.
Homeowners should not spray fragrances or disinfectants into vents to cover up the smell.
"While these products may temporarily mask the smell, they don't address the underlying moisture or microbial growth causing it," van Graan said, per Southern Living.
If the odor remains after a day or two, it is time to call an HVAC technician.
"Twice-yearly maintenance helps catch small issues before they become expensive repairs, improves system efficiency, extends equipment life, and provides year-round comfort and reliability," van Graan said.
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