The air produced by your air conditioning system ought not to have a smell.
If you notice a musty smell coming from your cooling system a few seconds after adjusting your thermostat to a lower setting, there is a problem with your HVAC system.
This could be caused by excess moisture in your air ducts or vents. Moisture build-up inside your ductwork or vents creates a favorable environment for mold and mildew to thrive and spread. Excess moisture in these parts can result from either a drainage leak or high humidity levels that go unnoticed. The musty smell could be resulting from frozen evaporator coils. Air conditioning systems function by absorbing warm indoor air, eliminating humidity from the air, cooling it, and recycling it back inside your home through the return vents. Proper airflow is crucial for this process, meaning having clean filters is mandatory. Clogged filters impede airflow, causing water from humid indoor air to accumulate on your evaporator coils. This water will then freeze on your coils as the air conditioning refrigerant cools the air, resulting in frozen evaporator coils. A full drain pan could also be causing the musty smell. A drain pan suffers regular wear and tear, making it less effective over time. An inefficient AC drain pan may accumulate mold, which produces a musty smell when your system is running. Another reason why your AC smells musty could be as a result of a blocked condensate line. The condensate line works to drain water from your drain pan to your storm drain or landscaping. It is the most conducive location for mildew, mold, and sludge to build due to its small size. This build-up can result in clogs and musty smells inside your home.