What is a common cause of odors from an A/C system?

Mold or mildew in the ducts is indeed a common cause of odors from an A/C system. When the air conditioning runs, it can create a damp environment, especially if there is moisture present in the ducts. This humidity can encourage the growth of mold and mildew, which can emit a musty odor. Such smells are often most noticeable when the system first turns on after being off for a period, as the humidity has had time to accumulate.

Furthermore, mold issues can lead to other concerns like the reduced efficiency of the A/C system and potential health risks for occupants, particularly for those with allergies or respiratory issues. Regular maintenance, which includes checking and cleaning the ducts and filters, is essential to prevent these odors and maintain a healthy indoor environment.

The other factors mentioned, while they could certainly cause issues in an A/C system, are less commonly associated with the generation of odors and affect the overall performance of the system in different ways.