Why are there stains on ceiling around air vents?

Stains on the ceiling around air vents are often caused by condensation or leaks in the HVAC system. When warm, moist air comes into contact with cooler surfaces, it can condense and create water spots. Additionally, if there are any leaks or drainage issues in the ductwork, it can lead to water dripping and causing stains. Over time, these stains can also indicate mold growth or poor air quality if not addressed.