Musty or Moldy Smell Coming From Air Vents — San Diego, California

A persistent musty or moldy odor that appears whenever the HVAC system kicks on is a clear sign that mold, mildew, or bacteria have colonized the ductwork, evaporator coil, or drip pan. Moisture naturally condenses on the cold evaporator coil during cooling cycles, and if the condensate drain is slow or the air filter is clogged, that damp environment becomes a breeding ground for microbial growth. Dirty ductwork lined with dust and organic debris provides additional nutrients for mold colonies to spread throughout the system, pushing spores into every room the vents serve. Beyond the unpleasant smell, airborne mold spores can trigger allergies, asthma flare-ups, and respiratory infections — especially in children and the elderly. An HVAC technician can inspect and clean the evaporator coil, flush the condensate drain line, sanitize the duct interiors, replace the air filter, and apply an antimicrobial coating to prevent regrowth.