AC Smells Musty

Moisture and dust accumulate on the indoor coil. Bacteria and mold grow in this biofilm. When AC starts after being off, the smell is at its worst — sometimes called 'dirty sock syndrome.' Common on systems that aren't serviced annually.

Smell is strongest in the first 5-10 minutes of AC operation, then fades (because biofilm dries out during runtime). Often worse after the system was off overnight.

Drain line clogged with algae. Water sits in the pan, grows bacteria/mold, and air blowing over it carries the smell into your home.

Look in the indoor unit's primary drain pan. Standing water = clogged drain. May see slimy buildup.

A moisture-saturated filter (often from a drain pan overflow) becomes a mold breeding ground. Air blowing through it picks up the smell. Less commonly, the filter itself is just very overdue and dust-clogged enough to host bacteria.

Pull the filter. Visible mold or strong smell = found it. Wet or damp filter is a major red flag.

Ductwork insulation has gotten wet (roof leak, condensation, flooding) and grown mold inside. Air flowing through carries the smell to every vent. Hardest cause to fix.

Smell comes from all vents equally. Visible water damage or staining near ducts. May be worse during humid weather.