The answer to 'why is my ductless system smells bad' is that there are several common causes, including mold/mildew buildup, a dirty air filter, or a dead animal in the ductwork. A musty or foul odor coming from a ductless HVAC system often indicates a problem that requires professional attention.
Mold and mildew can grow in the condensate drain line or indoor unit if there is excess moisture. A dirty air filter can also cause odors by trapping dirt, dust, and bacteria. In rare cases, a dead animal may have gotten into the ductwork, leading to a very unpleasant smell. These issues require cleaning, repairs, or replacement by a licensed HVAC technician.