The answer to 'why is my ductless system smells bad' is that there are several common causes, including mold/mildew buildup, pet odors, and stale air. Ductless systems can develop unpleasant smells over time if they are not properly maintained.
Some common symptoms of a smelly ductless system include a musty or moldy odor, stale air, and even a rotten egg-like smell from bacteria buildup. The good news is that many of these issues can be resolved with some basic DIY maintenance. However, in some cases it may be necessary to call in a professional HVAC technician to diagnose and fix the problem.