Musty odors that appear when your HVAC system runs may point to hidden mold in ductwork, drain pans, coolant-line cavities, attics, crawl spaces, or nearby walls. Because heating and cooling systems move air, a localized source can make the problem seem widespread and difficult to trace. This article explains how moisture, condensation, drainage issues, and poor maintenance contribute to HVAC-related mold—and why duct cleaning alone may not solve it. You’ll also learn how trained mold detection dogs help locate mold scent non-invasively, reducing guesswork and guiding more focused inspection, repair, and remediation decisions for homeowners concerned about their air quality.