The answer to 'why is my HVAC system smells bad' is that there are several common causes, including mold, mildew, or bacterial growth in the system, a dead animal in the ductwork, or a buildup of dust and debris. These issues can lead to unpleasant odors that circulate through your home when the HVAC system is running.
The symptoms of a smelly HVAC system include a musty, moldy, or rotten smell coming from the vents or air handler. This can indicate a problem that needs to be addressed to maintain indoor air quality and prevent potential health issues. In some cases, you may be able to address minor issues like a dirty air filter yourself, but more serious problems like mold growth or a dead animal usually require professional HVAC service.