The answer to 'why is my ductwork smells bad' is that there are several common causes, including mold growth, rodent infestations, and decomposing debris in the ducts. These issues can lead to unpleasant odors that circulate through your home's air.
Mold growth often occurs when there is excess moisture in the ductwork, such as from a leaky air conditioner or high humidity. Rodents like mice or rats can get into the ducts and leave behind waste and decaying organic matter. Over time, dust, dirt, and other debris can also accumulate and start to decompose, producing foul smells.