Dust

• Sometimes the culprit in smelly duct work is dust. Small dust and dirt particles are sucked into the intake system of the furnace and blown throughout the duct work. When the furnace and the duct work lie dormant or unused for a season, the dust settles. When the system is started again, the dust is stirred and heated. This causes a musty odor to emanate from the vents when the heat blows. Typically, the smell will fade away after several uses of the furnace as the dust is blown out of the duct work. If the smell persists after multiple uses of the system, you may need to clean the ducts.
  
  

Vermin

• Vermin such as rats, squirrels, bats and other small animals may find their way into duct work. They may enter through intakes, vents or holes in the duct work. They subsequently leave droppings and often die. The droppings will emit an odor that is carried by the airflow out of the ducts. This situation can be hazardous to your health, and you should call in a professional to address the problem. The vermin may also die in the duct work if they are unable to find a way out. The decaying bodies can cause intense smells. While the smell will eventually go away, clean the duct work if this occurs.
  
  

Mold or Mildew

• It is possible for mold and mildew to form in the duct work. Water enters through vents and intakes and lies in the duct work, creating a damp atmosphere ideal for the growth of these microorganisms. The result is a musty smell from the ducts. Moldy ducts can be a health hazard. You will need to have the duct work professionally cleaned and disinfected to ensure that mold or mildew are completely removed. Clean all the vents and intakes as well.
  
  

Corrosion

• As metal duct work ages, it is possible for corrosion to occur. Old heating systems may rust and begin to break down. When this occurs, the oxygenation process may cause a musty or bad smell to be released when the heating system is on. Replacing the duct work is the only solution to this problem.