Ducts get filthy.
Ducts should be professionally cleaned. A hot air system acts like a vacuum cleaner, sucking into the ducts household dirt, dust, pet hair, cigarette smoke, pollen, pesticides, cooking grease, even through filters. In the ducts bacteria and fungi breed and pollutants are stirred up and distributed by the ductwork through the air registers in every room. This spreads illnesses, allergens, and fumes.