Is there visible mold growth inside the ducts, registers, or other components of the HVAC system?

Do the ducts show signs of rodents or insects?

Do the ducts have excessive amounts of dust and debris and/or particles that are actually released into the home from the supply registers?

Has there been a fire or flood in the house?

Is there a musty or moldy smell?

Do you have to spend excessive time dusting, only to have the dust return the next day?

Has there been remodeling in the home - causing dust to recirculate?

If you have pets it is crucial that your air ducts be well maintained. Pet fur in the system, will provide germs, fungus and allergy causing mold and mildew something to live on.