As a technician that works on homes, I am constantly looking at broken and damaged air ducts-sometimes they are completely disconnected.  If you have an older house, with a crawl space or basement, it's a good idea to check the connections and the dampers to make sure they are set in the right position.

Occasionally I get a call from a customer who has dust everywhere in the house.  This usually means there is a problem with a filter or in some cases a broken cold air return in the attic-pulling all the loose insulation to the air handler.

If you get either of these two situations repaired, you will save a lot of money.  It's worth having your air ducts cleaned out because all the construction materials from the past have accumulated there...add the 6 pounds of normal dust and dander.  Well, you have a mess on your hands.