Why is my home so dusty?

If you have a fairly good (rated) filter in your hvac system it is unlikely that dust is entering your home from your supply registers. Many homes are equiped with a variety of exhaust fans.  These include the kitchen exhaust, all of the bath fans, and your cloths dryer.  Running these fans frequently puts some homes into a negative pressure condition.  (more air leaving the home). This may be allowing dust from outdoors to enter through windows, doors, and any other gaps in the building envelope.  To overcome this condition try running your furnace fan constantly.  Whenever your furnace fan is running it is  bringing a small amount of outside air into the home through the return system and a small vent somewhere on the exterior perimeter of your home.  This helps to pressurize your home with clean air, preventing the infiltration of dust from outside and continuously cleaning the houshold air through your filter.