Just like your body has a circulatory system that runs through it, if you have a central heating and cooling system, so does your home. Inside the walls and ceilings runs duct-work that is transporting hot or cold air to keep the temperature comfortable. To make sure that your system is cost effective, you want to make sure that your ducts are in good repair.
A common problems is leaks. You are more likely to experience leaks where the pressure is highest, and this means closest to the unit, but then again, leaks could appear anywhere. The problem is that a leak will not only leak out the air from the ducts, it could also take in air from unused portions of your home, bringing with it smells, dust, and also affecting the temperature of the air in the ducts. Many times the leaks will appear in the seams of the duct-work, since these areas are a weak spot.