Have your duct system tested for air leaks. Many think that windows and doors are the major cause of a home's air leaks. But according to recent research by the Department of Energy, gaps, cracks and disconnections in the typical home's duct system are much more significant. The DOE states that the typical duct system loses 25 to 40 percent of the energy put out by the central furnace, heat or air conditioner. Leaks are usually the biggest problem.