Most states have energy codes requiring more insulation in walls and ceilings than was commonly used in the past. That is the duct system. Most homes today have warm-air heat, and this requires some kind of enclosed channels to carry this air from the furnace to the rooms that need heat. The attic or crawlspace through which the ducts pass is almost as cold as the outside, so even the air that doesn't leak out still loses heat. To add insult to injury, the furnace fan often causes enough pressure changes within the living space that additional warmed air is lost through the walls and ceiling of the house itself.
Attic ducts are in the highest energy loss category with respect to location in the home.
This means that even with a furnace that meets today's efficiency standards, about half of the heat in the fuel never makes it into the house. High heating bills are not the only nightmare that poor ducting can cause. In many homes, one of the rooms is chronically cold in the winter. Often it's the one at the end of a poorly insulated, leaky duct run with more than its share of twists and turns. In order to be sure, you'll need to have it tested by a competent professional.