Leaky ductwork can be a major health hazard. Kinked or collapsed ducts will cause restriction of air flow. Duct systems lose energy in two ways: By conduction of heat from the warm surface, and air leakage through small cracks and seams.
Homes with forced air heating/cooling systems use duct systems to distribute the air through the house. The duct system is made up of a supply system and a return system. The supply system distributes the heated or cooled air throughout your living space. The returns bring the heated or cooled air back to the furnace or air handler to be reheated or re-cooled.
Leaks in your duct system can vary from small to catastrophic. Small leaks are usually in the seams and joints. These small leaks can add up to a very large leak and can significantly contribute to higher energy consumption/cost. Catastrophic leaks include disconnected ducts, or large holes in the duct system.
All leaks (even small ones) should be sealed. Look for holes large enough to see. Visible holes in ductwork would be a clear indication that the system needs fixing. The joints between duct sections should be sealed against leakage. If duct tape was used for this purpose, it often loses adhesiveness after a few years.
Duct sealing should be done using a silicon caulking or a special cement (mastic) with an imbedded fabric. If the ducts are in the attic or crawl space that is nearly as cold or warm as the outdoors, the heating or cooling is nearly completely lost if it is not insulated or sealed properly.