Blind-Alley Ducts
Occasionally found in duct systems that use joist spaces or other parts of the building structure to channel air flow, blind-alley ducts occur as a result of mistakes made during installation. A blind-alley duct leads nowhere (except possibly to the outside) while the register it was supposed to serve has no source of heat. The room containing this register is therefore cold. If a room always seems too cold or a register doesn’t seem to have any air flowing out of it, it may be worth investigating.
Source:Home Energy Library
[...] 1.Blind-Alley Ducts 2.No Return-Side Ductwork 3.Disconnected, Torn, or Damaged Ducts 4.Uninsulated Ducts in Unconditioned Spaces 5.Poor Duct Layout 6.Unbalanced Air Flow 7.Supply- and Return-Side Leakage 8.Health Hazards with Duct Systems [...]