Regional Jet 900 (CRJ900)
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical
Flight Crew Returned To Departure Airport
Aircraft
Climbing through 20;000 FT we received a left duct overheat warning message. Aircraft performed as expected and shut down bleed system. Aircraft started a slow depressurization. We descended to 8;000 FT and told ATC we needed to return.  Performed non-normal checklists and landed and deplaned at the gate. Cabin and flight crew performed as expected.
CRJ-900 Captain experiences a duct overheat warning climbing through FL200. The bleed system is shut down and the flight returns to the departure airport.