A crj-900 cas alerted left bleed duct with the left duct mon MEL'ed and simultaneously the right ble...

Narrative:

While enroute approximately 50 miles north east of our departure airport at approximately 17;000 ft; we received a left bleed duct warning. I immediately displayed the ecs synoptic page while the first officer pulled out the pilot operating manual abnormal checklist. We were about to try and change the bleed source due to a MEL left duct mon and the right bleed closed; leaving us in an unpressurized aircraft. We then donned our masks as a precautionary; declared an emergency and requested a descent to 10;000 ft. While the first officer accomplished the checklists I requested direct to the airport. Once at 10;000 ft; we removed our masks and proceeded unpressurized. Flight attendants and passengers were briefed of the situation when we had time and continued uneventful to a landing to our departure airport. After arrival at the gate passengers were sent to the gate next to us with a plane ready. I stayed and filled out the maintenance log and talked to; dispatch; maintenance; tower; and approach. I then went to the other aircraft and departed about 30 minutes later.possible memory item for a quick switch of bleed source; although I am not really in favor of flipping switches without following a checklist.

Original NASA ASRS Text

Title: A CRJ-900 CAS alerted L BLEED DUCT with the L DUCT MON MEL'ed and simultaneously the right bleed closed. So with a depressurizing aircraft an emergency was declared; a descent commenced and at 10;000 FT the flight returned to their departure airport.

Narrative: While enroute approximately 50 miles north east of our departure airport at approximately 17;000 FT; we received a left bleed duct warning. I immediately displayed the ECS synoptic page while the First Officer pulled out the Pilot Operating Manual Abnormal Checklist. We were about to try and change the bleed source due to a MEL L DUCT MON and the right bleed closed; leaving us in an unpressurized aircraft. We then donned our masks as a precautionary; declared an emergency and requested a descent to 10;000 FT. While the First Officer accomplished the checklists I requested direct to the airport. Once at 10;000 FT; we removed our masks and proceeded unpressurized. Flight attendants and passengers were briefed of the situation when we had time and continued uneventful to a landing to our departure airport. After arrival at the gate passengers were sent to the gate next to us with a plane ready. I stayed and filled out the maintenance log and talked to; Dispatch; Maintenance; Tower; and Approach. I then went to the other aircraft and departed about 30 minutes later.Possible memory item for a quick switch of bleed source; although I am not really in favor of flipping switches without following a checklist.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1114391
Time
Date	201309
Local Time Of Day	1801-2400
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Light	Dawn
Aircraft 1
Make Model Name	Regional Jet 900 (CRJ900)
Operating Under FAR Part	Part 121
Flight Phase	Climb
Component
Aircraft Component	Engine Air Pneumatic Ducting
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Critical

A crj-900 cas alerted left bleed duct with the left duct mon MEL'ed and simultaneously the right bleed closed. So with a depressurizing aircraft an emergency was declared; a descent commenced and at 10;000 ft the flight returned to their departure airport.