A B737 FLT CREW EXPERIENCED LOSS OF CABIN PRESSURE; DECLARED AN EMERGENCY; EXECUTED AN EMERGENCY DESCENT; AND RETURNED TO DEP AIRPORT.

Narrative

UPON LEVELOFF AT FL350 I FELT THE PRESSURIZATION CHANGING AND NOTICED CABIN ALT RATE OF CLB/DSCNT WAS VARYING +/-1000 FPM AND THE CABIN ALT WAS SLOWLY INCREASING. THINKING IT MIGHT BE THE CTLR; I SWITCHED TO ALTERNATE; BUT THE VARIATION CONTINUED. REFING THE QRH FOR RAPID DEPRESSURIZATION; I SWITCHED TO MANUAL BUT WAS UNABLE TO GET THE OUTFLOW VALVE TO MANUALLY CLOSE. THE CABIN ALT REACHED 10000 FT; THE ALT WARNING HORN SOUNDED; THE PLTS DONNED THEIR OXYGEN MASKS; AND I TURNED ON THE SEATBELT SIGN. I DECLARED AN EMER AND COORD A DSCNT AND TURN BACK TOWARD ZZZ. I REFED THE QRH FOR EMER DSCNT AND LET THE FLT ATTENDANTS AND PAX KNOW WE WERE DEALING WITH A PRESSURIZATION PROB. I SENT AN ACARS MESSAGE TO THE DISPATCHER NOTIFYING HIM OF THE SIT. AS WE DSNDED THE CABIN CONTINUED TO CLB; SO I RESELECTED 'AUTO' TO SEE IF THE CTLR COULD REGAIN CTL OF CABIN PRESSURE. THE CABIN ALT TOPPED OUT AT APPROX 14000 FT AND THE CABIN MASKS AUTOMATICALLY DEPLOYED. ONCE WE REACHED 10000 FT; THE PLTS DISCONTINUED USE OF OXYGEN MASKS AND I LET THE FLT ATTENDANTS AND PAX KNOW THEY COULD ALSO TAKE THEIR MASKS OFF. RETURN TO ZZZ WAS UNEVENTFUL AND WE TERMINATED THE EMER AFTER LNDG; ALTHOUGH I HAD NOTIFIED ATC ONCE WE WERE BELOW 10000 FT THAT SPECIAL HANDLING WAS NOT NECESSARY.