AN ACR MAINT MEL'ED A B737-500 PRESSURIZATION OUTFLOW VALVE HEATER AS FAILED WHEN A RUPTURED DUCT ACTUALLY CAUSED A HIGH CABIN ALT.

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WE WERE CRUISING AT FL360 (IF I REMEMBER RIGHT) AND THE CABIN ALT ALERT HORN WENT OFF. IT WAS INDICATING 9700 FT IN THE CABIN. WE GOT OUR OXYGEN MASKS ON; ESTABLISHED COM; AND DID THE PROC IN THE QRH CHKLIST AND GOT THE CABIN BACK TO NORMAL. WE DID THE CHKLIST; BUT IT WASN'T TILL WE PUT PACKS TO 'HIGH' THAT THE CABIN ALT STARTED GOING BACK DOWN TO 'NORMAL.' DURING ALL OF THIS; WE REQUESTED LOWER ALT; AND THEY GAVE US LOWER TO 12000 FT (IF I REMEMBER RIGHT). WHEN EVERYTHING WAS GOING FINE AND THE CABIN PRESSURE WAS ACTING NORMAL AGAIN; I ASKED THE CAPT; 'SHOULD WE LET COMPANY KNOW SO THEY CAN SET UP FOR MAINT TO MEET US?' SO WE SENT AN ACARS MESSAGE TO DISPATCH; WHICH WE FOUND OUT LATER DISPATCH NEVER RECEIVED IT. ON THE GND; THE CAPT TALKED TO MAINT CTL FOR A VERY LONG TIME. AFTER HEARING ALL THE DETAILS EXPLAINED; MAINT CTL SAID; 'ICING HAD PREVENTED THE OUTFLOW VALVE TO NOT CLOSE ALL THE WAY.' SO MAINT CTL AND DISPATCH GAVE US MEL 21.37 -- OUTFLOW VALVE HEATER GASKET. SO WE WERE DISPATCHED VIA MEL 21.37. WE FLEW THE NEXT 2 LEGS WITH ABSOLUTELY NO PROBS. NEXT DAY; CAPT COMMUTED ON THE VERY SAME PLANE. HE LOOKED AT THE LOGBOOK TO SEE WHAT THE FIX WAS. THEY FOUND OUT IT WAS THE 'AFT PRESSURE APU DUCT BULKHEAD BOOT;' AND NOT THE 'OUTFLOW VALVE HEATER GASKET' AFTER ALL. HE CALLED ME; AND TOLD ME IT WAS BEST WE DO A RPT. WE DID OUR BEST TO DO THE RIGHT THING. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT NEITHER PLT CONFUSED THIS HORN WITH THE TKOF WARNING HORN. BECAUSE THE QRH PROCS VERY QUICKLY BROUGHT THE CABIN UNDER CTL; THEY DID NOT DECLARE AN EMER. SUPPLEMENTAL INFO FROM ACN 683252: MAYBE I SHOULD HAVE QUESTIONED THE POSSIBILITY OF A DIFFERENT TYPE OF LEAK; BUT MAINT CTL SEEMED PRETTY MATTER OF FACT ABOUT THE CAUSE AND THE ICING OF THE OUTFLOW VALVE SEEMED PLAUSIBLE WITH THE EXISTENCE OF THAT MEL. IN THE QRH; PACK SWITCHES -- AUTO. WHY NOT GO TO HIGH IMMEDIATELY? I'M CONVINCED THAT GOING TO HIGH WAS WHAT STARTED THE CABIN DOWN AND WHAT COULD BE HARMED BY IMMEDIATELY GOING TO HIGH? SEEING THE CABIN DSND LOWERED THE STRESS LEVEL IMMEDIATELY BECAUSE WE WERE HAVING TROUBLE GETTING AN UNRESTR DSCNT TO A MUCH LOWER ALT.