AN AIRBUS 320 FLT WAS CANCELED DUE TO FAILURE OF MAINT TO PROPERLY LOCK THE L PACK VALVE CLOSED PER MEL REQUIREMENTS.

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THE ACFT ARRIVED AT THE GATE WITH SEVERAL WRITE-UPS -- WITH THE 2 MOST SIGNIFICANT BEING R WING A/I VALVE FAILURE TO CLOSE AND L PACK VALVE FAIL OPEN. MAINT INDICATED THE FIX WOULD NOT BE LONG; SO THE PAX WERE BOARDED ON SCHEDULE. SHORTLY AFTERWARD; THEY SAID THE L AIR CONDITIONING PACK WOULD BE INOP AND WING A/I VALVE DEFERRED OPEN. THEY WENT TO WORK ON THE WING AND THE AIRPLANE WAS GETTING HOT. AFTER 10 MORE MINS; 5 MORE MINS; ETC; THEY FINISHED WORKING ON THE WING AND SAID IT WAS OK TO OPEN THE APU BLEED AND OPERATE THE R AIR CONDITIONING PACK. (THE L PACK WAS SUPPOSED TO BE DEFERRED WITH THE PACK VALVE CLOSED.) WHEN THEY CAME UP TO FINISH THE PAPERWORK; IT WAS DISCOVERED THAT THE VALVE WAS NOT CLOSED (THAT WOULD TAKE AN ADDITIONAL 50 MINS AND IT WAS SHIFT CHANGE). I DECIDED TO ORDER MORE FUEL AND AFTER A CONFERENCE WITH THE AGENT AND ZONE; THE PAX WERE DEPLANED. UNFORTUNATELY; THE APU WAS FEEDING THE R WING A/I AND HAD BEEN FOR APPROX 35 MINS. THE FUELER ARRIVED AND FROM WHAT I UNDERSTAND BURNED HIS HAND. THE NEW SHIFT (MAINT) SAID THEY WERE GOING TO START ALL OVER AND THE TRIP WAS CANCELED. IT IS WORTHY TO NOTE THAT DURING THIS WHOLE PROCESS; WHICH LASTED APPROX 1 1/2 HRS; THE WHOLE LAVATORY SYS WENT INOP AND WAS REPAIRED. ADDITIONALLY; 2 MAINT CREWS WERE INVOLVED; WITH ONE THINKING THE OTHER WAS PERFORMING DIFFERENT TASKS.