THE APU WAS RUN WHEN IT WAS PLACARDED INOP.

Narrative

APU WAS PLACARDED INOP. CAPT CAME ON ACFT AND INITIATED START WITH AN INOP STICKER AND MEL NUMBER POSTED. THIS START POSSIBLY CAUSED DAMAGE TO FIRE BOTTLE FROM HEAT FROM APU CAUSING A THERMAL DISCHARGE OF THE FIRE BOTTLE ON THE #2 ENG. IT IS NOT CONCLUSIVE THAT THE DAMAGE WAS CAUSED FROM THIS START. DO NOT IGNORE MEL INOP STICKERS ON EQUIP. IF PLACARDED; DO NOT USE. SUPPLEMENTAL INFO FROM ACN 275812: THE CAPT WANTED TO START THE APU DUE TO THE HEAT AND HUMIDITY. THE APU WAS DEFERRED INOP DUE TO THE APU DOOR NOT OPENING. THE CAPT TURNED ON THE APU SWITCH AND THE DOOR OPEN LIGHT CAME ON. I SAID THAT I WOULD CHK THE APU DOOR TO SEE IF IT WAS ACTUALLY OPEN. THE APU INLET DOOR WAS OPEN. IT IS THE DOOR WHICH ACTIVATES THE APU DOOR OPEN LIGHT. THE CAPT STARTED THE APU. MAINT SAID TO SIGN OFF THE APU. MAINT AT DENVER DISCOVERED THAT THE APU EXHAUST DOOR WAS THE DOOR IN QUESTION AND IT WAS CLOSED. A FIRE BOTTLE HAD OVERHEATED AND DISCHARGED; DUE TO THE APU EXHAUST DOOR BEING CLOSED DURING APU OP. THIS DISCHARGE WAS NOT INDICATED IN THE COCKPIT AND THE FIRE LOOP TESTED OK.