MD80 FLT CREW REPORTS APU FIRE WARNING AND LOOP FAULT WHILE HOLDING ON GROUND WITH 100 DEGREE AMBIENT TEMPERATURES.

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WE WERE HOLDING ON THE GND BECAUSE OF A GND STOP AT IAH. APPROX 1 HR 50 MINS LATER WE GOT AN APU FIRE WARNING LIGHT ACCOMPANIED BY AN APU FIRE DETECTOR LOOP 'B' LIGHT. LOOP 'A' LIGHT WAS NOT ILLUMINATED. THE APU DID NOT AUTO SHUT-DOWN. WE SHUT DOWN THE APU AND DISCHARGED BOTH AGENT 1 AND 2 EXTINGUISHER BOTTLES. CFR WAS CALLED. THEY TOOK ABOUT 10 MINS TO ARRIVE. JUST IN CASE THE APU FIRE WARNING LIGHT DID NOT GO OUT; I INSTRUCTED THE FLT ATTENDANTS TO START THEIR CHKLIST AND BEGIN PREPARATIONS FOR A POSSIBLE GND EVAC OUT THE 2 FRONT DOORS ONLY. THE APU FIRE LIGHT TOOK A WHILE TO GO OUT BUT FINALLY DID. CFR WAS ASKED TO CHK THE BACK OF THE AIRPLANE FOR ANY INDICATION OF AN APU FIRE. THEY DID NOT SEE EVIDENCE. THEY THEN USED A THERMAL IMAGING DEVICE TO CHK THE SAME AREA AND DID NOT FIND EVIDENCE. WITH NO ENGS RUNNING AND NO APU; WE THEN CALLED FOR A TOW BACK TO THE GATE. IT TOOK ABOUT 30 MINS FOR A TOW VEHICLE TO SHOW UP. IT WAS ALMOST 100 DEGS OUTSIDE AND THE INSIDE OF THE CABIN WAS GETTING EXTREMELY HOT. I MADE THE DECISION TO HAVE THE FLT ATTENDANTS OPEN BOTH THE FRONT AND AFT DOORS FOR CROSS VENTILATION. SOME PAX WERE BEGINNING TO SHOW SIGNS OF HEAT EXHAUSTION. STRONG CONSIDERATION WAS GIVEN TO DEPLANING THE PAX OUT OF THE ACFT USING EITHER THE SLIDES OR THE AFT STAIRWAY BECAUSE OF THE EXTREME TEMP IN THE CABIN AND THE AMOUNT OF TIME IT TOOK FOR EITHER THE TUG OR PORTABLE STAIRS TO ARRIVE. SUPPLEMENTAL INFO FROM ACN 790617: WE WERE ON THE GND FOR 1 HR 30 MINS WHEN WE NOTICED AN APU FIRE LIGHT ILLUMINATED AND ALSO THE LOOP 'B' LIGHT WAS ILLUMINATED. CALLED TWR AND REQUESTED THE EQUIP WHILE LOOKING UP THE PROC IN THE QRH. SHUT DOWN APU AND FIRED BOTH BOTTLES. FIRE LIGHT STILL ILLUMINATED! FIRE EQUIP DID NOT KNOW WHERE WE WERE; THEY WENT TO TERMINAL! ARPT AUTH VEHICLES BEHIND US RPTED THEY SAW NOTHING. WHEN THE EQUIP GOT TO US WE REQUESTED A THERMAL SCAN OF THE AREA. THEY SAID EVERYTHING WAS NORMAL BUT SOME SMOKE WAS COMING OUT WHICH THEY PRESUMED WAS FROM THE BOTTLES BEING FIRED. WE STILL HAD THE FIRE LIGHT AT THIS TIME! WE CALLED THE COMMANDER AND REQUESTED A TOW BACK AND HE SAID THAT WE BOTH SHOULD CALL COMPANY. AFTER 10 MINS ON THE GND WITH NO AIR FLOW AND A VERY HOT ACFT; 1 PERSON WAS ON OXYGEN AND 5 OR SO MORE WERE HAVING PANIC ATTACKS. CALLED COMPANY AGAIN AND THEY SAID THEY COULD NOT GET TO US BECAUSE ARPT AUTH WAS NOT THERE TO ESCORT THEM ACROSS THE RWY! THE FIRE LIGHT FINALLY WENT OUT AFTER ABOUT 25 MINS AND TUG GOT TO US 35 MINS AFTER WE CALLED. WE WERE ON BATTERY PWR FOR 45 MINS BEFORE WE GOT TO THE GATE AND EVERYONE WAS EXTREMELY UNCOMFORTABLE.