Flight attendant on a B777 reported a cabin temperature reaching 100 degrees while sitting on the tarmac. After returning to the gate for further maintenance work the passengers were deplaned. The FA states management required the FA's to remain on board; anticipating re-boarding of passengers. The Captain later refused the aircraft.

Narrative

I am position XX at the very back of the 777-300. After being on the plane 5 hours with temperatures starting at 88 degrees when we boarded all the way to 100 degrees F. We had passengers and crew overheating on the aircraft. It was so bad with the young and the elderly. We kept the purser informed to let her of how bad the situation was getting in the back of the aircraft because it was cooler towards the front but still hot. Some crew was starting to get overheated during boarding as we were doing many duties in the cabin and I was working galley. We pushed back and the pilots kept saying it would get cooler once the engines were started. We ended up returned to the gate because something was wrong with the plane. It ended up starting to get hotter and hotter on the plane and running back and forth getting passengers cold compresses and water before they passed out. We ended up having all passengers deplane. We sat on the plane as management kept insisting we would be boarding shortly. The plane reached 100 degrees. Some crew told us it would not even show if it surpassed 100 in the cabin on the cabin temperature screen so may have been hotter.After 6 hours; Captain said he was refusing the plane. I kept record of the temperatures of the aircraft from when we were pushed back and coming back to the gate with passengers onboard. I kept updating everyone as the temperatures reached 100 degrees in the cabin. I have time stamps of the cabin temperature rising over the hours when they said the plane should be cooling down with passengers onboard. All is documented in the app for our flight.This plane has had this issue for more than a week. One of the crew members worked the same plane last week and had issues.