12:12 PM PT -- A spokesman for the stadium insists the heater is NOT broken ... and says there's a perfectly good explanation.
"There is NO broken heater – but we opened the garage to let buses and allow a few packages to be delivered to the stadium – and that allowed the freezing air to come into the stadium."
"It was like opening the back door and letting the record temps into the building."
"The heater isn't broken -- and fans can expect it to stay that way ... and should be able to enjoy the game with no problem."
It's 14 degrees in Philadelphia ... and it seems like the heater has broken inside the Wells Fargo Center -- so the visiting Indiana Pacers are warming up in full sweats and winter gear.
Unclear when the heater crapped out ... but multiple team sources tell us the same thing -- "It's f**king freezing in here."
One player tells TMZ Sports, "It's like we are outside playing ball ... this ain't cool."
Pacers' star Paul George is at the stadium ... and tweeted, "Crazy my guys have to practice under these conditions in philly! #CutThatHeatOnDamnIt."
The game is scheduled to begin in less than 6 hours ... and as far as we know, the game is scheduled to go on as planned.