This morning BA166 (TLV/LHR 777) was freezing as we boarded and the crew explained that the air was being pumped in from the terminal which is why it was so cold this morning.

Then on BA269 (LHR/LAX A380) the boarding was delayed due to problems starting the AC unit, so it was a sauna as we boarded.

How is cabin temperature set before boarding? I was surprised to hear about air being pumped in from the terminal, unless they just meant the flow coming from the jetway? And can a plane take off if the AC is on the fritz? Or is that a “mission critical” system?