Originally Posted by
trooper
In a sense they are turning the "air" OFF...not on... ambient air temp at altitude requires heating before being introduced to the cabin......
Actually thats not how the air system works on airplanes (except for the 787). In a simplified way the air is fed into cabin from engines (engine bleed air). That air is at approx. 250 degrees when it exits the engine, so it needs to be cooled down to ambient temperature before it could be used in cabin.
Google/wiki ''Environmental control system'', its quite interesting.