$\begingroup$ "Why can't bleed air, which is hot compressed air, be used as hot air?" Because bleed air comes directly from the compressor section, at temperatures around 250°C (482°F). Turning the cabin heat on would result in 100+ passengers experiencing the face-melting scene from Indiana Jones Raiders of the Lost Ark. $\endgroup$ – Ron Beyer Apr 19 '17 at 19:00