I'm no pilot, but my understanding is that airplane HVAC is driven by pressurized air from compressor bleed (running through a device that manipulates pressure

I'm no pilot, but my understanding is that airplane HVAC is driven by pressurized air from compressor bleed (running through a device that manipulates pressure and thus temperature, through various heat exchangers, to produce heat or cooling as needed), and doesn't run at all from the APU. Cool air on the ground is from a duct under the jetway. Once they push back, they're basically unconditioned until the main engines are at power.