Or you can do what I do and just reach up and start the APU yourself.
That is absolutely absurd having to wait until boarding with the cabin temp at 100 degrees + and a full boat.

I'm all for saving the company money where I can, but not at the expense of passenger comfort. Plus, I don't feel that I can execute my abilities to their fullest and operate a safe flight while I am pouring sweat on the verge of a heat stroke.