Re: I can handle this one
Close but no banana...
Defensive coding 101 - never assume side effects in functions, and avoid adding side effects to functions.
Else if (too_cold)
Plus there's no guarantee that turning the heater on turns off the AC, and vice versa. If the sample rate isn't high enough, transition from too hot/cold to too cold/hot could skip the Heat_and_AC_Off() call allowing heater and AC to be on together.