I think setting the temperature knob to LO provides the "vent" feature you want as an "ECON" mode, JimB, by admitting outside air with no heating or cooling. Then the only energy consumed is for the fan itself, which the Accessories display says is about 0.1-0.3 kW depending on speed (and it shows up as an "Other" rather than "Climate" load).
Could you please describe how you "use the 'lower fan speed' button to turn on the system"?
All I've found is that so long as MaxDefrost is off (because it locks on at maximum the cabin heater, the defroster, and the fan), the other ventilation options, in the bottom row of center-panel buttons, including the ordinary defrost one (defrost symbol with a person), can be used at any fanspeed. If you minimize fanspeed, the system remembers that setting even if you turn off and on the ordinary defrost or the car or both, but forgets the fanspeed setting and default to maximum at turn-on if you turn off the climate control panel.
If there's some workaround sequence that lets me make lowest fanspeed the new default, that'd be really nifty.
But I'm puzzled why, Gotea, you lost 10 miles of predicted range "simply by turning on the fan (no heater or AC)". Might you have had the temperature set on something above LO, so the range-prediction algorithm was thinking you'd be using cabin heat at times? That could indeed have the effect you describe. In fact, I just gained 9 miles of predicted range just by setting to minimum fanspeed and LO temperature, then turning the Accessories (via Start button without holding down brake pedal, all while parked) off and on.