I'm not sure why this is necessary? I do this myself everyday.
Heater without A/C: Turn A/C off (light on A/C button is off). Actually the A/C uses very little power anyway, about 200W when the compressor is running, but it cycles off very often when it is cold, so it is using even less. I never even notice the increase in power consumption when the A/C is running with the heat on. This in conjunction with recirc mode uses considerably less power than fresh with the A/C off, and keeps my windows from fogging.
A/C without heater: Turn the set temperature down to the minimum (60 degrees F).
Unless the cabin temperature is below 60 degrees this will keep the heater from turning on. Once the cabin temperature stabilizes the power use is about 200W -- very efficient. I usually use fan speed one or two to maintain comfort.
Vent mode: To have air blowing without either A/C or heat I turn the set temperature down to the minimum (60 degrees F) and turn the A/C off (light on A/C button is off). I usually use fan speed one or two to maintain comfort, leaving it in AUTO would generally mean the fan is blowing on too high of a setting.
Happy and comfortable EV motoring to all