- If I have 2+ passengers, or it's cold out (<10F), then I find that I need to turn-off recirculate (use 100% fresh outside air) or else the windows will fog.
- If I have a full car, or it's very cold, then I find that it is necessary to use Defrost (Heat+A/C) to keep the windshield clear.
One thing I have wondered is: where is the cross-over point, where you spend less energy doing Defrost with Recirculate, versus Heat with 100% fresh air. In the former, you are cooling the air enough to condense, then re-heating. In the latter case, you are just heating dry air, but by a lot (80F!).
Trying to estimate energy use, I'm going to ignore condensation: each resting adult should exhale between 15g/hr water vapor at room temp, and 20g/hr at -10C. A rough guess is that it takes only about 12 watts per adult to condense that water. Even with four adults, that's less than 2% of the 3000+ watts used in heating.
If the car has temperature set-point of 20C with 60% relative humidity (high-end for "comfort" humidity), then each cubic meter of air through the A/C system holds about 10 grams of water. If the HVAC system is cooling that from 20C to 12C (dew point), then condensing a bit of it, then heating it up to 20C again; and if the Leaf's heat-pump is 200% efficient vs inductive heat, then you'd spend about 120 calories per cubic meter of air through the HVAC system (plus a small bit for condensation, plus lots of fan motor load).
On the other hand, if you're drawing-in outside air at -5C, 90% RH, then there's only about 3 grams of water per cubic meter of air. You're heating it to 20C, which is a full 25C rise, but because there is so much less water, it should take only 75 cal per cubic meter, which is 37.5% less energy consumption (plus fan motor load).
Going back to Defrost, if the HVAC system is dehumidifying all the way down to 30% RH at 20C (2C dew point, below low-end for "comfort"), then there'd only be 5 grams of water per cubic meter, or 60 calories spent cooling+heating, which is 20% less energy than heating dry outside air.
Unfortunately, there are too many unknowns: What dew point does Defrost achieve, at what ambient temp? Does the defrost cycle run the same CFM? (the blower sure sounds louder on Defrost). You'd probably set the fan to "1" or "2" on the "outside air" case, but for Defrost, cooling is very inefficient at low airflow.
I think the only way to guess would be to test. With outside temps at (say) -5C, you could charge the car to 100%; then turn it ON and set it to Defrost+Recirc at 20C; and put an electric kettle inside (on extension cord) with 24cc of water and boil it off (to load-up humidity above dew point) then unplug the kettle. Then let the car run like that for 8 hours, then check how many watt-hours remain (then add 10 to 50 watt-hours per hour to account for condensation). Then do a similar the test using plain Heat with no Recirc.