Why is white smoke coming from air vents when I shunt down the cooling effect on my car AC?

There should not be any smoke coming from your car's air vents, regardless of whether you are turning on or off the cooling effect of your car's AC. If you see white smoke coming from your vents, it is important to stop driving and seek advice from a qualified mechanic as this could indicate a potentially dangerous issue, it could be from burnt wire loom on a heater element.