Some body told me to roll down the windows and turn on the air conditioning besides pushing the defrosting buttons in order to speed up defrosting; while others told me roll down the windows and have the heat on. Which is correct?
The a/c compressor runs when you use the defrost mode, as long as the ambient temperature isn’t too low. This serves to dehumidify the air. Opening the windows will just let in humid outside air, which will be counterproductive.
EDIT: Forgot to add: Move the temperature setting to the highest comfortable level to accelerate the process.
For defrosting, use heating filament in the window as ice and frost are usually on the outside of the window. If you don’t have that feature, it’s best to remove the frost / ice manually first before using the window “defroster” blower. On a real cold mornings, the blower type of defroster is overwhelmed.
For defogging, which usually occurs on the inside of the window, it is critical to introduce outside air as heat is turned on. When it’s really cold, take shallow breaths until the cabin air temp reaches above 60* F as it is you and the occupants causing the fog.
Come to think of it, why is it that the newer cars have such poor inside air circulation? A '73 Buick Century I killed after 180,000 miles of NE winter driving would carry 6 heavy breathing teenagers on a cold, wet night without ever fogging up!! A '71 Caddy was another fine example of American automotive manufacturing. Ah, the good ol’days… Were it ever real?
Not so much less circulation, but likely your current car has less interior volume.
Old cars were very leaky so it was the equivalent of rolling all your windows down slightly. Newer cars are much tighter.
It depends on how humid it is inside the car, outside the car and it depends on the temperature.