Why does my windshield fog up if the climate control isn't on A/C setting (even when using heat)?

Question

Like most people, my car - a 2008 nissan sentra- has three climate control dials. One for temperature, one for where you want it to blow, and one for how much air/how hard. The temperature dial has a range from cold to hot, but it also has a push button for Air Conditioning, which you have to use if you want really cold air.
Whenever I have the air set to blow on the forward facing vents (rather than on the feet or on defrost) my windshield always fogs up right away unless I press the A/C button and keep it on all the time. This happens whether I'm using the cold air, or if I'm using the heat. It doesn't matter if I have the air being circulated only within the cabin or not.
What effect does that A/C button have other than allowing the production of really cold air? In other words, what effect does it have when I have the heat on that keeps my windshield from fogging up? And is it bad for the car to have the A/C feature on when you are actually using the heat? Thanks.

Answer 1

the A/C works as a dehumidifier, that is why it clears the foggy windows...



on most cars the A/C compressor cycles on and off( button in or not ) in the defrost position, and whatever position that air comes out of those little vents that aim at the door windows...on my Saturn .. that is the floor heat position

It will not hurt your car, and probably says in your owners manual to use the a/c to clear foggy windows......

Answer 2

What's happening is "Condensation" causing the {Fogged Windshield}. A/C Drys the Air reducing the Condensation and [Actually heating the Cabin Faster, Don't ask! not sure how but it does]. No it won't hurt to run the A/C with the Heat.

Answer 3

You need to put it on ''defroster'' not AC or Heat. Im assuming your using the heat (cause its winter). What you need to do is, when you use the heat (or AC) and your windows start to fog up, just turn the dial to defroster for a few seconds. It will clear up in matter of seconds.

Answer 4

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Answer 5

AC system puts out dry-er air than without it.

To defrog your windshield rapidly,

use defrost on high,

fan on high, temperature on high,

with the ac on.

ONCE you have started your car and defrogged your windshield, it should not fog back up unless you have a plugged cowling drain and or a leak in your heater core!