I have a second hand car with an old slider to switch from left to right, with the same icons 1, 2 as in the picture below. But I do not understand it well.
Often when I am driving, my car windows become like mist (like the 'before' section):
How can I get rid of the mist, so that I have clear glass while driving?
In the two pictures/icons you provide the arrow is intended to indicate the flow of air with regards to the cabin. Where the arrow is coming in from outside the car allows air from outside the vehicle in through the HVAC vents. The circular (ish) arrow indicates that the air already inside the cabin is being recycled through the HVAC system and put out the vents.
If you are looking to avoid misting on the windows then you need to consider where the air is dryer. Given the fact that humans generally breathe out relatively warm, humid air leaving it on the recirculating setting will cause the cabin to fog up unless the HVAC system reduces the humidity of air passing through it. It's common for most modern air-conditioning systems to do this as part of cooling the air so in some cases it's better to have it on the re-circulation setting and let the AC dry it out for you.
Obviously if your vehicle doesn't have AC then it's pretty much always going to be better to have it set to allow fresh air in to avoid misting.
First, you need to know what each does. #1 allows Fresh air from outside the vehicle. #2 is Recirculates air already inside the vehicle.
Unless you are running the AC, you should have the selector to #1. This will help reduce the "misting" effect you are seeing.
Selection #2 recirculates air within the vehicle. This is helpful during hot days when the AC is running, as it recirculates the already cold air. During colder weather, this setting typically causes the windows to fog up, or "mist".
The owner manual for my car states this;
If you select RECIRCULATION while in the VENT, BI-LEVEL or FLOOR modes, humid air can recirculate inside the vehicle and allow moisture to form on the windows. If this happens, either press the A/C button to on or select DEFROST or the DEFOG modes.
Fog on the windshield happens when there is warm, moist air inside the car and a cold windshield (chilled by the cold, dry air outside). So when using your ventilation system to de-fog your windshield, you want to do three things:
Therefore, the proper ventilation settings are:
A/C on, heater on, recirculation off, windows open. Your gas mileage won't be great, but at least you'll be able to see.
Starting left to right, the controls you want to use are:
Other options that can help: Roll down your windows, start the car & let it idle, then come back in a few minutes when it's at running temperature. The open windows will let most any moisture evaporate. You can do the above procedure with hot air too, once the engine is warm, but I've found it sometimes makes the problem worse for 10-20 seconds before starting to clear up the windshield.
I believe there are water repelling products that provide a thin hydrophobic film on the windows, greatly resisting fogging as well, but I haven't tried them myself
TLDR: Never use recirc (position 2). The only time to use it is to squeeze extra performance out of your air conditioning. And A/C is very effective at clearing fog, even with heat too!
Air has water in it. Windows fog over when the air is very humid.
If it's raining, most likely outside air is at 100% of its capacity to hold water. Your car is taking that in. Also, you are adding humidity to the car from your breath, sweat and wet clothes.
Warm air can hold more water. So if you run your heater, it will reduce fogging. Of course that may be no fun on a 30C night.
The air conditioner makes the air quite cold. Remember, cold air can't hold nearly as much water, so the water falls out! That's how dehumidifiers work.
If you run A/C with the heat temperature high, the car will re-heat the air conditioned air. This will make it very, very dry.
If it's cold out, turn on the A/C and set max heat. This will defog the car in a few seconds.
If it's too cold out (below freezing) the A/C may not be able to run at all. The Freon system works fantastic at low temperatures, but the evaporator core will tend to ice up. Recirculation can help slow that down.
That lever decides whether to draw fresh air from outside the car (position 1), or whether to recirculate air already inside the car (position 2).
Generally you want fresh air most of the time -- for many reasons. If you don't have A/C, recirc is nearly useless. I never use it.
If you are not using A/C and simply using the heater to dehumidify, you need fresh air, otherwise your own humidity will add to the humidity already in the car and fog you up.
If you are using A/C to dehumidify the air, it helps to recirculate, because the air inside will be drier from already having been through the A/C.
