Should You Use Internal or External Air Circulation When the Windows Fog Up?
3 Answers
When car windows fog up, you should use the external air circulation. The button for internal and external air circulation in cars generally comes in two types: one with two separate buttons—one for internal circulation and one for external circulation—and another with a single button, where pressing it to light up indicates internal circulation, and pressing it again to turn off the light indicates external circulation. Internal circulation primarily serves to promptly and effectively block external dust and harmful gases from entering the car. Below is a relevant introduction about the car's internal and external circulation: The working principle of external circulation is to use a fan to bring outside air into the car and expel the air inside the car through the fan, achieving ventilation without opening the windows. The principle of internal circulation is to allow air inside the car to circulate, especially when using the heater in winter or the air conditioner in summer to promote airflow. In summer, the air conditioner operates using the compressor, so you need to turn on the AC button and then activate the air conditioner. Therefore, fuel consumption is relatively higher when using the air conditioner in summer. When the air conditioner is on, if the temperature inside the car is higher than outside, the windshield will fog up. In this case, you need to turn on the external circulation to achieve a defogging effect. In winter, the car's heater works using the heat generated by the engine during operation, and the engine's cooling is managed by the radiator.
Every time I encounter foggy windows while driving, I directly turn on the external air circulation combined with the air conditioning to defog. The principle is that moisture inside the car condenses into fog when it meets the cold glass. Using external air circulation introduces dry air from outside, reducing the humidity inside the car. In terms of operation, first start the air conditioning, press the windshield defogging button, and maintain the external air circulation mode. I've tried defogging with internal air circulation, and the effect was particularly poor—the humid air inside the car just keeps circulating, making the fog worse. Especially on rainy days or in winter, when there are more people in the car breathing and producing more moisture, it's essential to rely on external air circulation for ventilation. In summer, using cold air defogs quickly, while in winter, using warm air first warms up the glass to remove fog. Don’t forget to turn on the A/C dehumidification function, as it speeds up the dissipation of fog. For safety, if the fog severely affects visibility, pull over first before operating.
Having driven for over a decade, I never use the recirculation mode when defogging windows. Fogging occurs due to high interior temperature and humidity causing condensation on cold glass. The fundamental solution is to draw in dry outside air via external circulation. My routine: start the car, turn on AC, press the defog button, and switch to external circulation mode. In summer, cold air clears fog in half a minute; winter's warm air may take 2-3 minutes—don't panic when fog temporarily worsens upon initial warm air blast. Recirculation traps moisture inside, exacerbating fogging. During a heavy snow commute taking kids to school, two chatty backseat passengers exhaled enough vapor to instantly whiten the windshield—emergency resolved with external circulation plus warm air. Clogged cabin air filters also hinder defogging efficiency; regular replacement is recommended.
