Should You Use Internal or External Air Circulation When Defogging Car Windows?

[SadieAnn]

When you press the defogging button, the car's air conditioning system automatically switches to external air circulation because only external circulation can effectively remove condensation. External circulation means that when the heater is turned on, the air blown by the fan comes from outside the car, making the air inside and outside the car the same. Internal circulation, on the other hand, recirculates air from inside the car, creating a completely closed system where the inside and outside air do not mix. It's like having a fan blowing air inside the car. In northern regions during winter, the car's interior temperature is very low in the morning. Turning on internal circulation can help the temperature rise faster. Once the temperature increases, you can switch to external circulation to bring in fresh air. In extremely cold areas like Northeast China, if the temperature hasn't risen, frost or snow can quickly form on the windows, making it difficult to wipe off. Experienced drivers often use an old bank card or similar item to scrape it off cleanly. Some cars don't have an external circulation button—only an internal circulation button. When the internal circulation light is on, it's in internal circulation mode; when the light is off, it's in external circulation mode. Defogging method: During rainy or snowy weather, you don’t need to rely on cold air or internal circulation to defog. Simply prepare a defogging solution in advance and spray it on the windshield.

[VanLuna]

When encountering foggy car windows, I generally recommend using the external air circulation mode. This is because the external air circulation draws in dry air from outside the vehicle, which has lower humidity and can absorb moisture more quickly, preventing fogging. Using internal air circulation causes the air inside the car to recirculate, allowing moisture to accumulate and worsening the fogging issue. I've noticed this from daily driving, especially during winter when temperature differences are significant. In such cases, using the heater or air conditioning dehumidification mode along with external air circulation works particularly well. If fogging occurs frequently, you can also check the window seals or regularly clean the glass. These small measures can make driving safer and more comfortable by avoiding the risk of obscured vision.

[LeCamila]

As a safety-conscious person, I believe turning on the external air circulation to defog the windshield immediately is the best choice when fogging occurs. Fog obstructing visibility is extremely dangerous. The external circulation provides fresh air, reduces humidity, and clears fog faster. Using internal circulation closes the air system, allowing moisture to accumulate and making the fog worse. I always start by turning on the air conditioning for dehumidification or setting it to maximum airflow, then switch to external circulation—it clears up in seconds. This habit has helped me avoid many accidents. I also remind everyone to keep the car interior dry, such as avoiding opening windows on rainy days to prevent additional moisture from entering.

[DeAlice]

Turn on the external air circulation when fogging occurs, avoid using internal circulation. Internal circulation traps air inside the car, leading to high humidity and faster fogging; external circulation draws in drier outside air, making defogging easier. I've tried both hot and cold air—both work well, and using the defog button makes it even more convenient.

[GabrielLee]

From a cost-saving perspective, using the external air circulation for defogging is more efficient and hassle-free. Although internal air circulation can maintain temperature, the high humidity easily causes fogging, forcing you to use the air conditioner and consume more fuel. I insist on using external air circulation, occasionally adding a small amount of warm air when necessary. Once the windshield is clear, the air conditioner can be turned down. Over time, this reduces the frequency of fogging, minimizes air conditioner usage, and saves fuel. Keeping internal air circulation off when there's no fog also promotes better air circulation for a healthier environment.

[KyleLynn]

Fogging is mainly caused by temperature differences and high humidity inside the car. When defogging the windshield, using the external air circulation introduces dry outside air to reduce relative humidity, effectively preventing condensation. In contrast, internal circulation allows moisture to accumulate and fog forms quickly. Through my practical testing, using external air circulation combined with the air conditioning's dehumidification mode to blow on the windshield quickly removes fog. The principle is that the dry outside air balances the interior environment, maintaining clear visibility through the windshield and ensuring safer driving.