Do You Need to Turn on AC for Window Defogging?
2 Answers
Window defogging requires turning on the AC. The appearance of fog indicates that the air inside the car is very humid. Using humid air continuously for defogging is ineffective. Reducing air humidity makes it less likely for fog to form, hence turning on the AC for defogging yields the best results. Defogging does require turning on the AC, but no manual operation is needed. As long as the defogging switch is activated or the air direction is set to the windshield mode, the AC switch will automatically turn on, and the circulation mode will switch to external circulation. Turning on the AC and external circulation ensures the fastest and most effective defogging. Some cars allow manual deactivation of the AC, while others do not. If the AC is turned off in defogging mode, defogging can still occur, but sometimes the effect may be worse, with fog persisting for a long time or even worsening. Turning on the AC is not about using cold air to blow away the fog but rather utilizing the dehumidifying function of the air conditioning system. The AC switch controls the air conditioning compressor. Once turned on, a signal is sent to the compressor, which then starts working to cool under normal circumstances and stops when turned off. This compressor can only cool and cannot generate heat. Whenever it operates, it cools, meaning the AC is a cooling switch. Heating does not require turning on the AC, as the heat for warm air does not come from the compressor.
As a car enthusiast, I believe turning on the AC is essential for defogging windows, as it effectively dehumidifies and prevents glass from fogging up. The main cause of fogging is moist interior air condensing into water droplets on cold glass, which severely obstructs visibility and poses significant danger. The AC not only cools but also absorbs moisture to dry the air. The operation is simple: start the fan, increase the airflow speed, direct the airflow toward the windows, and then press the AC button. Even if the AC blows cold air in winter, combining it with the heater fan maintains a comfortable temperature. Before driving, I usually turn on the external circulation to introduce dry, cold air—this clears fog within minutes. For long-term car use, remember to check the cabin air filter to prevent clogging and ensure efficient dehumidification. Fog forms more easily in rainy or snowy weather, making the AC even more crucial for maintaining a clear driving view. I recommend making it a habit to act promptly to avoid compromising road safety.
