Is it necessary to turn on AC for defogging in winter?

[VonIsabela]

It is not necessary to turn on AC for defogging in winter. The methods for defogging in winter include: 1. Opening the car windows; 2. Using defogging agents; 3. Turning on the car heater; 4. Activating the car's defogging function; 5. Applying a mixture of dishwashing liquid and water or soapy water to the windows; 6. Placing dehumidifying boxes inside the car; 7. Wiping with a towel. The reason for fog formation is the temperature difference between the inside and outside of the car, where the higher temperature inside the cabin causes moisture to condense on the inner windshield, forming fog. Methods to prevent car fogging include: 1. Using specialized defogging products before driving; 2. Utilizing the air conditioning's cooling and dehumidifying function to reduce air humidity.

[LaEva]

In winter driving, you don't necessarily have to turn on the A/C when the windshield fogs up. I think it depends on the situation. Like me, having driven for over a decade, I usually first turn on the heater to high temperature, set the fan to maximum, and direct it at the glass. This way, the hot air can melt the fog, especially in the morning when the car is cold—it works quite well. Turning on the A/C is faster because it acts like a small dehumidifier, but in winter, the compressor works more, which might consume a bit more fuel. Try using the fresh air mode (external circulation) to avoid worsening the fog with recirculated air; for light fog, cracking the window for airflow can also help. However, on rainy or snowy days when the car is very humid, turning on the A/C for a minute before turning it off is more efficient. Overall, you can defog without the A/C, but using it speeds things up. Safety comes first—if the fog is too thick, pull over to clear it completely before continuing.

[StPiper]

From an automotive perspective, the key to defogging is reducing air humidity. The A/C switch controls the air conditioning compressor, which cools the air to remove moisture when operating, then reheats it to dry the windshield—this indeed clears fog quickly in winter. However, it's not mandatory to use it. For instance, simply using the heater fan to blow warm air can also reduce fog condensation by heating the air; outside air circulation is drier and more effective than recirculation. At low temperatures, A/C efficiency may decrease, potentially saving fuel. In practice, I've noticed some vehicles have built-in defog buttons that automatically adjust the A/C—manually activating it can speed up the process. The core principle is maintaining air circulation and temperature; A/C is just an auxiliary tool to be used flexibly.

[VanChristian]

If you want to save costs while driving in winter, you don't necessarily need to turn on the A/C for defogging. It increases engine load and fuel consumption, so I use it sparingly. For example, turning up the high heat and increasing the blower force while using the fresh air circulation mode can effectively defog the windows economically. For light fog, wiping with a cloth or opening the window for ventilation also works; the A/C can quickly dehumidify but avoid prolonged use to prevent wear. This approach is both cost-effective and safe.