Do You Need to Turn on AC for Defogging in Winter?
3 Answers
In winter, you do not need to turn on the AC for defogging your car. The AC is the compressor switch, which controls the air conditioning cooling. Methods for defogging a car include: 1. Open the windows. 2. Use a defogging spray. 3. Turn on the warm air from the air conditioning. 4. Activate the car's defogging function. 5. Apply a mixture of dish soap and water or soapy water to the windows. 6. Place a dehumidifier box inside the car. 7. Wipe the windows with a towel. Reasons for car window fogging include: 1. Temperature difference between inside and outside the car: The window glass is in contact with the outside and is colder, causing water vapor inside the car to condense on the glass and form fog. 2. Window temperature is below the dew point: At a certain air humidity, if the window temperature is below the dew point, condensation easily forms on the window surface, leading to fogging on the windshield. 3. Temperature difference between the two sides of the window: If the temperatures on both sides of the window are different, the side with lower temperature will have a lower saturation vapor pressure, causing water vapor to gather on the glass surface and form tiny droplets, resulting in fogging.
To defog your car in winter, you need to turn on the AC. The AC, or air conditioning compressor, can quickly dehumidify and remove fog from the windshield. When I drive in snowy weather, I used to only turn on the heater without the AC, and the fog would dissipate very slowly. This is especially dangerous when the windshield fogs up and obstructs the view of the road. Later, I learned a trick: after starting the car, simply press the front windshield defogging button, which automatically activates the AC along with the heater. Alternatively, you can manually turn on the AC and set the heater to maximum, directing the airflow toward the windshield. Using only the heater without the AC is also an option, but in winter, the warm and humid air makes it harder to dehumidify, resulting in much poorer effectiveness. It's recommended to use a bit of outside air circulation to draw in dry air when defogging. Once the fog clears, you can turn off the AC and just use the heater to maintain warmth. Safety is the top priority—don’t push through with blurred vision or try to wipe the windshield with a tissue while driving, as it’s too dangerous. I prefer adjusting the system before driving to save time and ensure peace of mind.
As a driver with family, turning on the AC is a must for defogging in winter. The air conditioning can dehumidify and prevent fogging, keeping the view inside the car clear. I usually start by warming up the engine for a minute or two, then turn on the AC and set it to blow hot air directly at the windshield—the fog disappears in minutes. If there are many passengers exhaling heavy moisture, I turn off the recirculation mode to improve air circulation. Defogging with just hot air (without AC) also works, but it’s much slower, especially on rainy or snowy days. Once, I tried not using the AC, and the fog grew so thick I almost couldn’t see the road, nearly causing an accident. Since then, I’ve made it a habit to check the defogging settings as soon as I start the car. Newer car models now have a defog button that automatically activates the AC, making things much easier. During regular maintenance, I ensure the AC system is functioning properly and that the condenser tubes aren’t clogged, which could affect performance. In short, turning on the AC for safety is the top priority—don’t skimp on a little fuel cost.
