Should AC Be Turned On for Defogging?

[DelClaire]

Defogging does not require turning on the AC. Additional information about AC is as follows: 1. The AC button is used to activate the air conditioning's cooling function, while defogging requires heating. 2. The AC button controls the air conditioning compressor, which is used for cooling; whereas the warm air function utilizes the car's internal heat circulation and does not require activating the air conditioning compressor. Therefore, to use warm air, you only need to adjust the air conditioning settings without turning on the AC. Additional tips: To defog, start the car, turn on the warm air conditioning, and set the air circulation to external to expel the cold air from the car. After 2 to 3 minutes, switch the air circulation back to internal.

[TuckerRose]

Our car gets extremely foggy on rainy days, and I always turn on the AC along with the defrost button. Once the AC is on, the compressor kicks in and removes the moisture inside the car, quickly clearing the fog on the windows. You might think the AC is only needed in summer, but it’s even more essential in winter when fogging occurs! Even if you’re using the heater, don’t turn off the AC, or the windshield will get blurry again. Once, when picking up my kid from school during a heavy downpour, I couldn’t see the rearview mirror at all without the AC—the condensation made it look like frosted glass. Now I’ve made it a habit: as soon as I turn the knob to the defrost setting, the AC light automatically turns on, which is super convenient. If the fog is heavy, cracking open the fresh air vent helps with ventilation, but don’t turn off the AC—that’s the key to dehumidifying!

[ODonovan]

The most annoying thing about driving in winter is the fogging up of car windows, especially when there are many people in the car. Based on my experience, using the AC to defog is the fastest method—just make sure the little snowflake icon lights up when you press the defrost button. The principle is simple: when the AC is turned on, the evaporator temperature drops, causing the moisture in the air to condense into water droplets and be drained away, resulting in dry air being blown out. Have you ever tried using just the heater to defog? It takes three to five minutes for the warm air to dry the glass, while cold air defogging clears the view in just 15 seconds. However, I recommend setting the temperature a bit higher—this way, it removes humidity without making you shiver from the cold. A while ago, on the way to skiing, my friend was shivering from the cold and wanted to turn off the AC, but as soon as it was turned off, the fog quickly returned, so we quickly turned it back on.

[SanJoanna]

A seasoned driver once taught me a trick: the AC acts like a car's dehumidifier. Although it consumes a bit more fuel, safety comes first! Especially during the southern rainy season, fogging on the windshield is even more dangerous than dim lights. Yesterday, driving back home on mountain roads, I encountered thick fog—seeing the AC light on the dashboard put me at ease. The compressor removes the moisture, and the hot air dries the glass, working together to clear the fog quickly. A reminder to new drivers: don’t be fooled by AC equaling cold air. Modern cars are smart—when you set the temperature to 28°C, the AC blows hot air. Wipers can clear water but not fog; you still need the AC! Sudden fog on the highway is terrifying—don’t wait for the heater to slowly dry it; AC defogging is a lifesaver!

[StChristian]

Experiment shows: on the same rainy day, using AC for defogging takes only 20 seconds, while waiting for warm air without AC takes 40 seconds! The reason is AC directly reduces air moisture content. Even if you feel cold, turning the temperature knob all the way to the right delivers hot, dry air. Some cars automatically start/stop the compressor when pressing the windshield defog button - super convenient. Remember last year driving a friend's EV - he turned off AC defogging and the glass got foggy, so I quietly turned it back on (running AC doesn't consume fuel in EVs!). Don't keep recirculation mode on too long - breathing in a closed space just puts moisture back on the glass. Occasionally switch to fresh air mode for better ventilation. Note: rear window defoggers use heating wires, different from the windshield method - no need for AC.

[EmersynFitz]

Physics teacher explained: Defogging essentially involves lowering the glass temperature or reducing air humidity, and the AC addresses both. Specifically, the compressor causes the evaporator to condense and absorb moisture, drying the air blown onto the windshield to prevent fogging. Turning off the AC means warm air first heats the glass and slowly evaporates the fog, which is particularly inefficient. Last week, during a typhoon while driving my wife to a prenatal checkup, using AC plus external air circulation kept the windshield so clear I could count raindrops. A hidden benefit: When the AC is on, the system also removes moisture from the ducts, reducing mold and summer odors. If you're really sensitive to cold, just turn up the temperature (red zone on the dial), but the AC light must stay on during defogging!