Why does defogging automatically turn on the AC?
2 Answers
The reason defogging automatically turns on the AC is that it requires activating the air conditioning compressor. Methods for defogging include: 1. When the fog is light, open the car windows—when the temperature inside and outside the car becomes similar, the fog will dissipate; 2. Turn on the car's cold air conditioning; 3. Turn on the heating before driving; 4. Apply a mixture of dish soap and water or soapy water as an anti-fog agent on the windows; 5. Place a dehumidifier box inside the car. The causes of window fogging are: 1. High humidity—when the window temperature is below the dew point under certain air humidity conditions, condensation forms; 2. Low temperature—when there is a temperature difference between the two sides of the window, and the surface with lower temperature reaches moisture saturation with vapor pressure lower than the surrounding environment, moisture gathers on the glass surface, forming fog as tiny water droplets.
When I first bought the car, I noticed that pressing the defrost button automatically activates the air conditioning. This is because windshield fogging is usually caused by high humidity inside the car, and the AC can quickly cool the air to remove moisture. When cold air passes through the evaporator, water vapor is absorbed and released outside, making the glass clear rapidly—especially effective in winter or rainy conditions with heavy fog. In fact, warm air can also defog, but it’s much slower and less fuel-efficient. Most car models are designed this way to minimize driver distraction, offering a one-touch solution for safety and convenience. If you prefer not to use the AC, you can switch to manual mode and rely solely on warm air for defogging, but AC auto-activation is the default setting. I’ve always found this feature thoughtful, preventing the need to fiddle with buttons while driving—particularly on rainy nights when quick fog clearance reduces accident risks. As an owner, my only concern is fuel consumption, since frequent use might slightly increase costs.
