Why does the AC automatically turn on when defrosting the front windshield?
2 Answers
The reason why the AC automatically turns on when defrosting the front windshield may be related to using warm air for defogging. Reasons for the AC automatically turning on: The AC refers to the air conditioning compressor, while the car's warm air is not provided by the compressor. Therefore, the AC does not operate when using warm air. In winter, when defogging with warm air, if the front windshield defogging function is activated simultaneously, the car's system will automatically engage the compressor. Some vehicle systems even have a memory function—if the warm air and front windshield defogging were used together previously, the system will remember this setting and automatically activate the compressor the next time warm air is turned on. Principle behind the AC automatically turning on: Window fogging occurs due to high humidity inside the car, where moisture in the air condenses on the windows when it meets cold surfaces. The car's warm air comes from the engine's heater core, and the air heated by the heater core tends to be warm and humid. To quickly defog the interior, dry air is needed. When the AC compressor operates, it has a dehumidifying effect—air cooled by the compressor becomes dry, and after being heated by the heater core, the air delivered into the cabin becomes both dry and warm. Blowing this dry, warm air onto the front windshield achieves better defogging results.
This design is very thoughtful. I noticed that every time I adjust the air vent mode to blow towards the front windshield, the air conditioning automatically turns on. This is mainly because the front windshield is prone to fogging up, especially on rainy days or when using the heater in winter. The fog on the glass is caused by the condensation of moisture inside the car when it meets the cold surface. Simply blowing hot air can only temporarily clear the fog, but since the moisture isn't removed, the fog quickly returns. When the air conditioning is turned on, it not only cools the air but, more importantly, removes moisture from the air, producing dry air. This dry air quickly clears the fog from the windshield, ensuring clear visibility and safe driving. I've driven several cars that work this way, and I recommend not manually turning off the air conditioning—letting the system handle it automatically is more efficient and hassle-free. Some older cars don't have this feature, making fog much harder to deal with. Remember, safety first—clear visibility is crucial for avoiding accidents.
