Why does the AC automatically turn on when defrosting the front windshield?

[DeTucker]

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.