Should the AC be turned on when using the car heater in winter?

[DiAllie]

In winter, there is no need to turn on the AC when using the car heater because the heating system utilizes the heat generated by the engine to produce warm air for heating purposes. The AC is only required when cooling is needed. If only the heater is used without the AC in winter, the windows, especially the front windshield, can fog up with condensation in less than five minutes. This happens because warm air contains more moisture than cold air, and when it comes into contact with the cold surface of the windshield, the moisture condenses into fog. Using both the heater and the AC simultaneously allows the heater to warm the cabin to the desired temperature while the AC, operating at low power, cools and dehumidifies the air to stabilize temperature and humidity levels, effectively preventing fogging. The primary function remains heating, with the AC assisting in dehumidification and cooling.

[DelRylee]

I've been driving for ten years, and the use of the AC button in winter depends on the situation. When you first start the car, the front windshield is particularly prone to fogging up, so you must turn on the AC compressor to quickly dehumidify. But strangely, just turning on the heater alone makes the fog worse. Once the humidity inside the car drops and the windshield clears, you can actually turn off the AC to save some fuel. However, on rainy days when humidity is high, or when the car is crowded with people breathing heavily, it's recommended to keep the AC on throughout to control humidity. Remember, the warm air comes from the engine's residual heat, while the AC manages the air humidity balance. This trick is especially practical and has been proven effective through personal experience.

[AndresMarie]

As a mom who often takes kids out, I find the car can get stuffy with the heater on in winter. When the windows fog up and obstruct the view, safety becomes a major concern. In such cases, pressing the AC button along with the heater works exceptionally well for defogging, with a comfortable temperature setting around 22°C. However, keeping the compressor running does consume more fuel, so after defogging, you can turn off the AC and switch to external air circulation to maintain airflow. For long drives, it's recommended to activate the AC for dehumidification about 20 minutes in advance, then switch to regular heating mode once the stuffiness is no longer noticeable.

[DiJaxon]

When I first bought a car, I was also confused about the use of AC in winter, but later I understood how the compressor works. Its function is to dehumidify and defog, especially in the humid and cold weather of southern regions—without AC, the warm air blown out would be full of moisture. My suggestion is to set the temperature to the highest and the fan speed to maximum when you start the car, while keeping the AC on to quickly dry the windshield. Once the interior temperature stabilizes, you can lower the fan speed to maintain comfort. Modern cars come with humidity sensors, and the compressor actually operates intelligently, turning on and off as needed, so it doesn’t run continuously, resulting in minimal additional fuel consumption.