Should You Turn on AC When Using Car Heater in Winter?
4 Answers
When using the car heater in winter, you should not turn on the AC. The warm air comes from the heat generated by the engine during operation: After the engine's cooling system dissipates heat from the engine, a fan directs this heat into the car cabin to provide warm air. When the heater is not in use, the fan stops, and the heat vent closes, allowing all the dissipated heat to escape into the atmosphere. Therefore, the car heater reuses waste heat and does not consume additional fuel. To quickly raise the cabin temperature in cold weather, start by allowing the engine to warm up after ignition. Once the engine temperature gauge reaches the midpoint, turn on the heater and set the air circulation to external mode. After 2 to 3 minutes, switch the air circulation to internal mode.
I've specifically researched this issue before. In winter, you can indeed turn on the heating without activating the AC switch. The heat generated by the engine itself is sufficient to warm up the cabin, and not using the AC actually saves fuel. However, sometimes windshield fogging can be quite annoying, which is caused by condensation due to the temperature difference between the inside and outside of the car. In such cases, I turn on the AC for a few seconds because the air conditioning system has a dehumidification function that can quickly remove moisture and restore clear visibility. Just be careful not to leave it on for too long, as the cold air mixing in might make the interior uncomfortably cool. Nowadays, many new cars come with an automatic defogging button—just press it, and it intelligently controls the AC switch timing.
As a commuter who spends two hours on the road every day, my experience is: there's no need to turn on the AC in winter. The heating relies on the heat from the engine coolant, so the compressor doesn't need to work at all. Turning on the AC just wastes fuel by running the compressor unnecessarily and actually reduces heating efficiency. However, when it's raining and the windows fog up, that's a different story—I quickly press the AC button for about 20 seconds to dehumidify, then turn it off as soon as the fog clears. Also, I make it a habit to turn off the heater and switch to outside air circulation before parking, which helps prevent fogging the next day and avoids having to use the AC.
This issue is directly related to weather humidity. On dry sunny days, turning on the heater doesn't require AC at all—the compressor only increases fuel consumption. However, during rain or snow, or when the car is full of people exhaling moisture, the windshield fogs up quickly. In such cases, I definitely turn on the AC, as it acts like a dehumidifier to remove moisture. Some drivers assume the AC automatically turns off when heating is on, but manual climate control systems require manual operation. Now, when I notice fogging, I first turn on the AC for 30 seconds to dehumidify, then turn it off once visibility is clear—this has almost no impact on fuel consumption.
