Do You Need to Turn on AC for Hot Air?
4 Answers
Hot air does not require turning on the AC. The warm air is generated by the engine's heat and circulated into the car through the air conditioning system, thereby increasing the interior temperature. If you turn on the AC switch while using the warm air, the compressor will also operate, which not only fails to improve the heating effect but also increases fuel consumption. The AC button controls the air conditioning compressor's switch. During summer, when the driver turns on the cold air, the AC button must be pressed simultaneously for the air conditioner to cool and circulate cold air inside the car. Drivers can turn off the AC switch during startup or rapid acceleration, as the air conditioning compressor is driven by the engine and consumes some of its power. Turning off the AC switch can prevent insufficient engine power.
I've been driving for almost ten years, and this is a question many new drivers ask. Simply put, you usually don't need to turn on the AC when using the heater, because the hot air in the car comes from the engine's heat, transferred to the cabin via the coolant—just turn the temperature knob to the hot setting. But why do experienced drivers sometimes say to turn on the AC? Mainly for dehumidification. In winter or rainy weather, when the air is humid, not using the AC can cause the windows to fog up, which is dangerous—blurred vision increases the risk of accidents. I’ve experienced it myself: driving on the highway in the rain when the windows suddenly fogged up. I quickly turned on the AC and the fan, and the air became much drier. For everyday driving, if there’s no fog or humidity, don’t use the AC to save fuel. But if there’s fog, turn it on—safety first. Remember, the AC is for dehumidification; the heat itself doesn’t rely on it.
I've always been interested in automotive technology and often study these small details. When using the heater, you generally don't need to turn on the AC, as car heating relies on waste heat from the engine coolant. The AC button is mainly for cooling or dehumidifying. If you activate the AC while using the heater, it will engage the compressor to remove moisture from the air, preventing window fogging or making the warm air less stuffy and uncomfortable. This technology is particularly useful in humid, cold weather, like during the southern rainy season—I've experienced situations where just the heater fan didn't work, but adding AC immediately improved comfort. Note that using the AC will slightly increase fuel consumption since the compressor has to operate, so use it sparingly in dry weather. The key is to decide based on your environment and not waste energy efficiency.
When I first bought my car, I was also confused about whether to press the AC button when turning on the heater. Later, I asked a friend and realized that generally, there's no need to turn on the AC. Think about it, the heat comes from the engine's warmth, so you can just turn up the temperature knob on the dashboard. The AC is for cooling or drying the air. But once when I was driving in winter and it was snowing outside, the car fogged up, and my vision was completely blurred. I panicked and quickly turned on the AC with the fan, and the fog cleared immediately. Turns out, the AC can dehumidify and protect your vision, but it's fine not to use it normally—no need to press extra buttons and waste energy. Simply put, heat comes on its own, and turn on the AC when fog appears.
