When the car air conditioning is off, should it be on internal or external circulation?

[StJulia]

When the car air conditioning is off, it defaults to external circulation. The differences between internal and external circulation in a car are as follows: External circulation refers to drawing air from outside the vehicle, directing it through the air conditioning system for heating or cooling, and then blowing it out through the vents. Essentially, external circulation introduces fresh air to improve the air quality inside the car. It is particularly useful for defrosting in winter or defogging in summer, as the air drawn from outside is richer in oxygen and drier, making it more effective for these purposes. Internal circulation, on the other hand, draws air from inside the car, creating a closed-loop airflow within the vehicle. Its primary function is to prevent external dust and harmful gases from entering the cabin. It helps warm up the car faster in winter and cool it down quicker in summer when the air conditioning is on. It is also useful during smoggy conditions. Internal circulation maintains the cabin temperature more effectively. However, it's important not to use internal circulation for extended periods while driving, as the cabin is a relatively sealed environment. Prolonged use can reduce oxygen levels, leading to fatigue and drowsiness, which can compromise driving safety. When to choose between internal and external circulation: Consider road conditions: On highways, use external circulation. Due to wind resistance and high speeds, opening windows is impractical. Prolonged internal circulation can lead to oxygen deprivation, impairing reaction times and judgment, which is dangerous. Additionally, air quality on highways is generally better, with lower exhaust concentrations, making external circulation a good choice. In congested urban areas or during rush hours, where exhaust concentrations are high, switch to internal circulation. Consider vehicle condition: New cars often have higher concentrations of toxic gases like formaldehyde. For the first three to six months after purchasing a new car, it's advisable to use external circulation along with opening windows to enhance air circulation and dilute toxic gas levels. Consider air quality: In areas with severe PM2.5 pollution (levels above 50), or when air quality is not good, it's best to use internal circulation to avoid inhaling excessive PM2.5 particles. Function of external circulation: External circulation ventilates the cabin, expelling stale, oxygen-depleted air and bringing in fresh air. In winter, if the heater is on, remember to switch to external circulation to prevent dizziness and chest tightness.

[DeAddyson]

As someone who frequently drives long distances, I find this question particularly practical. Whether to use recirculation mode when the AC is off depends entirely on your immediate needs. The most commonly used is the fresh air mode (external circulation), where natural wind blows in through the grille when the car is moving, giving a feeling similar to opening the windows but without the wind noise. I personally prefer using external circulation on highways for better air circulation. However, when parked waiting for someone or driving through dusty areas, pressing the recirculation button (internal circulation) can effectively block out external dust and exhaust fumes, and also slows down dust accumulation on the dashboard. Important reminder: prolonged use of internal circulation can easily cause windows to fog up, especially on rainy days—switching to external circulation immediately clears it up. My habit is to switch to internal circulation in traffic jams to avoid exhaust fumes and to external circulation on highways for fresh air—flexible switching is the most comfortable approach.

[OJoel]

Having driven a manual transmission car for over a decade, I've noticed many beginners confuse the difference between air recirculation and fresh air modes. Simply put: when not using AC, press the fresh air button - fresh air from the engine compartment enters through the vents, quieter than opening windows; recirculation seals the cabin air. Practical tests show that using fresh air mode + diagonally opened windows cools the car faster than AC after summer sun exposure. One winter, I kept windows closed with recirculation on, resulting in completely fogged windshield that nearly caused an accident. Remember these key points: always use fresh air when someone smokes in car, switch to recirculation in tunnels to prevent exhaust fumes, and rely on fresh air for defogging in rain. Mastering these two buttons eliminates the need for constant AC use and saves fuel.

[VanCooper]

Last time I was stumped when taking my kid out, so I specifically looked up the principle. Now the in-car operation is very clear: for daily driving, if the AC is off, turn on the external circulation, and the front windshield vents can blow natural air. However, there are two situations where internal circulation must be used—when there's heavy dust from roadside construction or when the car ahead is spewing black smoke in traffic. Once I forgot to switch, and my kid coughed for a long time. Note that rear window defogging automatically activates external circulation, so don’t confuse it with manual mode. Now I’ve developed a habit: after ignition, I first check the circulation light on the center console—a red light means the internal circulation mode is off, which is the safest.