Is it better to use the internal or external air circulation for heating?

[LaRhett]

It is more suitable to use the external air circulation when the heater is on. Below is an introduction to the functions of the car air conditioner's internal and external circulation: Function of internal air circulation: The main function of the car air conditioner's internal circulation is to effectively prevent external dust and harmful gases from entering the car. It helps the car warm up faster in winter and cool down quicker when the air conditioner is on in summer. It also reduces air pollution inside the car during smoggy days. However, prolonged use of internal circulation can decrease the oxygen level inside the car, which may lead to fatigue and dizziness due to lack of oxygen. Function of external air circulation: The external circulation brings fresh air from outside into the car, providing ventilation. The air blown by the fan comes from outside, and even when the fan is off, air still flows into the car while driving, replenishing fresh air. Sometimes, you may feel air coming in even when the fan is turned off, which is due to the external circulation setting. When driving in congested urban areas, the car may fill with exhaust fumes, which is caused by using the external circulation.

[ODonovan]

I love studying these little details while driving! When using the heater, if the outside air quality is poor—like in tunnels or during heavy traffic with exhaust fumes—switching to recirculation mode is cleaner and more fuel-efficient. However, remember to switch back to fresh air mode after 20 minutes to ventilate, otherwise, the car can get too stuffy and cause dizziness. If the windshield fogs up, you must use fresh air mode with the AC defogger—I’ve personally tested it, and turning off recirculation clears the fog super fast. The key is to switch flexibly based on air quality and fogging conditions. I usually use recirculation for short trips and alternate between the two modes for long drives.

[AmeliaLynn]

I did a special test on the highway the other day: using the recirculation mode when starting the car cold and turning on the heater really warms up the cabin much faster. The seats weren't even warm yet when the hot air started blowing out. However, when driving to the suburbs with good air quality, I immediately switch to fresh air mode, otherwise I always feel stuffy breathing. The most critical moment is during rainy days! Once I forgot to switch to fresh air mode, and the windshield became completely fogged up - it scared me to death. Now I've developed a habit: after getting in the car, I first turn on fresh air mode for 30 seconds to ventilate before closing the windows. Once it's warm, I switch to recirculation to maintain the temperature. If fogging occurs, a quick turn of the defroster knob solves it immediately.

[VonLillian]

In winter, using the recirculation mode for heating is definitely the top choice. Not only does it double the heating efficiency in a closed cabin, but it also reduces fuel consumption by 0.3L as shown on the gauge. However, remember two scenarios where switching to fresh air mode is a must: first, when carrying passengers for more than half an hour, otherwise high carbon dioxide concentration can cause drowsiness; second, when smells like hot pot or smoke cling to the seats, turning on fresh air mode for ten minutes is more effective than air fresheners. On smoggy days, avoid using fresh air mode—my in-car PM2.5 detector shows that opening the window for just 3 minutes sends the readings off the charts.

[McBella]

As a veteran driver with ten years of experience, I recommend a three-stage operation: use the internal circulation to accelerate warming up when starting; switch to external circulation to prevent windshield frost when the water temperature reaches 90 degrees; manually switch modes every 20 minutes on highways to prevent hypoxia. Pay special attention to the condition of the cabin air filter. Last month when I changed mine, I found the originally gray filter paper was clogged black with willow catkins—using external circulation in such cases just sucks in dust. Don’t slack off on rainy days; external circulation plus the AC button is the golden combo for defogging.

[OMicah]

The biggest fear when using the heater is stuffy air and high fuel consumption! I've experimented with various combinations: At -5°C in the morning, using recirculation mode for 10 minutes can raise the temperature from 5°C to 22°C, saving 40% time compared to fresh air mode. But an office colleague reminded me that prolonged recirculation can spike cabin humidity above 70%, which is bad for skin. Now I keep a hygrometer behind the vanity mirror and switch to fresh air mode when humidity exceeds 65%. Another trick is to tilt the sunroof when using fresh air mode - it creates airflow without compromising warmth.