Should You Use Internal or External Air Circulation When Turning on the Heater?

[LeDonovan]

When turning on the heater, you can use either internal or external air circulation. It is recommended to primarily use internal circulation while occasionally switching to external circulation as a supplement. External circulation means air is drawn from outside the vehicle, and the air blown out from the vents comes from the external environment, creating a connected space between the inside and outside of the car. Internal circulation, on the other hand, completely seals the car's interior, and the air blown from the vents is recirculated from inside the vehicle. This helps retain warm air inside the car without losing it. Therefore, based on the principles of internal and external circulation, if the outside air is heavily polluted, it is advisable to use internal circulation.

[LeAugust]

In winter, I usually start with the recirculation mode for the first ten minutes of driving, as the cold air blowing right after the engine starts is unbearable. Once the car warms up, I quickly switch to the fresh air mode. One time during the late rush hour after overtime, I forgot to change the mode, and the windshield fogged up instantly, making it hard to see the road—I almost rear-ended someone. However, in heavy traffic or dusty conditions, I switch back to recirculation to avoid exhaust fumes entering the cabin. I remember a road trip to Northeast China two years ago when I ran the fresh air mode for half an hour at -20°C, and the fuel consumption jumped by two points. It's essential to adjust flexibly based on the temperature.

[McEmma]

Don't rush to turn on the external circulation right after starting the car! The engine hasn't warmed up yet, and you'll just be blowing in cold air. At least wait until the coolant temperature gauge reaches the midpoint before switching modes. I usually set it to 23°C and use auto mode, letting the system balance between internal and external circulation automatically. Friends in northern regions should note that keeping internal circulation on for too long can cause frost to form on the windshield, especially on rainy or snowy days—switch to external circulation about ten minutes in advance to defog. I once read a study saying that using internal circulation for over an hour can triple the carbon dioxide concentration inside the car, making you prone to drowsiness.

[RowanDella]

From a thermal efficiency perspective, using recirculation mode maximizes heating effectiveness. This is because it reheats the already warmed cabin air, whereas fresh air mode continuously cools down the heating system. My hybrid vehicle tests showed: at 0°C ambient temperature, recirculation mode heats up twice as fast as fresh air mode. But EV owners should note that prolonged recirculation increases power consumption by 15%, especially for PTC heating models. It's recommended to combine with steering wheel heating for energy savings.