What Circulation Mode Should Be Used for Parking Air Conditioning?

[VivianDella]

Parking air conditioning should use the internal circulation mode. Below are the relevant explanations: 1. The correct way to turn on the car air conditioning is: first activate the external circulation to expel the hot air inside the car. After feeling a noticeable drop in the cabin temperature, switch to internal circulation. 2. Additional information: During city driving, especially in congested areas during rush hours or in tunnels, it is advisable to use internal circulation. However, when the car starts moving at a normal, steady speed, external circulation should be activated. Moreover, in dusty conditions, remember to close the windows and switch off external circulation to block external airflow. It is also best to set the air conditioning to internal circulation when parking, as external circulation can allow dust blown by the wind to easily enter the car, increasing the amount of dust inside.

[AlanLynn]

Parking with the AC on requires extra caution for safety. I've seen too many cases where people fell asleep with the AC on while parked and nearly suffered carbon monoxide poisoning. When the car is stationary, engine exhaust accumulates around the vehicle, and using the external air circulation mode essentially draws all those toxic fumes inside. In my opinion, you must use the internal circulation mode and crack the windows slightly for ventilation—that's the safest approach. Enclosed spaces like underground garages are especially dangerous. Last year, there was a news report about someone who fell asleep while waiting in an underground garage with the AC on and never woke up. If you really need to rest, I suggest turning off the engine and opening the windows, or parking in a shaded area. After all, safety comes first.

[MacHenry]

I usually switch to recirculation mode when parking with the AC on, as it cools more efficiently and saves fuel. However, you can't keep it sealed all the time - rising CO₂ levels in the cabin can cause drowsiness. My routine is switching to fresh air mode for 1 minute every 15 minutes, or simply cracking the sunroof open. Last time at a highway rest area, this kept me cool without feeling stuffy. Pro tip: watch your battery when idling with AC - older cars shouldn't exceed 30 minutes to avoid a dead battery situation.

[DelKaden]

Let me share a lesson from my last experience. During summer, while waiting for two hours in the car with the air conditioning on to pick up my child from tutoring, I initially used the external air circulation mode. However, the car kept filling with a gasoline smell, making my child complain of dizziness. Later, a mechanic explained that when parked, exhaust fumes blown backward from the tailpipe can be sucked back in through the air intake. He advised using the internal circulation mode to isolate outside air for safety. Now, here's how I do it: internal circulation + leaving a finger-width gap in diagonally opposite windows – keeping it cool without stuffiness. By the way, extra caution is needed for cars with auto start-stop systems, as the moment the engine restarts produces the highest volume of exhaust fumes.

[DeKaden]

Actually, this issue relates to the working principle of the air conditioning system. When the compressor is operating, the internal circulation directly draws air from inside the car for cooling, while the external circulation inhales fresh air mixed with high-temperature exhaust from the front of the car. When the vehicle is moving, wind pressure assists in exhaust, but when stationary, a negative pressure zone forms in the engine compartment, making it easy for exhaust gases to backflow into the air conditioning intake. I've tested it—idling with the external circulation on for three minutes can cause the carbon monoxide concentration inside the car to exceed the standard by five times. So, your physics teacher would definitely tell you: when parked with the air conditioning on, you must lock the internal circulation, just like sealing the hatch on a submarine.

[DeAdriana]

For a balance between comfort and safety, my approach involves three steps: start the AC and immediately switch to recirculation mode for rapid cooling; after reaching a comfortable temperature, maintain recirculation but slightly open the sunroof for air circulation; turn off the engine for a break if exceeding 20 minutes. This avoids the risk of carbon monoxide poisoning and prevents excessive battery discharge. Tested effective in last week's 40°C heat, with sunshades reducing cabin temperature by 15°C compared to outside. If prolonged idling with AC is necessary, installing a carbon monoxide alarm (available online for tens of yuan) adds peace of mind.