When to Use the Air Recirculation Button - Vol.384
Have you noticed the button in your car's AC control panel that looks like a circle, almost like a recycling sign? That would be the air recirculation button. When activated, the car's air conditioning system recirculates the air inside the car for cooling, instead of using air intake from the outside.
As to when to use the air recirculation button, the answer is most of the time when the AC is on but not when the heater is on. Recirculating air, especially on a hot day, will cool down the inside of the car quicker and put less stress on the car's blower motor and air compressor.
The only time that you should disable air recirculation is if you notice the windows and windscreen fogging up. This can happen on a cold day or when rain all of a sudden started to pour on a hot day. In this case, you should just hit the defrost button. Hitting the defrost button will disable air recirculation (if it's activated), turn off the air compressor (if AC is running), and blast the maximum amount of air onto the windscreen. The condensation inside the car can be cleared much quicker this way.
More Detailed Explanation
Ideally, the car's AC should work like the AC at home. The home air conditioner uses the air inside the room, cools and dehumidifies the air, and pushes the cooled air back into the room. The AC recirculates air inside the room so the air can keep getting cooler and cooler, instead of using air from the outside which would be much hotter. In fact, we also keep the windows shut so the hot air wouldn't mix with the cooled air.
The concept of recirculating air is the same for the car's AC system. The car's AC compressor and blower motor also happen to be expensive, especially when you consider the cost of parts and labor. So it is in our best interest not to make them work too hard, which would be the case if the blower motor has to move lots of air from the outside and the AC compressor has to cool the much hotter outside air.
Not only that, the air filter will also become dirty quicker if you keep pulling lots of air in from the outside. The blower motor would have to work even harder to pull in air when the air filter is dirty. You would have to change the air filter or the intake air volume will begin to drop.
No matter how effective the air filter is at purifying the outside air, it is nowhere near 100% efficient, so if the outside air quality is bad, which is usually the case with so much vehicle exhaust on the road, it will affect the air quality inside your car as well. This is why if you stopped behind a garbage truck, you would smell it unless you were recirculating inside air.
Debunking the Popular Misconception
A common misconception is that the occupants in the car will deplete the oxygen inside the car if the air recirculation button is on for an extended period of time.
This is false because the car is not a vacuum system. Outside air will leak into the car through the windows and AC system, especially if the car is moving, even if you are recirculating air. This is why you might smell a particularly foul-smelling garbage truck in front of your car even if the air recirculation button is on, though the smell is of course much fainter than if you were not recirculating air.
Many new cars don't even have an air recirculation button because they are equipped with electronic control to automatically turn on or off recirculation. Some cars also have an auto-recirculation mode. For example, press the recirculation button once to turn on recirculation and press it again to make it automatic. If your car has an auto-recirculation mode, you can use it to take any guesswork out of when and when not to recirculate.