How long do you let hot air blow out of your car before you roll up the windows?

Question

... because I know someone who runs the AC while keeping the windows down for 2, 5, 10 minutes, the whole d**n car ride.

We all know about AC vs windows down, but what is the mechanical/thermodynamic explanation of why you should keep your windows up if the AC is on? I mean, it's like 95 degrees out there.

Answer 1

OK, so you open the windows and start the car which starts the AC unit. The AC unit takes a period of time to actually start cooling the air flowing through it. Simply put, there is no reason to roll the windows closed until the AC is making cool air. When the car starts moving, the air in the car leaves, the moving car acting like the wind to blow air through, and if the car was parked, that air is quite a bit hotter than that outside, so it is to your advantage to cool things off some as the AC is coming up to working temperature. By starting to move with the windows open and then closing the windows as soon as the ac is putting out cool air, you have removed the most heat the quickest and spent the LEAST amount of time until you start to get cooler. Once the air cools to ambient from having the car moving with the windows open, it serves no purpose to keep the windows open any longer. Closing the windows then keeps the cooler air from the AC unit inside. In a thermodynamic sense, you have removed some of the heat load in the car so the AC unit has a head start and has less work to do to achieve a lower desired setting.

Answer 2

Well the law of thermodynamics states that hot air will move to cold air, so if you keep the windows down then it will not get nearly as cold as it should be if the windows were to be kept up.  You're wasting more energy to keep the car cool by turning up the AC while the windows are down.  If you keep them up then you won't have to turn up the AC as much = saves energy.

Hope that made sense.

Answer 3

  If the inside of the vehicle is hotter than the outside air, often true when parked, then opening the windows while starting the car and getting moving will reduce the heat load while the air conditioner gets to functioning and cooling down the hot metal and ductings.  But this only takes a few minutes.  

  When the AC is set on MAX it commonly recycles the air inside the car instead of bringing in hot outside air, but if the air inside is hotter than the outside, the purpose is defeated.  

Answer 4

I guess it's all personal experience. If you leave the AC on while the windows are down and it is still hot, common sense would say to put the windows up. I don't know the thermodynamics or anything, but years of experience have told me: When the AC is on, leave the windows up.