Why does my car heater/AC work this way?

[ZenBeam]

I really wish cars wouldn’t do this shit. Most of the time, warm air is sufficient to remove fog from the inside of a windshield, and I’m smart enough to turn on the AC to help if I need to. I generally avoid using the defroster in my car, I just point the dashboard vents upwards as much as I can. It’s slower, but it does work.

My wife’s car has another annoyance: When you turn on the Recirculate, it also turns on the AC. So you come up behind some smoking diesel truck, with your heater running because it’s cold out, and next thing you know, the car is blowing cold air at you. If you remember, you can turn it off, but I generally don’t until the cold air is hitting me. Then I’m distracted from the road because I’m fucking with the dashboard.

[Rick]

@Machine_Elf

You use defrost/defog when the windshield is cold and the air in the car is humid, such that frost/fog is forming on the inside surface of windshield. To get rid of that frost/fog, you need to hit the windshield with DRY air. The air outside of your car is typically less humid than the air inside your car, because the air inside your car contains the same moisture as the outside air PLUS whatever moisture you’ve added by breathing. This is why defrost typically does not allow you to use the recirculation mode: if it did, the air hitting the windshield would not be as dry, and so your windshield would not dry up as quickly as you might like, which is a safety concern.

It sucks that you have to choose between rapidly warming the car and being able to see where you’re going; this is a classic first-world problem. To enable recirc to be used together with defrost would require a more powerful A/C system to get rid of the larger amounts of moisture effectively, which means your car would be more expensive. Your friends, neighbors and fellow countrymen have already voted with their wallets: nobody wants their cars to be more expensive, so this is what we get.

It is a physical limitation of the AC system.
In the summer the AC system will cool the evaporator coils to about 28F. Any colder and they would ice over from humidity. Running the AC system in recirc is no problem as AC removes humidity.
When it is 0F outside the system is already colder than the coldest it will get when running. The AC system won’t engage so there is nothing to remove humidity.
So if it is freezing cold and you were to run the heater in recirc the windows will fog.

[GusNSpot]

Does all this not assume that the windshield is free of ice on the outside?

Recirculated air in a car will warm faster without -5ºF air being added all the time.

Let’s see, the car sat out in the sleet & snow all night. Has 3" of snow on top of ¼" of ice. So you jump in and go quick with no warmup???

Every time you leave the car off and spend 10 minutes cleaning the windshield, then start & drive away???

Always & never are dangerous words.