Auto a/c on and window down

The s.o. and I were driving along in his car with the a/c on; he stopped at the ATM, rolled his window down, and when he drove off, he didn’t roll the window back up. It wasn’t extremely hot outside - in the 80’s. I said “air conditioning the out-of-doors?” He said it didn’t matter if the window’s down - wouldn’t hurt the car. I suggested it makes the a/c unit work harder and uses more gas. He says, unlike the house, it’s not controlled by a thermostat, it’s just on, so no harm to anything. That makes sense, but it still bugs me. He did it while driving my car and I made him roll the window up. What say you?

However, your S.O. is correct in most cases. Some cars nowadays do have thermostatically controlled ACs, but most adjust cabin temp by mixing untreated air with the air that has been cooled by the AC unit. If your car is one of the ones that does not use a thermostat then in reality your S.O. would not be stressing anything in the automobile except you.


How does that go?

Don’t sweat the petty stuff…
or is it don’t pet the sweaty stuff?

There are two energy efficiency reasons beyond direct energy consumption to run the A.C. that you can use to object to this scenario.

  1. Opening the windows hurts gas mileage. Once the window is opened, you change the way air flows past and into the car, and that reduces the efficiency of the vehicle. I have often heard the claim that it typically takes less gas to drive a car with the A.C. on and the windows closed than it takes to drive with the A.C. off and the windows open - the increased drag from opening the windows outweighs the increased power consumption of the A.C.

  2. You are the thermostat. If you run the A.C. long enough, the inside of the car will be so cool you will decide to turn it off. That point will come sooner if you keep the windows closed (and try to cool a confined space, rather than the whole world). This argument will only hold water if you are travelling some distance - you won’t reach the point of turning the A.C. off if you are just making a trip around town.

Good luck.

Auto ACs are most definitely controlled thermostatically. You know that control you have with the blue at one end fading into the red on the other? That’s your thermostat.

The question is, where do you set your thermostat? I grew up thinking that there were only two appropriate settings for the temp control on a car’s dashboard: all the way blue in the summer and all the way red in the winter. This “all or all” mentality kept me ice cold in the summer and burning up in the winter. Just the way I thought I wanted it.

I’ve learned though that I can be much more comfy if I actually set it someplace in between. I usually keep my car’s AC on during the hottest months (normally in my part of the US, that’s March through October :)) but I regulate the actual temperature by using the knob control. I’m lucky in my anal-retentive way that the knob on my car as little … I don’t know what to call them exactly, but “hitches” I guess is close enough … places you can feel the knob catch as you are turning it. I like that because I know that if it’s not too hot, I can turn the knob all the way to blue, then turn it back two hitches. If I still feel hot, I can turn it down a hitch. If I’m still too cold, I can turn it back up a hitch. :slight_smile:

Hope this helps.

Well, no matter how your AC is designed, your car interior would be cooler if you closed the window, at least in summer. Or, with the window closed, you can turn down the air conditioner and still achieve the optimal temperature, thus saving some energy. Isn’t that enough reason to close the window?

First, I don’t know why I slipped into cockney for a second there. I meant to say, of course, that the knob on my car has these things. Secondly, the word I think I was searching for was “stops.”


Actually, unless you have automatic climate control, it’s not a thermostat, it’s just a valve that controls how much air blows through the heater box. In other words, if you don’t have that control down all the way, you’re actually air-conditioning the air and then heating some of it again to get it back at a comfortable temperature. The A/C compressor is running all the time your A/C button is pressed.

With automatic climate control, the system will turn the compressor on and off as needed.

Cecil covered the question What gets the better gas mileage: windows up, AC on, or windows down, AC off?

I disagree, but not enough to argue about it, so I’ll just stand corrected.

well I’ll disagree.
The a/c compressor has a cut off switch that will cycle the copmpressor based on freon temp mainly to prevent freezeup. if the compressor is working in a cooled car it will cycle on and off quite a bit (esp. if you have it set to circulate the air instead of ventalate). By intruducing hot air you will prevent this from happening causing the compressor to run continously.

Also you can’t get something for nothing - the compressor will work harder and consume more power ‘cooling’ higher temp air - I don’t care if the compressor is running at the same rpm’s it is using more power.

Also it took some time to get that nice cool air in the car - and he just let it go so you will have to run the a/c longer and harder to get it back

Also your SO is being annoying as hell opening the window on a hot day - but that’s not really your OP

*Originally posted by k2dave *

Well, the new Freon doesn’t get cold enough to freeze up… there’s no need for a cut-off switch. The compressor will run and run and run. Old cars with R-12 WOULD freeze up; they had no cut offs either. They, too, would run and run and run.

Nope, the compressor works on engine RPM. It will NEVER take more or less power. It will be more or less efficient for the fixed amount of power it does consume, though. The compressor compresses a fixed quantity of gas from a certain volume to a certain volume. The compressor does NOT compress air – it compresses freon. The freon is maintained at a relatively constant set of pressures. So, extremely hot air takes exactly as much as compressor effort as tepid air; the key is effort, not work. Obviously more work is done in heating a hotter car. More work comes through increase time running, not increased effort running.

Not harder, just longer, and having it run doesn’t hurt your car; it only wastes a tiny-itty-bitty-amount of gas compared to what you paid for the A/C in the first place!


The word you are searching for is detent. Detents are the spaces of relaxed tension between pawls. (In fact, detent has the same root as detente, a period of relaxed tension in political or personal relations.)


I stand corrected twice in one thread. :slight_smile:

Thanks, bughunter! I love learning new words!

…shutting the car’s engine off with all a/c controls still on, and then restarting it later under the same conditions? Hurt anything to do this?


well maybe the new freon but the old freon systems had these switches because I changed them - and when I bypassed them it would freeze up

true the compressor works on engine rpm’s but it will take more power depending on the quantity/pressure of gas it is compressing. IF the evaperator is at higher tempature (due to warmer air running through it) the liquid freon will boil faster. This increased amount of gas will raise the pressure and force more gas through the expanision tube. more gas will be entering the compressor and therefore the compressor will have to work harder.

yep, longer and harder

looks like we will need a 3rd party to settle our dispute.

Oh just shut that bloody window, or I warn you I’m stopping the car!

Um, just worked it out. Mr Sycorax was driving wasn’t he?

Well, try: “If you really loved me you’d close that bloody window!”

I’ve never actually been able to get this to work, but you might.

It doesn’t bug me enough to start an argument over it – if he wants to do that in HIS car, no skin off my nose. I posted this because what he said made sense, but I thought I had a case too. Thanks everyone for the input.

Also using a room a/c that exhausted into a closet (not my idea) that after a while the circuit breaker would trip as it was very hot inside the closet. Now this is the other side from what we are talking (‘hot’ side instead of the ‘cool’ side) but does show that a compressor at a constant rpm can consume more/less power depending on tempature.

You are free to duplicate my experiment.

Or it could be that the electrical circuits of the air conditioner heated up, causing more resistance, necesitating the use of more power to overcome said resistance, causing the breaker capacity to be exceeded.