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The other night, a few of my friends and I got into a cool (although some would argue it was heated) argument about the efficiency of cooling a house with air conditioning. I wanted to leave the windows open for 20 minutes or so, but they told me I was wasting energy.

My argument: As cold air enters the room, it will fall and create a small draft that will blow the hotter air outside.

It is hotter inside the house than outside.

Does science back my argument? Does leaving the windows open temporarily when air conditioning a house or a car cause for faster cooling?

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    $\begingroup$ Of course one should cool the house down to the temperature of the outside air, first, just not with the AC running at the same time. $\endgroup$ – CuriousOne Sep 8 '15 at 20:40
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    $\begingroup$ @CuriousOne the house I currently live in does not cool down to the outside temperature until about 2 or 3 am. It is constantly warmer than outside. $\endgroup$ – Jacob Raccuia Sep 9 '15 at 0:23
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    $\begingroup$ There are a couple of physically meaningful things one can do with relatively little effort (I am using them myself): stop radiative heating trough windows to minimize heating and force cool air trough the house with a whole house fan. If there is a poorly insulated attic, the temperature in there is usually far higher than the outside air temperature, which makes an attic fan a good idea. While this is not physics proper, I would say it's a good way to learn about heat transfer to actually do a few back of the envelope calculations about air flow etc. needed to cool a house. $\endgroup$ – CuriousOne Sep 9 '15 at 3:19
  • $\begingroup$ While the ac is switched on, by the second law of thermodynamics, it does work on the system or absorbs heat from sink i.e. the room of the house and releases the heat into the surroundings. So it is necessary to keep the windows closed all the time, else the efficiency of the process will automatically decrease and the released heated air will creep into the room making it less cool.The system and the surroundings must be kept separate. $\endgroup$ – SchrodingersCat Sep 9 '15 at 17:34
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It depnds on the inside and outside temperature.

If the outside temperature is higher than the inside one, keeping windows open will be waste of energy, trying to cool the ouside too.

If the outside temperature is lower than the inside, a fan is enough to help cold air come in. The airconditioning itself will only work as a fan so it makes no difference in the energy spent, except the fan energy which accelerates the natural convection.

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Turns out, you're a giga-savant whos brain works better than a trained physicist, who could tell you exactly how air conditioning is supposed to work but has never experimented with one empirically. See, air, much like water, flows in the direction of least resistance. Guess what happens to air when you pump it into a sealed room? Hot air, cold air: it doesn't matter... If you said something along the line of "it begins to push against itself until it finds an easier place to go," then you've got some mind! Just try it for yourself. shade your room with blinds, crack the window, maybe crack the doors open in other rooms. Make sure the other rooms have closed windows (remember, we're directing air at this point), and let the AC blast... Booya! Your friends were just getting on your case because their parents got on theirs, not because closing your window cools a room faster. Should you have all the windows open while running the AC? Heck no. Think of the ratio! how much air does your AC blast anyway? So, then, guess what you do when your room is finally nice and cold, and likely one of, if not the coolest room in the house? Yep. Close the window. It should take about 20 minutes!

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    $\begingroup$ Um... the first bit of that seemed like it was needlessly dismissive of physicists. It also isn't clear to me why you're talking about "pumping air into a sealed room". AC systems don't necessarily pump air at all; it depends completely on how it's set up. I can't really think of any AC units that work that way. Typically they just cool air that's already in the building. Or they expel some air and mix in some return air with fresh air. It seems like physicists ignored the things you discussed because they typically aren't relevant to the AC systems I've seen. $\endgroup$ – JMac Jul 16 at 18:20
  • $\begingroup$ Factor in ALL physics at play. you'll see I am not the one being dismissive. Various AC units may operate using different mechanics, but most work in conjunction with a fan driven air circulation system. Would you like more cold air to blow into your room and not the others? Give the air inside a place to go. One only need try it. Open the window in the room slightly, and make sure you are blocking the sun out. Don't throw the experiment by adding sunlight and opening all the windows in the house. You'll see we're right. P.S, it works in the car too but only takes about 1 or 2 minutes. $\endgroup$ – austin houlding Jul 16 at 22:03
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    $\begingroup$ Fan driven circulation doesn't pump extra air into the room. The air inside already has a place to go, because it started inside, and stayed inside as it circulates through the AC system. It's not even clear what you're saying you're "right" about. You need to be a lot clearer about what you're proposing and why you expect it to be good. Opening a window may or may not help, depending on the AC system, outdoor conditions, house layout, etc. None of what you said is universal, but you seem to be treating it as if it can apply anywhere. $\endgroup$ – JMac Jul 16 at 22:13
  • $\begingroup$ just try it! Physical science must abide by the scientific method like any science. Otherwise it's just theory. $\endgroup$ – austin houlding Jul 17 at 2:53
  • $\begingroup$ I'm aware how these things work. You just haven't even given a good explanation for why you think trying is worth it. There are definitely some circumstances where opening a window can help, for various reasons; but the reason you mention in the answer typically doesn't apply. Most AC systems are designed for a closed environment, and don't actually try to force air into a pressurized environment; they just move it around while cooling some. $\endgroup$ – JMac Jul 17 at 11:07

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