Close all the windows when the A/C is running, or just the low ones?
July 6, 2010 8:05 AM   Subscribe

I have opening clerestory windows at the top of my apartment's cathedral ceiling. Should I leave them open or closed when running the air conditioning, which vents near the floor?
posted by smackfu to Science & Nature (7 answers total) 1 user marked this as a favorite
AC is designed to operate as a closed loop. If you have the windows open, you are drawing in humid, warm outside air and forcing your AC unit to "condition" that air as well. If you close the windows, the AC unit recirculates the air in the house, removing moisture and cooling much more efficiently.
posted by cosmicbandito at 8:08 AM on July 6, 2010

That's a tough call without knowing about the air circulation dynamics of the room. It might make sense to have them open for the first few minutes that you run the A/C so any hot air is pushed up and out, but that depends on how well air moves. Most likely, the intake will simply suck in more air from the outside, making the A/C work double time. If you had some kind of separate vent fan that could exhaust hot air prior to the A/C coming on then, maybe. But it seems the conventional wisdom to keep the windows shut would prevail here.
posted by Burhanistan at 8:11 AM on July 6, 2010

Close them when running the A/C, but open them early in the morning without running the A/C, as those windows are designed to let the hot air get out of the place easily. You may find you don't need to run the A/C nearly as much, if you do this.
posted by davejay at 12:34 PM on July 6, 2010 [1 favorite]

If the air at the top of the room is hotter than the air outside, yes, a little bit. Unless it is really muggy out.
posted by gjc at 5:39 PM on July 6, 2010

Where's your air conditioner's intake vent?
posted by fantabulous timewaster at 4:51 AM on July 7, 2010

It's an in-wall model, so roughly 10 inches from the output vent.

I think my theory was that the hot air would rise and vent through the high windows, while the cold conditioned air would stick to the floor. I'm not sure it makes any practical difference though, since it might just make the ceiling air a few degrees cooler. Plus it would allow in humid air if there was any negative pressure at the open windows, which I really don't want.
posted by smackfu at 7:39 AM on July 7, 2010

Your theory is more or less right, but the hot air venting can't be the only thing that happens: all the air that vents out has to be replaced by air coming in from outside somewhere else. If you have multiple high windows you can let the breeze pass through them, but the main effect of that is to circulate outside, unconditioned air into your room. If you wanted to experiment with leaving the air conditioner off, you might open a high window on the upwind side and a low window on the downwind side, or vice-versa, and see if that tends to push the warmest air out of your apartment. But in general if the outside air is hot or humid enough that you want it cooled, then exchanging it for outside air is a losing proposition.
posted by fantabulous timewaster at 1:33 PM on July 8, 2010

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