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At night temperature outside is usually lower than temperature set on AC. So looks like conditioned air should not leak outside (if it does, it is replaced with even colder air) and it is fine to leave window opened. Is this correct?

Note: when window is opened but AC is off, temperature quickly rises because of hot walls and inside sources of heat.

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    This would generally depend on house and weather conditions. How many windows, temp outside, temp of surround walls (attic/basement), and the wind conditions.
    – DMoore
    Commented Jun 17, 2013 at 20:23

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If you're confidant that the air leaving the house through the open window is warmer than the outside air that will come back in to replace it, I suppose it's OK to leave the window open. But I think a better solution would be to shut the AC off and use a window fan to circulate the cool outside air through the house. A window fan will be quieter and more energy efficient than an AC, and probably faster at cooling as well.

Which direction the fan faces doesn't matter a ton as long as you're moving the air around, but if you think about the goal of getting the hot air out of the house and the cool air in, here are some ways to think about it:

  • If the wind is generating any air movement through a window, use the fan in the same direction. No point in fighting nature.
  • If you have a single floor, put one fan blowing in and one fan blowing out, at opposite sides of the house.
  • If you have several floors, put a fan on the lower floor blowing in, and a fan on the upper floor blowing out.
  • If you still can't decide, point the fan facing out so that the waste heat generated by the fan isn't pushed into the room.
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Most AC units have a simple fan unit that lets you blow outside air into the room. If it is cooler outside, set the fan only (the compressor won't run and you will reduce energy use). If you open an upper window, the fan should help repalce the warmn air with cooler outside air.

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    I don't like to use the fan-only function on my AC window units for several reasons: 1) they're loud and not very effective at moving air. 2) They actually use a lot of electrify. I measured mine at 90-130 watts in fan-only mode. Compared to my Vornado fan which is 30-60 watts and VASTLY more powerful. 3) I'm not sure they even circulate outside air. I think mine just uses the same air intake on the front of the unit.
    – Hank
    Commented Jun 17, 2013 at 20:36
  • AC is on a wall near window - "fan only" mode is ineffective, because it does not pump cold air from outside
    – Roman
    Commented Jun 17, 2013 at 20:38
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    There is an intake vent (a flap or louvers controlled by a level or wheel) that is either open or closed. When open, outside air is pulled in, bypassing the compressor radiator. When it is closed, air is pulled through the compressor coils. The fan is the same fan used to move cooled air. If you open the vent, you should get outside air with less wattage than with the AC on. However, a dedicated fan may be much more efficient and maybe quieter.
    – bib
    Commented Jun 17, 2013 at 20:42

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