Can you cool your house better by opening a window on the third floor?

Question

...because, when you think about it, less dense hot air rises over cool air, therefore it would seem warm air (that makes more a/c power needed) should enter the third floor and leave out the window. Meanwhile cool air, being dense, would supposedly fall below it, leaving the second and first floor needing less a/c (thus hopefully lowering energy bills).
Would this help and, if so, by about how much (in apx. % energy reduction needed to cool a house)?

btw...it's clear I need some clarity about this...I don't have "centralized air conditioning" but instead a couple of portable "window" a/c-s..one on the first floor the other on the second.

Answer 1

Hot air will only exit the top window if there is something to take its place below. So, you need to open a window on the ground floor too to get thermal convection. If the air outside is hotter than inside, though, you gain nothing. A window air conditioner pumping cooled air into the ground floor would be needed in that case. With recirculating central air, it's best just to close all the windows.

Noting from you followup you only have window air conditioners, opening the third floor window is a good idea if your air condition has a setting where air from outside is cooled then pumped inside. This will replace the hot air that exits the third floor window (assuming the interior doors are open to allow the flow). The setting would be called "vent" or something The most effective way to cool the room with the aircon in it, though, is to recirculate the interior air continuously. One reason why you might not want to do this, though, is that it might get stuffy.

Answer 2

If you assume that the air in some parts of the house are hotter than the air in other parts, the hot air will tend to rise, and if there is an available path to the third floor, it will go up there and force downward any third floor air that is cooler.

But your theory is that the hot air will rise to the third floor and go out the window. If that happens, then air will have to come into the house somewhere. You could open a first floor window to let air in, but that would be helpful only if the outside air coming in the first floor is cooler than the air going out the third floor window.

But why would some air in the house be hotter than other air? If all of the air is the same temperature, it is not suddenly going to separate into hotter and colder air. (OK, it DOES tend to separate a LITTLE, and the hot molecules will tend to rise and the cold molecules fall, but this separation will not be significant if you start with well-mixed air at one temperature.

Now think of what the usual situation is in a house in summer. The attic is very hot because the sun is beating down on the roof, and the attic is in contact with the roof. Similarly, the 3rd floor tends to be hot because it is in contact with the attic. Usually, the attic will be hotter than the outside air, and it is a GREAT idea to blow the air out of the attic with a window fan or a roof fan of some sort, and allow the outside air to replace it. This may also be true for the third floor (you can put in a window fan to blow third floor air outside, and open another window so that outside air comes in to replace the air that is blown out).

The percentage effect on cooling requirements depend on all sorts of things: house size and shape, number of windows, size of the various floors, current outside temperature, humidity, thermostat setting, whether it's cloudy, etc.
If you cool the third floor by just pulling in outside air, and cool the other two floors with an air conditioner, it's certainly conceivable that you could save more than a third of your A/C bill. If you put a fan in the attic to reduce the amount of heat conducted into the third floor, the savings would probably be much more modest (maybe 5-10% of the bill ... as a wild guess).

Hope this makes sense to you.

Answer 3

no,you cannot. but if you were to put a whole house fan in your attic,this will amke it easier to cool your house. but the fan should be mounted to draw out the hot air,thus making it easier to cool