How to cool a house with window fans

So, in this thread I was asking about whole house fans, but after reading a lot of the comments I started looking at info online about cooling the house with window fans. Sites like this one: say to have an equal number of fans blowing into the house as you have blowing out.

I was wondering exactly why that is the case. With a whole house fan, you simply rely on the open windows to bring the cool air in. Is it simply because window fans are not likely to be a powerful as a whole house fan? But if I prop this fan, with its 2400 CFM (about the same size as the whole house fan I was looking at) in an upstairs window, wouldn’t it be almost as good as the whole house fan?

I currently have two small twin 6 inch window fans that move little air, and I have also started to prop a box fan in an upstairs window blowing out as well. It helps, but it doesn’t cool the rooms as much as I’d like (I have several digital thermometers that display indoor and outdoor temp throughout the house).

And do you think it would make a great difference if I were to put the fans in the top of the double hung windows (which would involve either ordering a bug screen that covers the full height of the windows or working out a way to get the existing screens to seal the top pane when opened)?


I’ve found that blowing air out with a single fan cools the entire house (assuming it’s cooler outside) because the cool air comes in all the open windows. It is, however, slow.

Blowing air into a room cools it faster, but only cools that one room.

A whole house fan is designed to create negative pressure inside the home, which allows any open window to be a inlet for air.

With fans in windows there is no seal - and not much power, so while the fan may be blowing one way, the rest of that very same window is allowing air to get back the other way. You are creating wind more then a pressure difference. By placing fans so that air is moved both in and out of the house you create the same effect.

When I used to do this, I’d put a fan in my bedroom window blowing in and a fan on the opposite end of the house blowing out.

Worked pretty good!

This is a good explanation. There is one way to overcome the short cycling through the window. Most of the time the fan is smaller than the window opening. Blank off the open space between the fan and window casing. Also it is more important to get the hot air out than it is to get the cool in. On a 2 story house exhaust on the 2nd floor, and if you can through the upper sash window. Or open the upper window and put the fan blowing in on the 1st floor.
I use to put fans in the windows 1st floor blowing in and 2nd floor blowing out. I now have a whole house fan. So I open the windows in rooms that I want to cool and start the fan on high, when I go to bed I put the fan on slow speed, less noise.

I have a digital indoor/outdoor thermometer. When it is cooler outside, I use the fan in front of the window to suck in the outside air and exhaust the inside air.

It is a powerful fan, but very quiet.

If it is cooler inside, then no fan.

This thread brings back memories. My grandparents had a window fan in their front room; I’m talking a chop your arm off industrial type fan, which always blew out. My grandfather would demonstrate the air flow by blowing the smoke from his pipe in various rooms of the house.

For example, he would stand upstairs, blow his pipe smoke and we could follow it down the stairs, through the living room and out the industrial fan. Of course, this was back before he would have been arrested for smoking in the house in front of small children.

The fan was turned off from mid morning to early afternoon, when as others have said, it was cooler in the house than outside and therefore beneficial not to draw the hotter air in. This system kept you from sweating your ass off mostly, but it pales in comparison to air conditioning.

When we lived in Japan, we took various measures to reduce the amount of energy required. The house didn’t have an attic so the third floor would always get hotter than than outside. We would put a fan blowing out there and let the air in on the first floor.

It’s also important to work with the prevailing wind rather than against it. So if you open the window where you want to put your fan, and the wind is trying to blow into the room, then that’s the direction the fan should be pointing as well. If there’s no wind, which is fairly common at night, then as mentioned before you want to blow air out of upper stories and in on lower stories. Also, say you’re in something like a shotgun house with three rooms in a line from front to back. If you put a fan in the back room, don’t open windows in the middle room, just the front room. That way the air will be pulled (or pushed) through the whole house rather than just the rooms immediately adjacent to the fan.

Thanks for the replies. I see there are several people here saying they use whole house fans, an idea I am still kicking around, but it of course costs more money. If I were to mount the 2400 CFM box fan in an upstairs window (blowing out) and seal it so air can’t sneak around it, that would work pretty good for $25, correct?

Another problem I was having looking at whole house fans was that the ones in the CFM range I need are the units with the attached ducts that cost $600 and up. The $200 units mount directly to the attic’s floor joists, but the CFM is so high that my attic has no where near enough ventilation. What if I open my drop down attic steps, and mount the box fan there in a sheet of plywood and seal around it, as a proof of concept. That should work good, right? But then there is no way to adjust the speed. So not so good.

Never easy, is it.

I will sometimes use a box fan stuck in the window (with giant gaps on either side of it). I have it blowing out and then crack a few windows in other rooms. It appears to be working, the vertical blinds will move around quite a bit as long as the fan in the other room is on.

FWIW, I’ve found it works better to have the fan exhausting (rather than intaking) mainly because when it’s drawing air in, I’ll get bugs coming in with it. Plus, the screen gets dirtier.