Heatwave Filter

July 24, 2006 8:28 PM

Should I blow the hot air out or the suck the cool air in?

I don't have air conditioning in my tiny ground floor apartment. I do have a front door and a back door, windows in every room (all but one of which are on the same shady exterior wall) and fans. So far my efforts to utilise these to replace the hot air inside with the cool air outside come evening time have pretty much failed miserably.

Any advice? Rules of thumb from mefites in hotter climes?

posted by fshgrl to home & garden (18 comments total)

You should try & have a fan blowing in on one window, and a fan blowing out on another window, preferably on the opposite side of the apartment...though it sounds like that might not be possible. If it's not, do your front & back doors have screen doors where you can arrange the fans? I've found (after living in the top floor of a sweltering building) that getting a cross breeze going is the best way to cool down a room.

We used to live in an apartment with all the windows on one wall. My fiance and I argued to no end about whether to have the fans pointing in or out. He played the engineering/science-guy card and made the fans blow outward, to remove the hot air from the apartment. But in my humble opinion, scientific theory means jack when the apartment is still sweltering. So have the fans pointing in if you have to pick one way, and try & aim it directly at you so you can reap the most benefit from it.
posted by tastybrains at 8:37 PM on July 24, 2006

I'm no expert, but couldn't you have the best of both worlds by sucking cool air in the front door, and blowing hot air out the back door? (Or vice-versa. Use the shady side for intake, of course.) Basically, just cross-ventilation.

I've never tried it, but I saw the idea of hanging a wet sheet in front of a fan blowing in recommended as a way to cool things down a lot.
posted by fogster at 8:37 PM on July 24, 2006

You should suck air in (place the fan facing inside) from the coolest part of the outside. At my house it's the front yard because it is really shady and the sun sets in the back yard making that hot longer into the evening. Then blow the hot air out of your house towards the hot side of your yard.

Read about the stack effect if you have the time or inclination.

Open all your windows (if you can) all night when it is cooler. Then shut them first thing in the morning at the first sign that the heat is rising (here that's 5 o'clock).

If you can, put something in front of your window, outside, like aluminum foil, cardboard. The trick is to prevent the sun from hitting the glass to prevent it from heating the glass up.

I used to put black blankets as curtains in a few rooms to keep them cool but that isn't working this summer.
posted by cda at 8:39 PM on July 24, 2006

suck air in from the collest shadiest moistest area outside. Blow it out the hottest side. Of course it helps if your house is oriented north/south.
posted by singingfish at 8:40 PM on July 24, 2006

Try to create an airflow through your place. The Bernoulli effect is your (and prairie dogs' -- that's so cool that they make use of it) friend.

Actually, the little double fans that fit in a window and blow in or out depending on a switch setting I really like -- much more than air conditioning. If you set up one to draw in at lower altitude, and another across the apartment blowing out at higher altitude, you're golden. Much nicer than AC.
posted by orthogonality at 8:41 PM on July 24, 2006

tastybrains, that's funny. when we only have one fan or one option, my boys always say blow the hot air out. But I have always thought if you have one fan blow the cool air in (I'm a girl).
funny
posted by cda at 8:42 PM on July 24, 2006

Use the Bernoulli effect: Set a fan to suck cold air in from outside, and close everything else except one window at the other end: crack that one about an inch.

Wait 15 minutes.

I do this, on a small scale, in my bedroom, perching that door open on a plastic coat hanger hung from the doorknob.

That room ends up, by bedtime, 5 to 10 degrees cooler than the rest of the house.
posted by baylink at 8:49 PM on July 24, 2006 [3 favorites]

We used to live in a 100-year-old house with no insulation in the ceiling (in total, it was perhaps few inches thick, with dark shingles. If you closed it up, in the summer it would easily get 10 degrees above outside air temp, which put it around 110 or 115 at the hottest. The fans kept it around ambient, which was still very hot, but not as bad). In the summer, the only way we could keep the second floor out of heat-stroke territory was to have two fans in the windows, one sucking in on the east side, one sucking out on the west side, running 24/7. That worked well.

It helped that the two windows were a straight shot across from each other. I don't have that in the new house, which makes this technique much less effective in the evening (which is the only time we run the fans in the new house).

If you have a fireplace, open the flue.
posted by teece at 8:53 PM on July 24, 2006

Do you have double-hung windows? That is, the kind where there are two separate panes, top and bottom, and you can move the top one down and the bottom one up so that they overlap in the middle? If so, see this useful thread.

Another possible useful thread for general household heat beating.

The way your apt is laid out, can you set up a clear airflow path from any "in" opening to an "out" opening? (If so, encourage this flow. Eg, point a fan out the open back door, and open the front door which is in a direct line to the back door.)

Do you have ceiling fans, or a way to get the hot air down from the ceiling?
posted by LobsterMitten at 8:56 PM on July 24, 2006

D'oh. Must remember to use preview.
posted by LobsterMitten at 8:57 PM on July 24, 2006

Thanks for all the advice. I have problems generating a cross breeze because of the way the place is set up. My "front and back" doors are more like two side doors and all the windows are on the same side too except one tiny one. Also all the screens are kind of busted and the bugs are wicked here.

I am off to experiment with different fan configurations. The in-window fans sound like a brilliant idea for a place like this with no (normal) need for aircon, maybe I can sort of rig something similar............
posted by fshgrl at 8:59 PM on July 24, 2006

fshgrl writes "The in-window fans sound like a brilliant idea for a place like this with no (normal) need for aircon, maybe I can sort of rig something similar............"

No need to rig. I got mine for $20 (years ago) at CVS. They're rectangular, with pleated accordion extensions that can be pulled out to make the width fit in window, and the top and bottom are flat to fit the window frame bottom and the window bottom, and the top additionally has a flange that holds it to the window.

I used them even though I had central air, and now that I'm in another (also central air) apartment, I miss them. (Although now I have vertical blinds in the window, which swing against each other, making a sound like sailboats' rigging at a marima. I do love that un-patterned sound; it's like wind chimes, but without being jangly annoying.)
posted by orthogonality at 9:30 PM on July 24, 2006

Mount the fan in the most out-of-the-way window and have it blow out. This results in a nice gentle cool breeze coming in through all of the other windows. I think burying the fan in an out-of-the-way window trumps the hot-side / cool-side reasoning above.

I can think of at least three reasons why blowing in is wrong, but it's late and I'm going to get some sleep instead of typing it up.

Big ass thermostat driven window fans are awesome.
posted by intermod at 9:38 PM on July 24, 2006

All of my comments are anecdotal of course but I have to confess to being quite obsessive throughout my life regarding the cooling of living spaces.

First of all you need two box fans. These should be placed in the windows and any open space above or beside them should be covered (cardboard cutouts work). Keep an interior and exterior thermometer. Start the fans in the morning with one fan blowing out (the one on the sunnier side) and the other blowing in. When the exterior temperaure exceeds the interior temperature, have both fans blowing out. Maintain this until the exterior temperature exceeds the interior, then revert to the morning scheme.


What is essential in this scheme is that you cover open spaces around the fans, otherwise too much air escapes from these regions which render the fans far less efficient. Box fans are generally more powerful in cubic feet per minute their square shape makes them easier to seal around.

Keep blinds and curtains closed against any sunlight, or cover glass with foil.

If you can mount two fans in the windows, that's even better. I've stacked them in a sliding glass dooor to good effect.
posted by Neiltupper at 10:05 PM on July 24, 2006

Heh. I've never done it, but this is what I'd try if I were stuck in a hot apartment and feeling MacGyversh. Use the chimney/stack effect. No fans. No air conditioning. No energy use.

On the sunny side of your place, choose one window that you will turn into your stack. Get or make a tube of clear plastic for your stack. It should be as wide as the window and it has to stretch up above your window as far as you can put it. It could be a series of lightweight plastic hoops in a very thin plastic sheath. You want to turn the tube into a transparent chimney stack that starts at your window and rises straight up the outside of your building, probably stuck to the side of the building or suspended from a line hanging from above.

The sun should heat the air in the chimney and cause it to rise up and out of the stack. That would suck more air out of your apartment in a constant flow. To replace the lost air in the apartment, nice cool air will be sucked in through windows on the shady side.

A completely silent, ecological, inexpensive ventilation system. If it works, make and sell them to others. Build it from hoops covered with flexible plastic, so that it collapses for shipping and storage. Put them on all your sunny windows. Flip them open for a direct breeze when the sun moves on.

If it doesn't work, maybe building it kept your mind off the heat.
posted by pracowity at 1:43 AM on July 25, 2006 [1 favorite]

The hottest air in your apartment will rise to your ceiling, and unless you've got ceiling fans to mix it all up, will form an ever-thickening layer of hot air. When the boundary between that layer and the cooler air underneath drops down to about your head height, you'll feel very hot very quickly. If your window openings are below that boundary level, you'll find you can move quite a tremendous amount of air through your living space without making much of a dent in the trapped hot air pocket.

The fastest way to get rid of it is to build a corrugated cardboard duct the same width as your least picturesque window and about a foot deep, running up the wall from the inside of that window to within six inches of your ceiling. Paint something decorative on it or cover it with posters so it doesn't look so completely shite.

If you've got a shady wall, the air at ground level outside will be the coolest outside air available to you (especially if you've got a damp garden bed outside that wall). You can get at that air by mounting a window fan in one of the shady-side windows, and running another box duct from the outside of that window down the wall to ground level. Corflute and duct tape are good construction materials for the outside duct.

If you assume your apartment is essentially a sealed box, it wouldn't matter whether you mount your fan in your intake window blowing in, or in your outlet window blowing out. But since it's pretty rare to find a perfectly sealed building, and since the air at ground level on the shaded side really will be cooler than random ambient air, you're probably better off pumping the cool air in than the hot air out.

If you've got serious heatwave conditions and even the ground-level shady-side air is too hot in the daytime, you should do the air exchange at night; then, in the daytime, seal everything up and turn your living space into a cool dark cave by installing blockout blinds in all the windows - when buying these, prioritize windows that get direct sunlight. Any sunlight entering your apartment will heat it up, but direct sunlight is about five times as potent as indirect.

Blockout blinds are generally opaque and white, so that incoming radiant heat just bounces right back out again. Opaque external shutters are even better, but they're expensive and inconvenient, and you'll be amazed how effective well-fitted interior blockouts can be.

Paint the outside of any door that gets direct sun white. A dark colored door in direct sunlight will heat a small interior up quite quickly.
posted by flabdablet at 5:50 AM on July 25, 2006

Years ago, I think in Consumer Reports, the recommendation was to blow the hot air out and let the the cool air come in another window.

They also said that the way to get the most air out of the window was to put the fan a few feet back, and not in the window frame. The flow of air spreads, and the turbulence sucks up air from the room and carries it out the window (or something like that).

The only windows in my bedroom are side-by-side. To get cross-ventilation, I open the (apartment) door into the hall, which has open windows. This pulls cooler air through the apartment and gets rid of the heat quickly.

The ideal solution, if you have access, is an attic exhaust fan, which pulls the hot air from the top of every room.

In Brooklyn, however, I can't live without a room A/C. I use a fan only to get rid of the hottest air before turning on the A/C.
posted by KRS at 10:44 AM on July 25, 2006

One really cheap way to keep your place cool during the day so that you don't have such an oven to deal with, is to go to your local art supply store and get a big piece of white cardboard. Then cut it into a shape that will cover your window. Place it in between panes if you can, if you can't place it on the inside. It's a poor person's blind! Reduced my inside temperature by 10+ degrees...
posted by storybored at 6:31 PM on August 4, 2006