In or out? This sucks, that blows

July 15, 2008 11:11 AM

Regular Floor Fan Filter: My wife swears that it's best to use a fan to "suck the hot air out of the room" first before turning the fan around to draw in the cooler evening air. I say the only hot air in the room is...well, help us figure this out!

We have a very hot second story loft that never cools off from the air conditioning that cools the first floor so nicely. We keep the shades and windows closed during the day, and at night (as the outside world starts to cool off) I've been putting a fan in front of the screen door to our roof patio, hoping to draw in the cooler evening air into that hot room.

On more than one occasion, I've gone up there to see the fan facing TOWARDS the door blowing air (supposedly) out of the screen door. My wife's logic is that she's blowing the hot air out of the room, then will reverse the fan later, and draw the cooler night air into the room. This seems silly to me, and I promised I'd poll the hive mind to see if one or the other of us is "more right."

My logic is that if the floor fan faces OUT (blowing the air inside the room OUTSIDE), then you are taking the coolest air in the room (the air from the floor - which, let's be honest, isn't very cool, but at least cooler than the air eight feet above it) and blowing it out the door.

The fan is not huge, but it should be able to pull (or push) a little air around. Any thoughts? Who has it right?

posted by Spyder's Game to home & garden (27 comments total)

generally, when i set up a fan to blow air out of a room, air comes in through any other open window or space. so it really becomes a question of whether i want to remove hot air faster or bring in cool air faster. in the situation you described. i'd have the fan blowing out.

but overall i see it as a matter of preference.
posted by lester at 11:22 AM on July 15, 2008

I can't directly answer your question, but my gut feeling is that a fan isn't going to have that much of an effect on the air pressure in the room.

I solved a heating problem in this way: I created a "tunnel" with 2 rooms and 2 open windows. It went sort of like

|window|--|room|--|room|--|window|

and I used the motion of the wind to drag cooler air in. It seemed to work OK. I've tried using fans, but I've never known it make much difference. I do live in quite a windy area though (on top of a hill), so perhaps the fan was fighting the natural flow of the air?
posted by Solomon at 11:26 AM on July 15, 2008

30-minute experiment (or longer, if needed).

Note the room temperature at mid-day (when the sun isn't changing angles fast).

Turn the fan outwards, wait 15 min, and record the temp.

Turn the fan inwards, wait 15 min, and record the temp.

That should end arguments, even if it isn't 100% thorough.

I've been wondering this myself, frankly.
posted by IAmBroom at 11:26 AM on July 15, 2008 [3 favorites has favorites]

You're both wrong. She's a little more wrong than you, though.

Where is the air conditioning intakes and output vents? That's the wild card here. Hot air rises. If your intake is downstairs, and all your vents are downstairs, you're just cooling already cooled air.

If this is the case, you need to vent hot air out of the upstairs. But blowing the fan out the window won't help.

Put the fan downstairs and point it up, or point it toward the stairs. Open the upstairs windows. Attempt to circulate cold air from the downstairs to the upstairs.

If you want to really solve the problem, get an attic vent.
posted by Cool Papa Bell at 11:27 AM on July 15, 2008

I debate a friend on this frequently- he likes to blow the hot air out, I like to suck the cool air in.

The bottom line, though, is that you ultimately need both. Whether you suck air in or blow it out, you need a way for the inside air to get out- like opening a window across the room.

The difference lies in what space you're trying to cool. If you're trying to cool your whole house, then you open a bunch of windows and stick as many fans as you can in some of them blowing out. That way the other open windows will draw in outside air while the fan sucks out the heat.

If you're trying to cool one room, you're better off sucking in outside air and letting the hot room air escape on its own out another open window.

The first will evenly cool your whole house. The second will quickly cool a single room.
posted by Thrillhouse at 11:28 AM on July 15, 2008

We have no air conditioning, a skylight above a lofted space, and regular windows on the 1st floor. After working through the combinatorial possibilities, nothing cools off our place faster than 1st floor windows open, fan pointed straight up towards the open skylight, which both pulls cool air in and blows hot air up and out.

I think you're both half right. It's not really about conserving cool air or adding cool air - it's more about convection cooling and getting all the air circulating. Ideally, you'd have a source of cool outside air blowing in on the first floor, and an opening up top to vent heat.
posted by NoRelationToLea at 11:29 AM on July 15, 2008

I think your wife is right.
posted by BobbyDigital at 11:35 AM on July 15, 2008

In my experience, blowing air OUT will cool the room faster. sorry.
posted by The Light Fantastic at 11:39 AM on July 15, 2008

Forget the fans. Just open as many windows as possible both downstairs and in the loft.

Hot air will rise out of the house and draw in cooler air through the downstairs, but it will still take some time for the loft to cool down because there is so much heat stored in the walls, roof, etc.

When I do this in my house, there is a very obvious draft circulating upwards, especially if I try to close the door to the loft.
posted by randomstriker at 11:51 AM on July 15, 2008

I think blowing hot air out would work. Just like to help stop your car overheating don't you turn on the hot air and have it blow out, sucking it away from the engine? Or is that an old wives tale too?
posted by evilelvis at 11:52 AM on July 15, 2008

st like to help stop your car overheating don't you turn on the hot air and have it blow out, sucking it away from the engine? Or is that an old wives tale too?
posted by evilelvis at 2:52 PM on July 15 [+] [!]

Not an old wives' tale - I've used it to nurse an overheating car along. You can see the engine temp drop when you blast the heat, in those cases.
posted by IAmBroom at 11:55 AM on July 15, 2008

Oops - caveat: it only works if your heater is based on engine heat. Electric air heaters won't affect the engine temp, obviously.
posted by IAmBroom at 11:56 AM on July 15, 2008

I have a neato double window fan that can blow/suck in either direction. I usually have one side set to blow cool air in, and one to blow hot air out.

Anywho, I'm with trillhouse. It depends on where the air is circulating to/from besides the window. To really settle this one, you'll probably need to keep a temperature log.

Alternate days using her method/your method. If you alternate less frequently than every day, you're more likely to have outside temps that differ more for each method, making the results less valid. Record the temperature outside and in the room at set intervals. Also record the pressure, for good measure. If there's during or within a couple hours before you record temps for one day, better just discard the results, as the pressure inside and outside your house is more likely to be very different, causing air flow to be abnormal. Make sure that if you're opening and closing the window during the day, that it's done the same time each day, so pressure differential inside/outside the house is comparable between days. Make sure to note things like if the sun was shining into the window during recording or not, if people were frequently in the room or not, or other things that could affect the temperature. Make sure the door to the room is in the same position (open/closed) for all recordings, as well as windows. Try to keep other doors and windows in the house in the same positions also. If possible, also, keep all window shades, except for the ones where the window is open, closed, so that the sun doesn't heat the house up too much. And above all, take the results with a grain of salt. Loser buys ice cream.
posted by gauchodaspampas at 12:06 PM on July 15, 2008

I'm voting for your wife.
posted by greta simone at 12:13 PM on July 15, 2008

Sucking cool air in and blowing hot air out are exactly the same thing. Either way you are exchanging inside air for outside air.
posted by Stonestock Relentless at 12:17 PM on July 15, 2008

Neither. The best cooling effect of the fan is to create air movement across your bodies where it can act to evaporate sweat and/ or cause you to lose body heat through convection.
posted by Gungho at 12:22 PM on July 15, 2008 [1 favorite has favorites]

Sucking cool air in and blowing hot air out are exactly the same thing.

The bottom line, though, is that you ultimately need both.

So, when you're blowing the hot air out you need a place for cooler air to come in (outside, air-conditioned downstairs) or, conversely, when you're blowing cooler air in you need somewhere for the hot air to go (preferably outside, not the air-conditioned downstairs).

My preferred set-up has two fans at opposite ends of the house - one pulling air in, one pulling air out. The fan coming in is set up nearest the most occupied part of the house as that's where the cool air comes first and it "create[s] [greater] air movement across [our] bodies where it can act to evaporate sweat and/ or cause [us] to lose body heat through convection."
posted by GPF at 12:29 PM on July 15, 2008

Whichever way the fan is pointing, you're going to have an equal amount of air moving in the opposite direction. I.e., if it's pointing out, all the hot air getting pushed out will get replaced immediately by outside air coming in--from somewhere. You are not going to build up any significant pressure differential between the inside and the outside--as I'm sure you've noticed--so this much has to be so.

So the question is really, in which direction is the fan able to move the most air from one side to the other? I tend to believe that it is able to push more air out through a screen than it can pull in, given identical placement otherwise. Suppose the fan is 2 inches from the screen. When it's facing inwards, a lot of the air it sucks seems to come from inside the room, from the space between the screen and the fan. But when it's facing outward, all of the air comes from the inside, and most of it goes out. There is some bounceback from the screen, but my seat-of-the-pants measurements tell me that the net movement is greater with the fan pointing out. Maybe the intake area is more diffuse that the output, so the concentrated output has the advantage?
posted by bricoleur at 12:30 PM on July 15, 2008

6 of one, half dozen of the other.

That said, I'd lean more towards bringing the air into the room from the outside. When I've pointed the fan outwards, there's not a pronounced air current. Sucking air inside from the out of doors, though, does increase the breeze inside, which is another way to cool down bodies.
posted by Stewriffic at 12:47 PM on July 15, 2008

Perhaps you could apply the logic from this Lifehacker post to your situation?
posted by jroybal at 12:49 PM on July 15, 2008

In terms of air exchange, if it's a window fan, either one works. You're moving the same volume of air.

However, the point isn't to bring down the house temperature in an absolute manner. You want to increase the comfort level of the inhabitants. This will be accomplished best by drawing in the cooler air and blowing it into the room/directly over the people.
posted by explosion at 12:52 PM on July 15, 2008

you may be more correct, but youre wife is always going to be "right", especially in a case where there is no clear correctness. let her win.
posted by fumbducker at 1:10 PM on July 15, 2008

If the outside air is cooler than the inside air, have the fan draw air into the room. Reverse if the opposite is true. If you are in the room, the greatest cooling benefit is if the fan is blowing air directly you.
posted by battlecj at 1:35 PM on July 15, 2008

Explosion has it. Convection is much more efficient for heat transfer than radiation. So which is it, Spyder's Game? Do you want to know what cools the temp of the room air better, or do you want to know what keeps the occupants of the room cooler? Your wife may be right about the room air temp, but even if she is correct, it's probably a small effect compared to what air moving across your skin does.
posted by Slarty Bartfast at 1:48 PM on July 15, 2008

Oh, and your wife is always right. You must be newlyweds.
posted by Slarty Bartfast at 1:49 PM on July 15, 2008 [1 favorite has favorites]

Thanks for all the input. Keep 'em coming! The goal here is to cool down the room temp, thereby increasing the comfort of the people who are working and hanging out in the loft. We are rarely in the path of the fan when we are upstairs.

My bias probably comes from the fact that I can feel the flow of air in the room when the fan is pointed inward, so I think it's doing a better job of moving the air.

One other issue to consider: there aren't any windows up there aside from the screen door leading to the roof. The only other outlet is a (what I've heard called in the past) "swamp cooler." It's basically a hole to the outside with a fan inside. I think it can blow inward or outward. It sounds like a fleet of jet planes taking off, so we rarely use it. As it's set up high in the wall, I think it would be good to draw in cool air on the floor (with the floor fan) and "suck out" the hot air (near the ceiling) with some short bursts of the swamp fan, as long as we can stand the noise.

Still doesn't sound like a definitive answer one way or the other, though, on the effectiveness of a floor fan drawing air in, or pushing hot air out. Might be a tie. ;P

We've been married for just over six years. She's a smart cookie. So am I. We usually are both great about admitting when we are wrong. And I'll be sending her a link to this page when I'm done, so she can see for herself! Thanks again!
posted by Spyder's Game at 2:11 PM on July 15, 2008

Fans cannot cool air; they can only move air.

Air moving across your skin evaporates faster, and so you feel cooler. So if the goal is to cool off folks sitting within range of the air being perceptibly moved by the fan, blowing it in is the answer.

If you're trying to move air to exchange warm air for cool air, then the question depends on a couple of things:

1: is the fan in a frame sealed against the edges of the window? That is, is it sealed?

If so, then the direction doesn't matter, so long as you have a window open at the opposite end of the room(s) -- the air pressure is slightly impacted by the fan blowing air in or out, and either the cool air comes in through the window, or through the fan. If you're sitting right by the open window, such that you could feel the incoming/outgoing air, the fan should blow outward (as the air coming in will move across your skin.)

If not, then then direction should definitely be in, because, say, a standalone fan in a doorway isn't going to have any impact on the pressure of the room, and any attempts to pull air out will cause that very same air to circle 'round and come right back in, mixed with a small portion of the cooler air. Do it the other way, blowing in, and the same problem occurs, but mixed with a small portion of the warmer inside air. The difference is negligible, though.

Finally...

2: Is the fan high in the room or low?

If low, the previous advice applies, but if high, blow it out to vent the hot air.

there, now the beans have hit the fan
posted by davejay at 3:38 PM on July 15, 2008