Window Fan Direction and Cooling

Window Fan Direction and Cooling

Window Fan Direction and Cooling Question: My wife and I have a different opinion on how to cool a room at night using a single fan. We live in Southern California — in the summer months it can get hot but always cools down at night. So, the inside temperature in our home is warmer than the outside temp at night. I feel the best way to cool our bedroom is to place the fan in the window blowing inward (so the cool air from the outside enters the room). She feels the fan should blow outward to draw the hot air out of the house. By the way, we do not have any other windows open in the house during this time. So, what is the best way to cool the house?

Replies: Hello Greg:

Perform at least four experiments, monitoring change in temperature vs. time, to see which configuration works the best:

1. Fan direction in; any other window in house or room closed.

2. Fan direction out; any other window in house or room closed.

3. Fan direction in; any other window in house or room opened.

4. Fan direction out; any other window in house or room opened.

We can hypothesize ad infinitum, but only the experiments will answer the question. These tests are just starting points, you can make your experiments as complex as you would like to optimize the cooling efficiency of your fan. However, remember, change only one parameter at a time in order not to bias any result. Good luck.

Hope this helps,

Joel Jadus


From a purely theoretical perspective the cooling of a room should depend on the difference in the two temperatures and how rapidly the temperatures get exchanged. In both cases, blowing air in or out, the temperature difference is the same and the air exchange (again theoretically) is the same.

Blowing air in allows air to come into a room, but since air can not build up in a room to a high pressure from simply a blowing fan, air will have to leave through gaps in the doors, windows, etc. Similarly, blowing air out of a room will (initially) in a drop in air pressure in the room, but then air (from the outside will have to come in). So, from purely theoretical perspective, and only from the point of view of air exchange, there should be no difference.

However, cooling does not come only from air exchange. There is also cooling from sweat evaporation and forced air movement in the room. Sweat evaporation will cause cooling because as the water evaporates it draws heat from the skin and fanning speeds up evaporation. From this perspective, it may be better to have the fan blowing in -as long as the forced air hits you.

(There may be other factors I have not considered, so see what others in this forum might say.)

Greg (Roberto Gregorius)


I have had same discussion. Open a second window, put the fan in it, blow out. Close off other rooms. You want to make a tunnel to move the air.

Warren Young


Between your idea and your wife’s, I have two distinctions to offer.

You seem to be trying to get heat exchange through one opening. (If no other window is open elsewhere, the house outside your bedroom will hardy be cooled at all, no matter what the direction.) In cooling your bedroom through one window, you must in effect be splitting this opening in half: half of the area has inwards flow, half has outwards flow. Thus the hot air gets out and cooler air replaces it inside.

If the fan can be mounted very close to the window and is smaller than the window, this can work in either fan-direction: blowing in or blowing out. The part of the window not covered by the fan is the part that carries the reverse air-flow which completes cooling-cycle for the bedroom. Put your hand there; you can probably feel it.

Window Fan Direction and Cooling

The distinctions:

(1) If the fan _cannot_ be mounted very close to the plane of the window-frame, point it out the window, offset slightly to one side. The fan’s blowing stream is more directional than its suction pattern. The suction pulls air in from all directions just behind the fan, but the blown stream will go some distance as a «narrow beam». This can be used to «poke» an air stream out through the window when the fan cannot be mounted immediately in the window, but is instead about one or maybe two feet back. Blowing outwards may also help the ejected air get farther away from the house, so it cannot become the same air that is sucked back in through the other half of your window on very still nights. (iffy)

(2) Even for the same temperature, moving air feels cooler than still air. So if your fan _can_ be stood directly within in the window’s frame, then pointing inwards may be preferable because it will cool the far side of the room sooner and also blow across your bed or move the room air in circles. A fan pointed outwards does considerably less of these things. However, one can instead use a second fan to do this interior circulation. If you have this second fan, then (1) may work a little better than (2), and perhaps both fans can be reduced to a slightly lower and quieter power level.

(3) However, I too live in So.Cal. What I have learned is: there must to be two strong openings, «a gozinta and a gozouta» A single window, with no equally-big opening in another part of the room, is about ten times slower and does not usually cool the room in one night, regardless of fan position.

So my house has at least one other window wide open somewhere down the hall, as well as having our bedroom door largely ajar. Measure the area of such openings by eyeball, and compare. It probably matters. They are resistances in series; the smallest one limits the results.

Once I have two openings, I find that cool air coming into the bedroom from outdoors works at least twice as fast as inside air going out through that same window, simply because the whole house is full of heat already. (At least my house is.) All that warm air would need to be completely replaced rather slowly by my fan, before any of the air entering my bedroom starts to feel helpful. Even worse, probably the thermal mass of the house walls needs to be drained too. So I pull-or-push air inward through my windows, get some relief and some sleep right away. After I enjoy the cooler air, it goes on down the hall and just about cools the rest of the house in a night. A very productive arrangement. Much less trade-off between cooling the house for tomorrow and cooling yourselves for tonight.

Given all that as well, I can make pushing or pulling both work. Pulling takes a little more technique. Sometimes I place my biggest fan in the open bedroom door, blowing air down the hall. This pulls air in through the windows of the bedroom behind it quite noticeably. Be aware that all fans not mounted in a portal (i.e. window-mounted) are mostly short-circuited by air going in short paths from front to back. With my 20″ box-fan on the hall floor, and I can feel lots of air going backwards over the top of it. So I tilt it upwards a little, so its downstream bluster «stuffs» the hallway from top to bottom. About ten feet down the hall I can feel the air moving all one way. The airflow I feel well down my hallway seems about equal to the airflow being sucked in through the windows behind the fan. When doing this I can sometimes notice the inertia of the air’s mass: Once I apply the fan properly, it takes a number of seconds for the air to accelerate to its final gentle-breeze speed coming in through a window. After all, the mass of the air filling several rooms is some kilograms, and how hard does a fan push on your hand?

Doing something like this, you can put the biggest, noisiest fan in another part of the house, instead of with you in your bedroom, and still get a good breeze in through your bedroom windows.

When using two openings I do have to consider whether the fan is pushing with or against the existing breeze outside the house. Against does not work too well. A rather mild breeze can greatly diminish the net air flow. One needs to find one good arrangement that adds to the prevailing breeze most days of the year.

I am considering mounting a fan in my hall ceiling, pushing air upwards into the attic. This would not only flush hot air out of the attic (so the attic’s stored heat cannot radiate downwards on me all night), it would immediately and strongly pull fresh air in through all the windows. The living-room would be cooled at the same time as the bedroom. And the external breeze would interfere a little less.

Learn how much airflow 300 cfm (Cubic Feet per Minute) feels like, both in the turbulent stream in front of a fan, and in the quieter flow down the hall. Net flow less than 100cfm might be pretty weak cooling for one room, and 1000cfm might decently serve a few rooms simultaneously. Most fans have a rating printed on the box, and some percentage of that number can be coupled to your room-air-exchange problem. 100% for a portal-mounted fan, such as a window-mounted fan, or my ceiling-inserted hall-to-attic idea. Guessing 30% for bluster-stuffing a hallway, door-way, or window-frame. Much less if you do not set up something like that. You might be surprised how little real air-exchange you are getting with your existing methods.

Look how much of an art can be made of it. Still some things work much better than others. Use your senses, try things, learn a lot. Of course, you still might not be able to convince your other half, for a while.

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