Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

If it's really hot inside, but cooler outside; what is the best way to place a single fan to try and cool a room down?

I always assumed it would be better pointing inwards (and this thread suggests the same).

However; today I had a thought - if the room had a bad smell in it, we would probably expect a fan blowing air out would remove the smell faster than a fan blowing inwards. Is a smell really any different to heat? Would it be more efficient to remove heat by blowing the fan a different way to a smell?

share|cite|improve this question
up vote 22 down vote accepted

From a purely temperature point of view, not human perceived level of hotness, it is better to point the fan outward. This is because the fan motor will dissipate some heat, and when the air is blown outwards, this heat goes outside. This is all assuming the room has enough ventillation cracks and the like that the pressure inside is still effectively the same as the pressure outside regardless of what the fan is doing.

Human-perceived hotness is quite different because humans are a heat source themselves and have a built-in evaporative cooling system. Air flow will help with the cooling process and remove heat from the area around the body. A human sitting in a chair in the room with the fan blowing in will feel cooler than with the fan blowing out due to the higher motion of the air in the room.

If the point is to make you in the room feel cooler, blow the air in. The extra power from the fan motor is a miniscule effect in the overall scheme of a normal room in a house and the kind of airflow such a fan would create. Worrying about the fan motor power is really nitpicking, but can be significant for things like cooling chassis of electronics.

Another issue is where the air comes from that enters the room if the fan blows outward. If it is coming from other parts of the same house that are also hot, then that may technically be the most efficient for bringing down the temperature in the whole house, but less useful for just the room in question.

share|cite|improve this answer
The goal was to reduce the temperature more than make it feel cool, though it was really just theoretical; I was curious. The windows in my hot room are in the shade in the afternoon/evening, which might actually change things; since blowing air out would drag air from rooms with sun shining on windows! But I think I'll let the fan blow over me in reality! – Danny Tuppeny Jul 19 '13 at 21:29
But if you have the fan outside the room, on the balcony blowing in through the door, there will be no heat from the fan and cool air entering will displace through cracks the hot air ( or even through chimney or planned outlet). that is how airconditioning works after all: the compressor outside with the heat and the cool generated air blown in. If it is colder outside it is a natural air conditioning. – anna v Jul 20 '13 at 10:54
I'd argue that the best way to use a fan to feel cool is to have the fan blow directly at you, assuming you don't move around the room much. – Thomas Jul 22 '13 at 15:18

If this all reasonably happens in a steady state, so the air pressure inside the house is constant, then any flow of air out of the window has to be matched by an equal flow in somewhere else, so either way the room is going to cool down: a parcel of cool outside air will replace an equal mass of warm inside air, and the direction of the fan doesn't matter.

The fan is going to set up a current of air through the house regardless.

If there is a heat source inside the house, then a parcel of inside air will still carry the heat from the source to the outside regardless of direction, as long as the source sits in the current of air set up by the fan, but there is a crucial difference in how long the heat from that source remains in the air in the house:

If the house has two open windows and a heat source is near one, clearly a given amount of heat will stay in the house longer if the air current is towards the window that's farther away, as a parcel of air has to travel a longer distance before it's outside; so the heat has more time to dissipate, heating the air that's not directly in the current.

So the house would reasonably be cooler if the fan was pointed at the window closest the heat source.

So if you're sitting by a window with your PC, you're reasonably the largest heat source in the room, and the fan should pointed towards the window closest to you so your heat escapes the house quickly. More effective than pointing the fan out the window would be positioning it so you are in between the fan and the window, since the air current will have a cooling effect on your skin.

If the fan is the heat source, then yes, point it at the closest window. But as a general rule, point it towards the window closest to the heat source.

share|cite|improve this answer

In addition to the already accepted answer of Olin Lathrop, I'd like to mention that for cooling a room on a hot summer evening it is not only important to cool the air inside, but also to cool the wall structures. A fan directed inside will not only improve the heat transfer from human skin but also from the wall structures. Therefore, I assume that the walls would cool faster with an inward directed fan. This would lead to a more effective cooling of your hot room. For concrete walls the heat conduction inside the walls will be weak. So, it is better to cool the walls from inside than from outsinde.

share|cite|improve this answer

Directing a stream of cooler air AT a hotter object will definitely cool it faster than trying to suck air AWAY from said object. I used to have this argument with my wife (when I had a wife). So, if your indoor environment is hotter than the outside air, blowing the cooler air ONTO the things in the room will cool them faster than just passively trying to suck heat away from them. I think anyone who has blown on hot food knows this. Who sucks at the food to cool it? (This is nothing to do with humans sweating, it works for any object.) If you are using a whole-house fan, it usually draws outward, because then it pulls air INTO all rooms using a single large fan. OK? If the fan IS the heat source (who thinks this is likely?) then for the love of God, turn it off!

share|cite|improve this answer

protected by Qmechanic May 29 '14 at 15:32

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.