<story><title>Ask HN: Do I place the fan so that it blows air inwards or outwards?</title><text>I am positive that there&amp;#x27;s a huge difference between the theoretical model and reality here.&lt;p&gt;Let&amp;#x27;s assume it&amp;#x27;s 30 celsius inside the room and &amp;lt;20 celsius outside.&lt;p&gt;Enjoy your Monday my thermodynamics friends!</text></story><parent_chain><item><author>cbanek</author><text>Like others have joked about, it&amp;#x27;s really about where you want the air and temperature to go.&lt;p&gt;Let&amp;#x27;s say you have three fans in a computer case, one at the back, one at the front, and one on top of the CPU.  If they all blow out of the case, then you&amp;#x27;ve basically created a negative air pressure zone, and you will be sucking air through the vent holes elsewhere, and through any gaps in the case.  This may have effects for where the dust is taken in.&lt;p&gt;If all of them blow into the case from the outside, you&amp;#x27;ve created basically a high pressure zone, and air will be taken from the room and pushed out through the same cracks and vent holes elsewhere.&lt;p&gt;A fan if it&amp;#x27;s pushing against high pressure will inherently push less air because it&amp;#x27;s pushing against air that isn&amp;#x27;t moving as well.  A fan that is trying to pull air out can also become inefficient if it is working hard to suck air in to eventually push out.  Therefore it&amp;#x27;s best to have matching airflow - what you push out, you should try to also pull in somewhere else in more or less an equal CFM (cubic feet &amp;#x2F; minute).&lt;p&gt;For a room, it&amp;#x27;s best to have one area that is your intake and the other side of the room be the exhaust.  This way you aren&amp;#x27;t just exhausting air you just pulled in.  If you just have one fan, I suggest blowing out, which will draw air in from other spaces, such as under the door, because of the negative air pressure by pushing air out.&lt;p&gt;(I&amp;#x27;ve worked these problems with indoor gardens and HVAC systems, where there&amp;#x27;s a lot higher CFM fans and spaces, but the theory is basically the same)</text></item></parent_chain><comment><author>colanderman</author><text>I strongly disagree on the setup with only one fan. Having only an exhaust fan in the room will draw nasty air in through cracks in the house&amp;#x27;s structure, possibly stuff with e.g. mold spores. My wife could never remember which way was which for our window fan; I could tell by smell when she had left it exhausting for more than an hour.&lt;p&gt;If you have only one, set it to intake and leave another window open a bit (maybe like half the fan size). You can confirm that air is exiting that way by hanging some strips of tissue paper in it.&lt;p&gt;Positive pressure (blowing in, from outdoors) is also the right thing to do if you are trying to keep smells from other rooms at bay. Whereas negative pressure (blowing out, to outdoors) is best to keep a smell confined to one room (e.g. this is how bathroom fans work).</text></comment>
<story><title>Ask HN: Do I place the fan so that it blows air inwards or outwards?</title><text>I am positive that there&amp;#x27;s a huge difference between the theoretical model and reality here.&lt;p&gt;Let&amp;#x27;s assume it&amp;#x27;s 30 celsius inside the room and &amp;lt;20 celsius outside.&lt;p&gt;Enjoy your Monday my thermodynamics friends!</text></story><parent_chain><item><author>cbanek</author><text>Like others have joked about, it&amp;#x27;s really about where you want the air and temperature to go.&lt;p&gt;Let&amp;#x27;s say you have three fans in a computer case, one at the back, one at the front, and one on top of the CPU.  If they all blow out of the case, then you&amp;#x27;ve basically created a negative air pressure zone, and you will be sucking air through the vent holes elsewhere, and through any gaps in the case.  This may have effects for where the dust is taken in.&lt;p&gt;If all of them blow into the case from the outside, you&amp;#x27;ve created basically a high pressure zone, and air will be taken from the room and pushed out through the same cracks and vent holes elsewhere.&lt;p&gt;A fan if it&amp;#x27;s pushing against high pressure will inherently push less air because it&amp;#x27;s pushing against air that isn&amp;#x27;t moving as well.  A fan that is trying to pull air out can also become inefficient if it is working hard to suck air in to eventually push out.  Therefore it&amp;#x27;s best to have matching airflow - what you push out, you should try to also pull in somewhere else in more or less an equal CFM (cubic feet &amp;#x2F; minute).&lt;p&gt;For a room, it&amp;#x27;s best to have one area that is your intake and the other side of the room be the exhaust.  This way you aren&amp;#x27;t just exhausting air you just pulled in.  If you just have one fan, I suggest blowing out, which will draw air in from other spaces, such as under the door, because of the negative air pressure by pushing air out.&lt;p&gt;(I&amp;#x27;ve worked these problems with indoor gardens and HVAC systems, where there&amp;#x27;s a lot higher CFM fans and spaces, but the theory is basically the same)</text></item></parent_chain><comment><author>kenhwang</author><text>Just to add, for computer cases, you want it to be a high pressure zone because intake fans generally have filters in front to catch the dust before it gets into the case. Negative pressure means dust gets sucked in from the cracks.</text></comment>