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Suppose I have two fans in my two story house, one in an upstairs window and another in a downstairs window. I want to cool my whole house; it is cooler outside than inside. Is it more efficient for

  1. the upstairs fan to blow cold outside air downstairs, with the downstairs fan blowing it back outside,

or

  1. the downstairs fan to blow cold outside air upstairs, with the upstairs fan blowing it back outside?
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    $\begingroup$ So we're all on the same page: what definition of efficiency do you want to use? $\endgroup$
    – BMS
    Commented Oct 29, 2014 at 6:30
  • $\begingroup$ @BMS: lower cooling time → higher efficiency $\endgroup$
    – Geremia
    Commented Oct 29, 2014 at 15:00

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Since hot air rises it will be hotter upstairs than down. You need to expel the hot air from the top storey and suck in cold from the bottom. Otherwise, you are trying to send hot air downwards.

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  • $\begingroup$ Yes, but you have gravity to fight against trying to get the cold air upstairs, no? $\endgroup$
    – Geremia
    Commented Oct 29, 2014 at 15:45
  • $\begingroup$ Not really because if both top and bottom were open the hot air would naturally exit and suck in the cold. This is just speeding up the process. $\endgroup$
    – user56903
    Commented Oct 29, 2014 at 16:30
  • $\begingroup$ I wonder how large of an effect this plays in real-world homes where there are typically air currents that may mix air masses. No idea. $\endgroup$
    – BMS
    Commented Oct 29, 2014 at 17:59

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