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cooling fans and thermodynamics

Books + Ideas — July 2018

There seem to be some common misconceptions about the thermodynamics of fans going around. Running a fan in a room will not cool the room down. In fact it will heat it slightly, as the electrical power going into the fan (on the order of 50-70 watts for a typical standing room fan) will almost all end up as heat. The moving air from a fan will cool people down, by speeding up evaporation from the skin and the cooling associated with that (heat is transferred from your skin into the water as it evaporates), but it makes no sense to run a fan in an empty room.

Similarly, if there's a breeze blowing it may feel cooler outside than inside even when it's 30° outside and only 25° inside, but it makes no sense to open the windows in unoccupied rooms, as the moving 30° air will come inside and become stationary 30° air... To make the best use of convective heat transfer in hot weather, open the windows as much as possible overnight, and especially in the early morning, and then shut them when the outside temperatures start to climb. If your curtains have any kind of light-coloured backing, keep them closed during the day, especially if they will be in direct sunlight - that will reduce radiative heating of the house by reflecting as much light as possible. (Shutters outside the windows are much better, of course, if you have them.)


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