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Me and my dad were arguing last day about this question: suppose it is a very hot day, and the temperature in my room is about $30$ Celsius degrees. There is no air conditioner on, no fan, nothing. The room has a window on which the Sun is shining, and the window is closed with the roller shutter raised halfway. There is only a door.

Now the question: I state that is I put a fan inside the room and I turn it on, and I close the door leaving the window closed too, what the fan will do will be just to move the (hot) air inside the room, and the temperature won't change (or it will change in a very subtle way, like $29.5$ Celsius degree after a while).

My dad instead said that the fan will make the room less hot due to the air movement by the fan.

Who is right? I believe I am right, since the air in the room won't change its temperature just by being moved, and the fan doesn't produce any cold effect (on the contrary, by Joule effect there is a little amount of heat produced by its engine, right?).

What is the correct reasoning?

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I would say you are correct. The reason a fan cools you down is due to evaporation of sweat. A fan does not lower the temperature in any way it does however as you mentioned increase the temperature due to all energy from the fan in the closed room will eventually be converted into heat.

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    $\begingroup$ It does not matter whether the fan is efficient or not. All the consumed energy is converted to heat. $\endgroup$
    – Anixx
    Commented Aug 9, 2021 at 11:04
  • $\begingroup$ Good point Anixx, I agree. $\endgroup$
    – ludz
    Commented Aug 17, 2021 at 6:05
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The fan will heat up the temperature in the room, proportionally to the electricity consumed. All consumed electricity is finally converted into heat. So, you can use any electric device to heat up your room, be it a fan, a heater, a TV or a fridge. And you can easily calculate how much energy will be turned into heat by looking at your electricity counter.

The efficiency of the fan does not matter as all the energy goes to heating.

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In fact, evaporation does not merely make you "feel cool", it actually does cool YOU. A fan blowing on a PERSON cools the PERSON (through evaporative cooling) but adds heat to the ROOM. Fans not directed at persons will add heat to the room and may even make it hotter by disturbing and mixing in the layer of trapped heat at the ceiling. Fans going in unoccupied rooms - well, that just adds heat.

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The fan uses electricity to operate and won't be 100% efficient, it generates heat - so the temperature of the room would increase gradually.

The fan might have an apparent cooling effect though, i.e. it helps with the evaporation of sweat, perhaps that's what your Dad meant.

Best to block out more sun, open the door slightly and then make a choice as to whether the fan makes it feel cooler or is just generating more heat.

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    $\begingroup$ It does not matter whether the fan is efficient or not. All the consumed energy is converted to heat. $\endgroup$
    – Anixx
    Commented Aug 9, 2021 at 11:03
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    $\begingroup$ Also, the main cooling effect of the fan is not due to evaporation of sweat but by moving cooler air (say, at 30 C closer to your skin, which is at 36.6 C) $\endgroup$
    – Anixx
    Commented Aug 9, 2021 at 11:08
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The fan would not make the room cooler i.e. the temperature of the room would not go down.

But the fan would make it FEEL cooler because fan helps in evaporation of sweat , which is what produces the cooling feeling in our body.

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