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Recently my office manager said this, after I requested a desk fan:

...using a fan causes a draught, which then leads the air conditioning to believe it’s cooler than it is and it then blows out even more hot air

I responded saying this is incorrect as fans simply move air, they don't make it cooler. It feels cool to us, but would make no difference to the air temperature measured by the air con.

I thought this would be an interesting question to post here, and would of course give me back-up. (For those interested, I got fed up waiting for a fan so I just bought my own.)

So my question is, is there any reason why the use of desktop fans in my office would make the air conditioning think it was getting cooler?

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  • $\begingroup$ Your response was right. $\endgroup$ – TZDZ Nov 28 '14 at 9:45
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A fan moves air around. It makes people feel cooler, by causing evaporation of skin moisture (sweat). A fan's motor also gets hotter.

Air moving over a thermostat would have no affect (thermostats don't sweat), but the increase in temperature of the fan's motor, would increase the air temperature slightly, causing the air conditioning to work harder.

If the fan was directed in such a way, that it was moving air from a place with a different temperature, that may also have an affect. For example, a fan directed such that it was aiding warm air move from outside to inside, would increase the inside air temperature, and make the air conditioning work harder.

If you blow a fan over a bucket of water, then it would aid evaporation. Since evaporation is accompanied by cooling, it may help the air temperature cool, which would help the air conditioning work less.

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Since warm air resides above cooler air, when a fan is switched on, it will dissipate the warm air molecules in the room thus slightly increasing the temperature and hence the ac will blow more air out. But after a time the warm air molecules loose their kinetic energy and hence their heat. This results in slight cooling of the room. This cooling, coupled with the ac cooling, will make the room more cooler than it would have been without the fan.

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Another possibility is that a fan would "de-stratify" the air in the room. If the temp sensor was fairly high up a fan would mix the hot and cooler layers of the air in the room making the sensor a bit cooler than it would normally be.

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