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Why does a table fan eventually stop circulating air? I noticed this with every fan I ever used. The fan will work great for about 2 hours blowing air at full speed, then I notice that, even though the blades are still spinning, the amount of air output has been greatly reduced to the point that I can barley feel any air coming off the blades. If I turn it off for a couple hours it will still go through the same cycle. Why does this happen? Everything works fine, just no air circulates out.

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I don't notice any such phenomena. – dmckee Jul 16 '12 at 14:29
Perhaps you have a mirical fan that does not obey the second law of thermodynamics... – Killercam Jul 16 '12 at 15:33
Get a second fan and try to duplicate your results. Have a measuring device to determine wind speed. Size of your room and windows, doors must be kept constant. – Argus Jul 16 '12 at 16:25

My previous answer was based on physical principles. This answer is based on biological principles.

I noticed in the question @JoeBlack stated "I feel air" meaning you are using your own biology to sense the air flow. Air flow can be felt by pressure of turbulence on the skin. It can also be felt by perceived temperature. Usually a person turns on a fan when he is hot. By then, the person has perspiration. If the air is reasonably cool (< 90 F) and dry, the air from the fan will evaporate the perspiration faster and feel cooler than if the fan is not running. As the air dries all of the perspiration up, it will not feel so cool as it did at first.

Then, when the fan is off for another couple of hours, that time allows more perspiration to build up on the person. When the fan is turned back on, the same cooling effect returns.

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It is possible that the fan motor is slowing down as it heats up. If you can supply data such as motor temperature and RPM, that will verify my answer.

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+1 Very possible much older fans can start to wear out usually by slipping of the clutch due to heat stress. – Argus Jul 16 '12 at 16:29

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