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Adjusting iMac Fan Speed
As always, follow this guide at your own risk. Increasing the speed of your fans in your iMac may lead to a voided warranty and/or decreased fan life. That being said, I have not heard of anyone significantly decreasing the life of the fans in their iMac by increasing the speeds at which they run (within reason). There have, however, been many instances where the internal components of an iMac have been damaged from excessive heat. This excessive heat can be caused by environmental factors (room temperature, ventilation, etc.), prolonged high CPU load (long periods of video encoding as an example) and, in some cases, faulty hardware (a faulty power supply heating up other non-faulty components).
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Whatever the case, if you are in need of a relatively safe and easy way to increase your iMac fan speed to decrease your component temperatures you’re in luck. There are two freeware programs that I would recommend taking a look at to accomplish this task.
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The first program I would suggest you look at is “iMac Fan Control" and is developed by Derman Enterprises. IMac Fan Control is free and unsupported by the developer, but as of the writing of this article, works perfect on my 24-inch aluminum iMac. The program installs a preference pane in Mac OS X system and the options are fairly straightforward. You can set a low and high temperature threshold and a minimum fan speed for your CPU, hard disk and optical drive. The preference pane also displays your desired / current fan speeds and the temperatures of the major components in your iMac.
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The second program, and my program of choice for controlling the temperatures / fan speeds of my iMac, is smcFanControl. SmcFanControl offers all the options of iMac Fan Control with the addition of a few “advanced" options. These options include a menubar readout and the ability to create favorite profiles which can then be applied for different situations. Personally I have two profiles saved, one for times when I'm putting my iMac under what I consider to be a normal workload, and another for when I'm encoding audio/video. Under a normal workload my fans run at 300-500 rpm over default and while under a heavy workload my fans run at 750 rpm over default.
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A couple things to keep in mind when deciding what speed(s) to run your fans at is the default and maximum fan speeds for your iMac. For example, the iMac aluminum 24-inch has a default HDD and CPU fan speed of 1200 rpm, while the maximum fan speeds are 5500 rpm for the HDD and 3300 for the CPU. My personal recommendation is to set your HDD and CPU fan speeds to 300 - 500 rpm above the default values. This should give your iMac a small boost in cooling power while not creating an abundance of fan noise. Your mileage may vary … don’t be afraid to experiment to obtain the best possible settings for your personal needs.