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iMac running hot; what is it DOING?

4155 Views 10 Replies Latest reply: Oct 30, 2012 5:20 PM by motrek RSS Branched to a new discussion.
motrek Level 1 Level 1 (25 points)
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Oct 29, 2012 3:53 PM

I understand many people are concerned with how hot iMacs run. I'm not too worried since I'm sure Apple has stress-tested these computers and engineered them such that they will run for many years even if you're doing 3-D renders in Death Valley. Just because a human is uncomfortable at 150 degrees doesn't mean a computer chip will stop working.


But what I don't get is what the iMac is DOING. Sometimes I will be doing a lot of intensive stuff (I check Activity Monitor frequently) and the top of my iMac will only be lukewarm, and other times I leave it idle (doing absolutely NOTHING) for an hour and I come back and the top is HOT.


This is a complete mystery to me. Has anybody been able to figure out what causes these computers to heat up?!

iMac, OS X Mountain Lion (10.8.2), 27", 2011
  • rkaufmann87 Level 8 Level 8 (40,615 points)
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    Oct 29, 2012 4:04 PM (in response to motrek)

    Please post the temps your machine is operating at. I'd recommend downloading and installing iStat Pro, you can find it using a Google search.


    A normal operating iMac will get warm, saying it's hot or overheating normally means nothing. In most cases iMacs do not overheat, there are extremely unusual exceptions but most never overheat.  Apple engineers them to run for years, mine is 5+ years old and run just fine and has never overheated.


    Your computer has heat sensors and fans inside, if the computer senses the computer is warm the fans will promptly cool it down. So when you are feeling it hot, check 10 minutes later and it will probably be just warm again. Forget about it and enjoy using the computer.

  • John Galt Level 7 Level 7 (33,085 points)
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    Oct 29, 2012 4:05 PM (in response to motrek)

    Run Apple Hardware Test and make sure your iMac's fans are OK:


    Using Apple Hardware Test



    I have tried and failed to get my iMac to overheat using Geekbench stress tests. The CPU stabilizes at 65° C or so and won't go any higher. It's a 21.5" model though.


    An SMC reset may also be indicated for anything fan-related.

    MacBooks  iMacs  iPods  AirPorts, OS X Mountain Lion,  27 years Apple!
  • inandoutofgrace Level 3 Level 3 (520 points)
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    Oct 29, 2012 4:09 PM (in response to motrek)

    Yes, download iStat Pro or iStat Menus. Then you'll be able to see exactly what your machine is doing at any given time and the associated temperatures and fan speeds.


    I have an 2.8GHz i7 27" iMac. These are the temps I see under most conditions (per iStat Pro):


    HD: 90F

    CPU: 115F

    GPU Diode: 120F

    GPU Heatsink: 117F

    Mem Controller: 103F

    Optical Drive: 85F

    Power Supply2: 115F


    The hottest part of the case is the top left corner most of the time.


    When my machine is really cranking (when encoding video, for example) I've seen the CPU and power supply temps get up to around 130-140F.

  • MichelPM Level 5 Level 5 (7,140 points)
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    Oct 29, 2012 4:23 PM (in response to motrek)

    Coming from an electronics engineering background, I can tell you most assuredly that the heat generated inside the current iMac design enclosure is not good for the electronic components over a period of time.

    There are a lot of posts here of hard drive failure, CPU, GPU and even logic board failures.

    I am convinced that all are heat related issues.

    Heat is the enemy of all small electronic components, period!

    Component failure and component "brown-outs" can be caused both by fluctuating electric current and/or heat!

    Electronic components like to operate in as cool a temperature as possible. Conputer components, at one time were cooled by either water, anti-freeze-like fluids, liquid Nitrogen, etc.

    In earlier iMac models, heat was vented by having a case with that has ample venting built-into the case design.

    The new iMac designs IMO, do not have enough sufficient, passive venting to vent all of the heat the iMac components generate. I feel the aluminum body conducts as much heat on the inside of the enclosure as it convects outward from the enclosure with an insufficient net reduction in overall internal temperatures.

    Apple has definitely NOT done long duration stress testing of its current designs.

    It refreshes its product designs every couple of years or so. So why do long-term testing on a design that will be redesigned and refreshed every few years. I believe they do not.

    Apple engineers were simply concerned much more with the complete quiet operation of the iMac design than with internal heat buildup issues within the current iMac enclosure designs.

    I and many other users use a software fan control to ramp up the iMac's internal fans as well as use an external fan on the backs of our iMacs to further cool them down more directly.

    The result is an additonal 15% to 20% reduction in the iMacs overall running temps.


    As to your main question, when running OS X applications, OS X is doing things, while the apps you are also running are doing things.  These things use CPU, GPU cycles, swapping commands and data in/out of RAM, maybe using hard drive cycles, etc. These processes work different hardware and software components of your iMac and, in turn, increase the components temperatures. Typically the CPU, GPU and hard drive ( and in the case of getting data on and off of discs, the optical drive) do heat up appreciably and heat up even more when the application tasks get more and more demanding of the Mac hardware.

  • MichelPM Level 5 Level 5 (7,140 points)
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    Oct 29, 2012 5:04 PM (in response to motrek)

    Maybe your iMac's internal fans are running correctly.

    Look at this post and try the suggested solution at the end of the post .


  • MichelPM Level 5 Level 5 (7,140 points)
    Currently Being Moderated
    Oct 29, 2012 5:09 PM (in response to MichelPM)

    Darn iPad,

    I meant to type that your iMac's fans may NOT be running correctly.


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