I have noticed my iMac (Retina 4K, 21.5-inch, Late 2015) tends to run hot & noisy when watching some simple videos. I have tried whatever light duty diagnostics I can manage, such as activity monitor and the built in diagnostic tool, but nothing seems to be apparent.

The only thing I can think of is that my iMac is connected to two external monitors: one is a semi-retired older iMac in target display mode, and the other is a Phillips monitor.

Is it possible that supporting external monitors causes the iMac to run hotter?

  • I know this occurs on my 2018 MPB. Commented Aug 11, 2022 at 14:46
  • Have you looked at the GPU History in Activity Monitor? It's expected that larger/more displays will require more work (and thus more heat) to process.
    – benwiggy
    Commented Dec 9, 2022 at 14:26

2 Answers 2


It could... I would suggest getting a can of compressed air and blowing out the fans. If you haven't done this recently, it could be the cause of the extra heat. When blowing out the fans DO NOT LET THE FANS SPIN!!! I have killed a laptop while doing that so be very careful when blowing out the fans.


My hunch is the added load on the GPU for having a second display is negligible and won't matter in your hardware. On portables, they tend to have less metal and airflow, so that second screen tends to make the overall heat load measurably higher.

You can test this by playing the video with only one screen to verify that the CPU+GPU for that activity are generating the heat load and not the second display.

Apple has their own CPU and GPU now to combat the power needs of the Intel / traditional GPU manufacturers that went into your era of iMac. You can look at efficiency settings, but they will be unlikely to move the needle on that era hardware.

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