iMac fan not speeding up

I have an iMac Retina 5K and I have noticed that the fan never goes higher than 1200rpm regardless of how hot the machine gets (in particular the GPU), being like over 90C, unless I increase the speed manually with for example iStat menus. Have reset SMC. Anyone else come across this?

rogerinlondon said:

I have an iMac Retina 5K and I have noticed that the fan never goes higher than 1200rpm regardless of how hot the machine gets (in particular the GPU), being like over 90C, unless I increase the speed manually with for example iStat menus. Have reset SMC. Anyone else come across this?

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That does seem a little high, and there is a discussion on Apple's forums with others raising similar concerns.
https://discussions.apple.com/thread/6641477

Before doing anything, be absolutely certain that you have iSTAT Menus set to ONLY monitor your temps and NOT manage the fan speeds. Click on the menubar item and hover your mouse over "Active Set" at the top of the popup menu. You'll get a new popup that says "Fan Control" up top. There should be a checkmark on "Off". If it is not set to off, then there is the possibility that the "problem" is one of your own creation by overriding the iMac's ability to manage the fan speeds itself. Set to "off" and then reset the SMC.

If the high temps persist, I would follow the advice given in that discussion and report your concerns to Apple. If there proves to be a problem, Apple will issue a firmware update to address it eventually. If the heat does in fact cause you a problem, you are under warranty and though I would recommend getting the extended warranty, Apple historically has covered issues like these that are "defects" out-of-warranty.