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It's fair to say that iMac is not designed to run at top processing power all day, every day (and nobody is suggesting you've done anything like that). But barring some kind of defect (of which there is zero evidence thus far), your iMac will cool itself just fine, even under heavy load. The machine will simply shut completely off by itself if it ever gets so hot that it can no longer protect the processor. If this hasn't happened, then everything is working as it should and don't worry about it.
I wouldn't say that is fair at all... I would say it is fair to believe that any computer an individual buys should be able to handle whatever job the owner wants to accomplish with it that the hardware is capable of doing - including encoding video 24/7 if that's what the owner wants to do with it. And I believe, the iMac can do just that.
With over 10 yrs experience overclocking in the other camp by the time I got my first Mac, I was astounded (and stressed out) that it was running temps in the 90-100°C range (194-212°F) while gaming and encoding video. The goal on my custom built air cooled overclocked rigs at the time were temps below 30°C at idle and I rarely saw anything above 50°C under stress. Holy Cow I thought - what now?
You can't go in and pull the CPU and lap it. You can't upgrade the CPU cooler nor the fans. You can't add liquid cooling to it. You can't overclock it <-- the primary reason temperature monitors became vogue in the home PC market, as the cooler you could get the system to run the higher the overclock you could achieve.
I can't even begin to tell you how many hours my '06 MacBook Pro has spent encoding video, not to mention the 6-10 hour sessions spent gaming on it - all with the temps sitting in the 90+C range. (I may still have the red marks on my legs to prove it.) It's still here and still being used. Bought the kid (he's an art museum curator) an iMac that same year. He spent a few years putting together videos on that machine and is still using it today as his primary iTunes library server.
The higher the percentage of the processing power being used by the CPU and GPU - the higher the heat is going to be - period...
Now I'm not saying you should expect to use most computers in an area where the air temp is 100+°F and not expect to have some issue running CPU/GPU intensive tasks on it. But, if the air temp is below 75-80°F, you shouldn't have an issue.
Bottom line - I did all the stuff - Want my advice? Trash the temperature app and forget you ever heard there was such a thing. You'll be much happier. Use your machine to do what you want to do with it. If/when it starts having heat issues, it will let you know.
What a joke - an app that recognizes when you're actually doing some work on your computer and then pops up to tell you if you shut it down it will cool off - duh!!!! - what a freaking asinine statement and waste of space - get rid of that stupid app and use your computer to do the things you bought it for.