iMac 2010 back panel overheating

The hot-air exhaust vent is on the back panel. That could the why.

Your critical cool-air intakes are on the bottom edge of the case. Anything between them and the workstation surface will cause a rise in internal temps. This owner of this iMac posted it asking us why his iMac was running hot ( I added the red to highlight the problem area):

A little housekeeping on the owner's part resolved his overheating problem.

You iMac's internal temperature is managed. A thermostatic system uses sensors to monitor temps and, on finding a component that is too hot, will signal the fans to speed up to remove the extra heat. At idle (~1200rpm), the fans can seem nearly silent except in a very quiet workspace. If you are hearing the fans run at high speed, the temps are truly rising.

These and more will work the CPU and GPU quite hard, causing increased internal temps:

— gaming

— streaming video

— video conferencing

— useless fearware (ex: anti-virus and so-called "cleaning" apps)

— rendering operations

— compiling operations

Even some poorly-coded web pages can use too many resources and run up temps.

So more detail on when your iMac gets hot might help us further help you.

When (if ever)has the thing been opened and vacuumed out?