iMac 2019 (21.5") fan on almost constantly for no good reason

Genozel wrote:

Thanks, I will check things and do the SMC reset. Did this lots of times in the past when I had the on/ off switch issues. but haven't for a while now.

It might be a software issue like you mentioned. Is it possible it has something to do with Time Machine as well?

I'm not too keen on Time Machine being a cause unless you have a defective external drive. Left alone, TM tends to be rather unobtrusive. If you suspect Time Machine, simple eject its drive and turn it off using System Preferences, then test.

You can check to see if it is software or processes in your current user account by setting up and testing from another "dummy" acct with no outside encumbrances:

Set up users, guests, and groups on Mac - Apple Support

If the problem goes away, then that narrows the search from hardware to software in your current users acct. If nothing changes, hardware is likely the issue.

Eliminate any peripherals as suspect (other than the mouse and keyboard) by testing with peripherals disconnected.

What is the OS version you are running. Apple will recommend you to run the latest one.

I would try an SMC reset first. Instructions here:

How to reset the SMC of your Mac - Apple Support

You do NOT have the T2 security chip so use the "Reset the SMC on other computers" instruction set.

If that does not help, check for anything that can compromise the vents for the cooling system. Cool air goes in through slots on the bottom edge of the case:

Place NOTHING between the intake vents and the desk. It takes very little to reduce the required air volumes. I was able to see a 10-15C increase in exhaust air temps by reducing the case-to-desk distance by half.

Also mak sure nothing INCLUDING the stand interferes with the EXHAUST vent on the back:

If the screen is tipped fully forward, it will put the top of the stand too close to the exhaust and temps will rise. I've duplicated that on my iMac in working through another user's heat issues here.

Beyond that, we are into software issues.

• Google Chrome tends to run hotter and use more system resources that Safari or FireFox..
• The uncalled for addition of anti-virus and/or so-called "cleaning" apps WILL run up the temps as they compete with elegant self-maintenance routines you paid Apple to program into into the OS. Like cats, Macs clean themselves and have for over two decades.
• Any streaming video—videos services; YouTube video, web conferencing; etc—will run up the temps to levels approaching those of many video games and compilers.