I do not know for sure whether I can help you. I can only explain what I did and what worked for me. Try it out at your own risk, it is certainly not a standard procedure, let alone Apple-approved. No guarantees or liability on my part. And please forgive me for not describing the process in every detail, but only in rough sketches
I needed two programs, both downloadable from the Internet:
smc fan control 2.5.2
mac fan control (1.3.2).
I downloaded both and installed them, then quit them. Both programs are so designed that you can raise fan speed above factory settings, but not lower it beneath Apple's minimum setting - which varies from machine to machine.
The former program (SMC fan control) generates a respectively named section in the application support folder of the library (visible only when pressing "alt" in the "goto" menu.
There, I found a file named machines.plist.
This file contains the factory MSC fan settings for a lot of Macs, and also an auto-generated entry and the end named iMac17,1. I deleted the reference to auto-generation and edited (or, as you might say, manipulated) the values for min and sel speeds to 1000, and saved the file.
Afterwards, I went to the applications folder and right clicked the package smcFanControl and let the contents of the package show. In the resources folder of this package, I took the original machines.plist file and moved it to a safe place. then, I copied the edited version of the machine.plist file into this folder, so that it is now in both places.
I then opened SMC fan control and chose the command to reset to factory default. I restarted the Mac. After starting the program, I could see a new minimum value of 1000.
But this still did not change the actual RPM, the fan was still too noisy for my taste.
This is where the second program Mac fan control came into play. I opened it and chose the panel where you can set a fan speed by hand using a slider. Unlike before the measures described above, it was now possible to set the slider to 1000 RPM, which I did. Appearantly (and I do not know why and how), Mac fan control accesses the machine.plist of the competing smc fan control program.
Bingo! The fan slowed down to 1000 RPM.
I chose to auto start the Mac fan control software (not smc fan control). At every auto-start, the software will now set the fan to 1000 RPM. Sometimes, this does not happen after the Mac wakes up from sleep. In these cases, I just quit the program and restart it.
At 1000 RPM, the iMac can still be heard a little bit in a totally quiet environment, it is not completely silent like a MacBook air normally is. Nevertheless, it is almost silent, the difference is significant.
Naturally, the iMac runs a bit warmer at 1000 RPM. This is easy to monitor because Mac fan control rests in the menubar of OS X and can show you the CPU temperature sensor's reading as well as actual RPM.
At normal room temperature, my machine normally runs at between 38 and 48°C when the fan whirs at 1000 RPM, depending on cpu load, of course. I have not seen it exceeding that range so far. Also, no other sensor on the machine shows higher readings.
I have not done any testing as to the difference in temperature at 1200 RPM, but I think it may be between 5 and 10°C.
I am no expert, but I think I have read on the Internet that the CPU of my machine (i-6700K) can stand far higher temperatures (e.g. when overclocked).
Also, like I said before, I applied roughly the same procedure to my wife's iMac14,1 two years ago. It is still alive and kicking (and silent at that).
Besides, I have two Windows machines constantly running at temperatures in this range for years.
IMHO, Apple's engineers, like many of their colleagues at other manufacturers, were just a bit to conservative/protective when defining minimum fan speed on iMacs with dedicated GPU - but that is just my
Of course, I can not rule out the possibility that the lifespan of your CPU is shortened, and, like I said, I will not accept any liability for adverse effects if you try to do what I did. Enter at your own risk.
I did so myself. I just thought, chances are that I will replace my machine for other reasons well before its CPU dies on me for getting "too" warm constantly. And, in any case, I will have had the pleasure of near silence when using it (as opposed to not wanting to use it for the noise it generates).
Cheers, Marius...