Hey, Steve!
I own this computer and noticed a few months back that the fans were running an abnormal amount even when the computer was at idle. My first thought was that there must be dust blocking some of the air vents thus preventing proper cooling.
There is a big vent behind the iMac’s stand but I found that that there was almost no dust present. I cleaned it out anyway and it made no difference with the fans.
It was a couple weeks later that I found out that there is a thin strip of 13 air vents at the very bottom of the display. These vents are responsible for the air intake and cooling where as the vent on the back is for air out.
The vents on the bottom of my machine were packed with dust and once I cleaned them out with an anti- static brush (or a trusty old toothbrush) my Mac’s fans returned to their normal RPM!
Give this a try and let me know if it works!
Good Luck!
Milo Firestar