iMac used for Target Display Mode runs really hot I have a 2014 15" retina MBP that I've hooked up to my old 2009 27" iMac using it iMac as a monitor via Target Display Mode while the MBP is in clamshell mode.

My problem is that the iMac turns really hot very fast. A lot hotter than it turned when I was using the iMac as my primary machine.

The iMac is running a fresh install of OS X Mavericks so there shouldn't be an issue with a lot of background apps running.

Any ideas on why this could be?
I'm experiencing the same problem.  I have a current generation 5K iMac which I use as my main computer, and a 2009 iMac which I use as a second monitor using TDM.  In TDM mode, the older iMac runs painfully hot.  Switch back to regular mode, and that same iMac runs cool.

Typically, I don't even bother to login to the older iMac - I just turn it on and off with the power switch when I want to use it in TDM.  Hence, there can be no question about background apps running hot.