Three of my UPSes run hot (where "hot" means too hot to lay my hand on it. The other two run cools. The cool ones are Compaq, the hot ones are APC, but it's very obvious that they're all the same hardware. Maybe the firmware is different?
So I came up with a great idea to make the hot one run cool; I have a couple of big old eight inch fans (I don't remember where I got them), and I've laid them on top of the UPSes.