You can't have a powerful phone that doesn't produce some heat. And phones of course are too small to have active cooling systems. If you had done a search you'd find that some heat is normal. You're assuming that the phone is overheating when there is no basis for that. You just didn't expect to notice any obvious heat.
You don't need a thermometer, you can install an app e.g. an Xposed module to report the processor temperature. Idle temperatures of ~ 35C are normal. Light load, perhaps 45C, heavy load ~ 65C. And if you are doing something processor intensive e.g. transcoding or some games and have a bulky case that causes your phone to run hot, you might see temperatures approach 80C. There is no (public) published TDP figure for the Snapdragon 800 series, but it's believed to be about 5 W. So these temperatures should not surprise anyone.
Auto shutdown i.e. what the manufacturer considers overheating is I believe 100C. Advances in semiconductor lithography do produce devices that radiate less heat in general but these temperatures are not out of line with other processors e.g. I have measured desktop CPUs that routinely operate as high as 90C.
Last edited by fffft; 7th September 2014 at 11:29 AM.