I hve also the same problem in my HTC820... It's get hot ....because of tht.I hve the many problems....
I wnt to kno tht.. wht is the reason behind it.........
Some solutions of prevent warmness of cell phone....
Please suggest me...

I using HTC desire 820
But there is some issues with this
Because when I using my phone it's automatically get in lock screen.please answer my question.

Could you please tell me how to change the color temperature (cool/warm) of the screen? The whites seem a little yellow to me. I have a samsung galaxy R where i have the option to change is Screen mode from Dynamic/Standard/Movie. Thanks! :)

htc 820 does have some heating problems,which in my case happens near the primary camera.the problems can be the battery or the processor.most probably it would be the process because as have observed that the phone is slim therefore there is no space for heat to radiate out of phone.

so to minimize the heat coming out of phone go to setting>power>Power saver> and then check if CPU power usage is conserved.then go to quick setting and toggle the saver(remember not the ext saver).this should help.
the above steps may hamper the heavy gaming habits but still its a small price to pay for a slim phone.

the above steps worked with and i am quite sure that even after using the 'saver mode', other process,games and multitasking were snappy and good

and if you want to conserve only cpu usage and want other functions to run normal then again go to setting>power>Power saver> and uncheck all other values other than the "cpu usage".

i dont know about any charging problem with my device,still it charges slowly that's not a problem with me.

recommended : download du battery saver to find problems with your charger and temperature if battery temperature is less than 50 C then its fine, if it rises above it quickly power off your device