Try re-reading the post. I said 'decorative fireplace' which 98% of the ones people want to mount a flat screen over have. I have had fireplaces in every home I've ever owned. None of them were fitted with a fireplace insert or any sort of blower arrangement to produce heat. None of them made more than a few degrees of extra heat in the area a foot or two over the fireplace. If there was a significant updraft of heat in that area that somehow wrapped around and over the little mantle piece, it'd be carrying carbon monoxide and other combustion contaminants into the room.
The vast, vast majority of heat and combustion gases in the majority of fancy fireplaces in fancy homes that have big flat screens mounted over them goes right up the chimney. If its working properly, and you're lucky that is.
As a public service announcement, if you have a regular open face/glass door wood burning fireplace without any sort of heat exchanger, insert or blower arrangement and you can feel enough heat a foot over the top of it on the inside of your house to even remotely disturb a piece of well vented electronics, call someone to check on your chimney and flue, because they're not working properly.