In general, I agree that above fireplace mounting is not the ideal scenario. Mostly because of the chiropractor bill and not because the HDTV would necessarily burn up ![]()
Seriously though- there are some dynamic mounts available that are pricey, but allow you to pull out and lower the HDTV enough where comfortable viewing is possible. Truth be told, most folks don't use their fireplaces much, so the crossover is not frequent. Unfortunately, the price of this type of mount is normally several hundred dollars. See prior posts on this here. Youtube also has several portrayed. I believe Monoprice has an 'economy' version though which could potential work in your home. YMMV.
Search for 'pull down HDTV mount'.
I would like to mount my 50" HDTV over my fireplace, Where do I start?

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