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It really depends on the fireplace and the mantlepiece surrounding the fireplace. A wood burning fire can give off a lot of convection heat. This heat will tend to rise up and out ouf the fireplace and up the wall - right where your TV is located.
What you should do is start a fire and get it established - coals and flame. Then measure the temperature at the TV. if it is over 100 degrees, then you need to take measures to reduce the temperature.
I have a 42" LCD above a wood burning fireplace. To keep the temperature down, I have a 48" x 12" x 1/4" piece of laminated flooring which I made specially. It sits on the mantle and is heald in place by my center speaker, it sticks out about 10" and deflects the heat out away from the TV and into the room. It is so thin that it is hardly noticible and when there is no fire, I place it behind the sofa.
You could use a sheet of aluminum or something similar, the laminated flooring gets very hot but can withstand the temperatures created by the fire.
One other thing to consider: check the temperature behind the TV on the face of the chimney. Mine doesn't get warm at all because I have a flue but an old brick fireplace may get hot. I'm not sure what you could do there - maybe get a small fan to blow air across it.
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The first thing to do is to see how hot the