Does heat ruin an LCD television?

Well per say, the heat from the chimney will not ruin your lcd. Of course you do understand that electronics specially need to keep a cool to ambient temperature air draft current for them to correctly work and wont lag. The internal components will run at a certain temperature especially with a certain voltage. What is the wall that you are setting your tv on, made off? Brick is a great insulator, it will keep your tv cool on it. I would not recommend it myself. Ads put them on top to make it look niced, and like i mentioned before people that place them above fireplaces, are usually made out of brick.