Personally, I hate them. I do so because they trap moisture in the top of the masonry chimney. If you think about it, warm air rises. If you are going to remove your existing damper that is right above your fireplace and install a new one at the top of the chimney, the interior flue for that fireplace is going to sweat a lot when its cold out. This in time can transfer into the cavity of the chimney and start to cause the masonry to prematurely fail. And this is going to be an unwelcome expense for you should it happen. If there is any way to fix the damper down below, or use your glass doors as a damper (which isn't the best), I would do that before installing a top mounted damper.
General advice. Contact licensed chimney expert for your particular situation.
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