Depending on the BTU output of the gas fireplace, the heights can differ. Refer to your gas fireplace manual, and it should be stated in there. For ours, an 8" wood mantel need a minimum of 22" above the top of the heat outlet. However, a noncombustible mantel can be installed anywhere above the heat outlet. You should be able to find your manual by doing a quick google search of your fireplace.
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ceiling like ours
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