Dale Miller wrote:

Pete C. wrote:

If it's a new house there is a rather good chance that the fireplace
wall will be of conventional frame construction and the fireplace will
be a factory built zero clearance gas fired unit with a double wall pipe
chimney. No clearance / wall depth issues at all with that construction.

Well actually there are clearance issues. I work in this field and I
have heard of all kinds of problems with Plasma units above a fireplace.
Even with gas fireplace units, there are clearances from combustibles
for the mantel and anything else surrounding the unit.
All manufactured wood fireplace units have clearance issues, even
thought the unit maybe be zero clearance, there is usually a 1/2 inch of
clearance around the box to combustibles. All wiring should be at least
2 inches from the pipe and unit, more is better. You would be surprised
at how hot a gas fireplace unit that is not "furnace rated" gets.
All manufactured gas and wood fireplace manuals include the
specifications of clearances in them. It is best to thoroughly read the
manual that comes with it to makes sure the clearances are met,
otherwise it could be dangerous to yourself, family and home.

That is not relevant to the subject at hand, installing an electrical
box and LV box in the wall surface above the mantel. The point is that
with the common factory built ZC fireplace unit, there is no issue of a
thin veneer type wall surface an inch away from solid masonry and a
normal depth box can be easily installed.