Hope this question isn’t too far afield of the GBA mission.  I have lots of respect for people who comment here so hoping someone has run into this question before.

I’m looking for code, or physics illustrations, about heat dynamics above and behind a fireplace.

Situation: the front of a combustible (in this case a TV)  is set back X inches from the back of a “zero-clearance” fireplace (in this case a direct vent, no fan).  A mantel (quartz or steel) with min. 1” air gap could be placed above the fireplace unit if truly impactful. 

The  manual only speaks to and shows clearances directly above the unit, and in front of the unit.  They do not speak to the area behind the unit except the 3” clearance from vent pipe (a non-issue in this case).

My hypothesis is that the area both behind and above the fireplace is zero-clearance.  Otherwise the wall the fireplace is mounted on would require specifications to be non-combustible for at least 15” above the unit, which does not appear to be required at all. 

Taken literally this would mean that combustibles set back just 1 mm behind the back of a zero-clearance fireplace could be 0mm from the top (but still 1mm from the back) of the unit.  But common sense tells me that radiant heat doesn’t arbitrarily stop behind the fireplace.  So it seems there may be (should be?) a guideline something like:

TYPICALLY, unless otherwise noted by the manufacturer, for every inch that combustibles are set behind the back of a zero-clearance fireplace the required vertical clearance is reduced by X%. (In reality would likely be non-linear drop-off rate, but you get the idea.) A diagram of typical heat range that includes the air behind the fireplace would be ideal.

I’ve been trying for hours over days to find clear guidance on areas behind/above fireplaces and can only find  anecdotes and generalized advice unsupported by code or science or numbers.