Sizes stack diameter and height from a physical draft model, driven by mass flow rather than CFM — the only way an altitude deficit shows up at all.
Areas are computed internally — never enter a square-inch figure here (D3).
Limited by throat (0.54×) — stack (0.74×) and firebox (0.62×) are also subpar. A pit runs at its worst subsystem, not its average. See the bundled fix below before cutting anything.
A chimney is a density-difference engine. At altitude the draft that drives flow weakens — but so does the inertia of the gas being moved. The two effects cancel exactly in velocity: your stack gas moves at the same speed it would at sea level. They do not cancel in mass. At 7,300 ft you're delivering 76% of sea-level oxygen mass through an identical stack at an identical velocity — the deficit is invisible in CFM and shows up only when you compare mass flow, which is why this calculator never compares CFM to CFM.
Stack undersized — 0.74× required mass flow. Step up to 8″ NPS pipe, or add height: draft scales roughly with the square root of stack height, so ~12″ more height recovers some margin without a diameter change.
Throat is the choke point — 120 in² against 224 in² required. Nothing downstream fixes an undersized throat; it starves every calculation after it.
RF gap reads Optimal only because the throat is starving it. Open the throat alone and the gap ratio falls from 1.09× to 0.58× — Unoptimized. Widen the gap to at least 224 in² (≈7.7″ at the current 29″ plate width) in the same pass.