Design review before I build: 6” J-tube greenhouse RMH with oven, bypass & return bench

I’m designing a 6-inch J-tube rocket mass heater for a 12’ × 20’ greenhouse near Knoxville, Tennessee. Before I pour the dedicated slab or mortar any of the heater, I’d really appreciate a critical review from experienced RMH builders.

The available heater footprint is about 49.5” × 86”. I already have a 24” stainless removable-lid drum, a salvaged wood-stove door/firebox frame, roughly 128 standard-size firebricks plus five larger refractory slabs, and 2” ceramic-fiber board for insulating the hot core.

The combustion core is intended to be a fairly conventional 6” J-tube. The unusual part is downstream of the barrel. Hot gases descend into a collector, then rise to a raised cooking oven. I’m considering splitting the flow around the oven and recombining it afterward. From there the gases descend into a thermal-mass bench, travel to the far end, turn, and return along the opposite side to the chimney.

Because the chimney is near the barrel end rather than at the far end of the bench, I’m also proposing a short startup bypass from the collector area directly to the chimney. Once draft is established, the normal route would be through the oven and mass bench.

I’ve attached a short PDF showing the proposed arrangement, materials, known dimensions and the questions I’m hoping reviewers can help answer.

Nothing permanent has been built yet. The foundation is excavated, but the slab has not been poured. So if the geometry, oven arrangement, bench return, bypass, chimney arrangement, clearances—or the whole idea—needs changing, now is the time for me to hear it.

I’m particularly interested in criticism of the gas-flow path and restrictions. I would much rather redesign it on paper than discover a drafting problem after several hundred pounds of masonry are in place.

Thanks for taking a look.