The drawing, sketch (pic.11)
dated: 25/25/18, 09/26/2018
Without measures again! Otherwise normal. It could have the airheating mantle all the way down.
There is ash inside the hearth wall, so no serious cooling! And it’s comming back.
Your last notice about the nozzle measure = 5/16" = 7,8125 mm.
The (bolted) seam between the silo and and hearth? Silicone or gasket?
But first a “grand question”! The silo lid! When did you a smoking backflow test last time?
Shure not at last drive! You have the Kirby forward/backward blower…
Temporary woodplug, if no backflap. Don’t waste tape! (At the air intake)
Don’t fill-up more than 2" over the nozzles before you can confirm that the nozzle-blast REALLY works!
Are your nozzle-nuts properly welded? FULL SEAM?
It would not hurt to check the ash-lid too!