First photo is the firebox (which is 30 inch dia x 30 inch long) and fire-tube heat exchanger that sits above the firebox. The firebox is insulated with firebrick on all 4 walls, there is a top ss baffle plate which directs the flue through a pass, then through the fire-tubes and out the stack. The idea is to have the fire burn hot and have heat exchange in the fire tubes.
Second photo is the front and rear walls of the firebox. The rear wall also has water tubes to pick up radiant heat from the fire but still allow the fire to burn hot.
Third photo is the firebox and fire tubes sitting in the boiler shell.
The firebox is surrounded by water on all 4 sides, including the front wall. I would not include a front water wall on build 2.0. It has really complicated things that I would like to do with air flow.
Fourth photo is the more or less completed boiler, showing access doors to the tubes, which I brush weekly, firebox, and ash pit door at the bottom. Water capacity is around 225 gals. Wheels were to move it from shop to its building.
Fifth photo is inside the boiler building. You can see the 300 gallon storage tank, and above it an expansion tank. Air is supplied from a retrofitted oil furnace burner. The fan is on a timer and only comes on once a day for 30 or so minutes when I first start the fire, after that it's natural draft. I try to burn the fire hot enough to keep the stack temp around 300 and especially to have no smoke! That's it in a nutshell. I start the fire at 4 pm most days, water temp is around 120 F, burn it hard for 4 hours, and then shut it down (close air dampers) around 8 pm with the water temp around 190 F. Next day repeat. It is very different than the "store bought" units and not everyone's cup of tea, but it works well for me.