Our firebox is 3/8″ with the water jacket 3/16″ like all other manufacturers. The front and bottom of the water jacket are 3/8″. Most manufacturers sold today have just 1/4″ fireboxes.
We have found the pump being in the bottom to not be any better as the water has to move from the intake to the return no matter where the pump is located. We have found that the particulate matter which always ends up on the bottom of the stove can and has harmed the pumps as well as the possibility of trash going into the system further down the line and clogging the heat exchanger.
We did a test with dye before a top was welded on to see how the water worked around the firebox and found it really did not matter where the pump was placed. In minutes all the water was the same color. Temps also taken at different locations varied very little. Our pumps are approximately 12 inches from the top of the water jacket.
All HyProTherm furnaces utilize Thermal Ceramics Superwool Plus R low bio-persistent high temperature insulating wool as the firebox insulation. Superwool Plus R contains no binder to burn out and is mechanically needled to maintain structural integrity at continuous temperatures of up to 1832 F. , indefinitely. The higher density and higher fiber index of Superwool Plus R yields very low thermal conductivity values at temperatures above 200 F. compared to other fibrous insulations. Superwool R fibers are used in a variety of applications from aerospace and automotive heat shields, industrial furnace linings in power generation, metals processing and petrochemical applications, as well as many household and commercial cooking appliance applications.
We have a complete document about the history of insulation in our furnaces, you can view that by going here.