Thanks for the Guelph seminar in September. Things froze up here too soon to get much done. Am working on plans for a heating wall as Ernie suggested to me. Just wonderin', what thickness of cob from heat pipe to exterior would be the minimum you'd suggest?
If you want to follow the ASTM standard, it would be 5" cob (or 5" total masonry between the heat-exchange channel and the exterior surface), plus a 4" air gap to combustibles.
For just practical / structural purposes, about 4" of cob is about the thinnest most people can build successfully to support the hollow pipes inside a wall or bench. If you think of those 8" pipes as formwork for a masonry arch, you know you want the sides of the arch to be stout enough to keep that pipe from collapsing outward.