Well I have over 22" of cob loaded with large pieces of slate over the 8" pipes in mine. But A better answer is 6" or more. You must protect those pipes from being crushed, and you want as much mass as possible.
I see no rock in your mass? Rock is denser than cob so it holds more heat longer (this is a good thing ) Using as many rocks as you can will help your bench stay toasty warm all night. It also means you make less cob (also a good thing).
Jeremy, I remember Ernie Wisner saying something like you heat about an inch per hour and dissipate the heat about the same which gives you an estimate of how long you want to tend the fire and how long the heat will last. Cob or rock or both, I'm not sure.
Also, it depends on what you plan on using the bench for. If for sitting, most chairs are designed to be about 18 inches from the floor. If for a working bench of course much higher depending on how tall you are. The top of the mass should also be much thicker than the sides which don't capture as much heat.
I think with the 8 inch(?) you built, you'll find that your rocket is really gonna roar and heat the mass up very quickly. But of course, the great thing about cob is that you can just add or remove if you find what you put down is not working for you.
I agree with thomas also about using rock in your mass.
Silence is Golden
posted 1 year ago
Awesome thanks. It is an 8 inch. I plan on collecting some more rocks tomorrow. I also have a 2'x4' piece of granite counter top i may set on top and cob over it.
Jeremy: While scrolling thru your photos I saw something . What dimensions did you use on your feed tube (and the rest of the core) ? It sure looks like your feed tube is 9" wide ? Tell me I'm wrong....
Not all who wander are lost... J.R.R. Tolkien
posted 1 year ago
Feed tube is 6x8 inches. It's slightly smaller than optimum for 8 inch pipe but ran pretty good during my test fire.