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Decided to stop waffling and get on with the core casting mould now that I have decided on the refractory formula. I was going back and forth between an 8" vs 6" system but really wanted to try using the two elbows approach to see if such a simple approach would yield good results. Hope the burn chamber isn't too short!

Looks promising Rhett. What do you have for wall thickness there between the inner mold and the outer? I think you could probably pare that down a bit and save some material, and save you some serious mixing and lifting. All that volume in the containing box adds up quick, a reduction of an inch all around would save you a lot. If you are already in the 3"-4" range then I probably wouldn't go much thinner, but it looks like you have more than that.

Nice man, sounds like you have a good handle on your plan. They make fantastic "instant on" radiators! Can't wait to see your build.

I'm really curious to see how the elbows will perform, both from a burn perspective and a feeding perspective. I know you said you had seen or heard of people doing this but I have not seen a video or write up on it so I am eager to see if it works well. It sure seems like the shape is a natural as a feed tube, as it should funnel the wood coals and ash into the burn. It has a built in curve to direct the fire up smoothly. And if the riser is tall enough and insulated enough it seems like it should get good, thorough combustion without introducing turbulence that might fight against good drafting. Well at least that's my theory!

Anyway thanks for all the help. You guys have been great. Long live team castable!

Well, at the last minute I decided to go with 8" pipe elbows. Holy crap this stuff is huge! It def took out some of the mass of the concrete. I needed to tack a little onto the top mould to make it deep enough. I can def see why people say the 8" systems can put out more heat. You can just pack a ton more wood into there.

Yeah, it's almost twice the heater. I know you aren't going to have mass, but for folks who do, the eight inch tubes through the mass provide an exponential increase in heat exchange surface area. It's a big step up in every way.

I can believe it. And while this first one is for a shop that I am only in a few hours at a time on the weekend, if the experience is good I would like to convert my firepit into a rocket stove powered warming bench and that one will need mass.

First of all, no fair. You guys didn't warn me how much work mixing and packing this stuff would be! Maybe I kept the mix too dry but working it wore me out!

Second, nothing has gone right today. It feels like everything that could go wrong did.

All that said, I have a cast core and it's drying away. On to the riser.

Nice! Yeah, it will tire you out for sure. If you go back and read the first page or two of my heater for my home thread, you'll see I had just about everything go wrong when I cast my core. Can't wait to see it burn.