My first rocket mass heater. How off am I?

I am just about to cast the burn chamber and heat riser for a 5 inch CSA rocket stove/heater. It is going into a reasonably well insulated 128 square foot/896 cubic foot house. Yes, tiny is the word of the day. Anyway, I am looking at 2 inches of refractory around the burn tunnel and feed tube and 3 inches around the heat riser. I have a feed tube length of 7 inches, a burn tunnel length of 14 inches and a heat riser of 28 inches. These measurements being from edge to edge. If using Peterberg's centerline to centerline measurement method they would 4.5-9-25.5 respectively. I am planning to use 1 part refractory mortar, 2 parts ball clay (it was free and my profesional potter friend said it could be fired to a cone 12 safely), and 8 parts vermiculite. I would appreciate any comments anyone has. I did model this configuration sans the insulation and it burnt and drew well, though my experience is mostly with stoves and not heaters. I do plan to add some mass to retain some of the heat though space will dictate how much. Right now I would appreciate some heating during the burn, but I am mostly after a good stove to cook on. Well, appreciate is a weak word, I would love the heat. I'm getting a little tired of running the 1500 watt electric heater which is currently heating the space. By the way this is virtually my first post here and what I consider my first real rocket mass heater build. Thank you in advance for your comments.

Hmm... I just found a couple hundred pounds of mason's sand. My workshop is filled with many wonderful surprises, read I'm a but of a picker and a pack rat. LOL Anyway, could/should I add that to the refractory, maybe 1-2 parts? To help with the shrinkage and to increase it's ability to withstand abrasion in the feed tube.

Jim, just to give you ideas

https://permies.com/t/35569/rocket-stoves/Range-retrofit

http://donkey32.proboards.com/post/16159/thread

Thank you, Satamax. The one thread I had read. I have an old stove sort of like that, that I love. But I can't take up that much space in this version of the tiny house. I will scour the other thread this afternoon. I appreciate it.

Does anyone have anything to say about my insulation core/heat riser recipe. I'm hoping a healthy percentage of refractory mortar to clay ratio will help with shrinkage, crumbling, and wear. It is worth 60% of project cost to add from what I've read, is the dry mortar material not as robust as the wet castable refractory in the buckets? Is 8 parts vermiculite unnecessarily high? I want it insulative, but is that too high of a proportion as to make it too crumbly?

Jim, i would do the inner layer with no vermiculite or perlite. But that's just me. Check the range retrofit for my method of building if you like.

Max, I'll think about how I could go about casting an inner refractory mortar only piece and then a clay and vermiculite insulating layer around it. I see what your saying, basically creating a hotface and putting a cast insulative layer around it.

As for your stove, I had read all about before. I love those stove and have a giant in storage among e everything else that I hope to use someday in someway. I really wish we could get those refractory flue pieces over here. All we have are clay flues which don't much like the serious heat the rockets generate. My stoves will take them out, I'd worry about what a real heater would do.

Thank you for the suggestion about the hotface. Would you think an inch of the refractory mortar surrounded by 2 to 3 inches of insulation will work well?

You could also make your refractory mortar flues yourself

But about one inch of refractory, yep, that would work!

I did a test of "buttering" about 1/4-1/2" of fireclay/perlite mix onto an inner form, then filling with castable insulating refractory. I tried three different ratios, 3:1, 1:1, and 1:3 fireclay/perlite. The 3:1 had a bunch of shrinkage cracking and tended to separate from the refractory, but the others did fine and were stronger than the insulating refractory surface after firing all the test bricks to cone 06 in a kiln. So that is one possibility.