Dimitris Tsigopoulos

Here's some first thoughts...

a) The barrel could protrude into the tub, rather than sit outside of it, in order to transfer more of its heat directly to the water
b) The stratification chamber can warm both butts and water
c) The whole structure could be surrounded by insulation, such as perlite-cob for the floor and hempcrete or other lightweight fireproof (and hopefully fire-friendly) insulation

Thomas Tipton wrote:Excellent!  I hope you have all you need to put it all together now.  Looking forward to seeing it in the real.

Yes, me too! I feel fully equipped now. It's only a matter of time and money to get this done.
Thank you to everyone from the bottom of my heart. I'll keep posting all the updates here.

Thomas Tipton wrote:What is the length of your barrel v.s. the length of your burn tunnel plus feed tube?  Barrel could be coated with a rustproofing material such as hi temp black spray paint.  if the burn tunnel is surrounded by sand/gravel, it won't get that hot at the barrel.  Since you will already be cutting and welding barrels, seems like the natural thing to do.

I can see on the SketchUp model that it fits just fine, gotta put a bit more than half-barrel is all. It's a very good proposal. And I will coat it with an anti-rust hi-temp spray paint as you said. Thank you!

Thomas Tipton wrote:Dimitris.

Perhaps you could use another of your half barrels to nest your feed tube and burn tunnel into.  My thinking is that you will then have a barrier to the infiltration of soil moisture that is impervious to heat.  Just turn it open side up, build your J-tube inside of it and pack any unused space with sand for your insulation.   The barrel will keep moisture out, even if the ground water level rises.

I really like your way of thinking! Making a barrier towards the soil with an impervious membrane-like thing, such as metal. It won't be too close to the fire so it'll be safe from spalling.
I am attaching some pictures, it looks elegant but steel in the ground worries me.
The big problem is that my soil is heavily acidic (between 4.7 and 5.8 pH). So I think it would corrode pretty soon.

Are there other continuous materials that resist heat and are good to use in contact with soil and moisture? Firebrick (or firebrick slabs) has good qualities, but is !@#$% expensive and has many joints...
Any other metals perhaps, such as bronze?

Happy week everyone. I've been puzzling over this issue for days now (how to build a sunken burn tunnel), and I'll show you what I've managed to come up with.

According to the advice I've been given by the refractory materials guy in Greece, firebrick can manage fine under the ground, while insulative materials cannot (their pores fill up with water). I've managed to come up with the following alternatives:

Option A: Simply having the entire feed tube and burn tunnel completely uninsulated, made of normal firebrick. I've seen old builds of RMHs whose cores are built entirely of firebrick, where they just heavily insulated the riser, so it's probably doable. I imagine, however, that more insulation in the burn tunnel makes for a cleaner burn, which might be why many people have stopped the firebrick-only style, so...
Option B: A Matt Walker - inspired core with Ceramic Fiber Board (CFB) burn tunnel and heat riser, with firebrick slabs at the feed tube, and more firebrick slabs surrounding the whole burn tunnel (contacting the soil and proecting the CFB). And, for extra oomph, a 1" ceramic fiber blanket wrapped around the CFB riser (probably fastened with a metal mesh).

I am really liking Option B. I am also wondering about what I should use as "glue" between all of these elements.
There must be a way to do this. Looking forward to hearing your criticism and alternative solutions!

Thank you for this advice, Thomas. Glad you liked it.

I spoke with a vendor of refractory materials here (he actually took the time to understand everything I wanted, and was excited too) and he told me that it's a bad idea to put insulative firebrick in direct contact with the earth (it can wick moisture from the soil). So I have a number of options, and if anyone wishes to suggest something I am all ears:

1) Making the whole feed tube and burn tunnel underneath the 5 minute riser out of normal firebrick. This seems like the best solution, safe under the ground, but I am not sure if it would still draw sufficiently.
2) Making the insulated part of the burn tunnel and heat riser out of firebrick splits and insulated firebrick splits (firebrick on the exterior touching the soil, insulated firebrick in the interior). He does not sell splits, and I would be afraid that the two layers would detach from each other at some point.
3) Put regular red building bricks up against the insulated fire bricks (no idea if that's an option).
4) Put some refractory cement or something else up against the insulated fire bricks as a waterproofing layer. I doubt that this can actually hold water out, and might even keep it in.
5) Open to suggestions.

EDIT: The same problem of wicking moisture also concerns the insulative slab underneath the core.

Hello wonderful people. I've made a few updates to the rocket mass heater bed, and I'd love your input on them.

1. I'd like to sink the j-tube down in the ground. That way, I can have a lower barrel top (waist-high, much easier to cook on), simplify the manifold (no need for any extra cuts for the j-tube) and increase the clearance to the top of the tent (protecting the tent roof canvas from the hottest part of the system, the barrel top).
2. Putting a big piece of hard, fire-resistant insulation under the base of the core. That way, heat from the bottom of the burn tunnel will not be lost to the ground so easily. I can also level the whole system much more easily if it's all sitting on one big slab.
3. I've surrounded the barrel and exhaust pipe with adobe bricks, with the following advantages:
- The sleepers on the bed are protected from the barrel
- the canvas of the tent's sides is protected from the barrel
- the adobe gathers heat from the 2nd hottest spot of the system, the barrel sides (the absolute hottest being the top of the barrel) without compromising the initial draw and without delaying the comfortable radiant heat that the barrel initially produces.
- the chimney exhaust gets even hotter, even faster, due to the reflection and radiation from the adobe, further improving draw.

My main concerns are:
1. Considering the sunken core solution, I'm afraid that the soil around the feed tube will store heat, increasing the temperature of the feed tube, causing a smokeback due to a "competing chimney" effect. Moving air around the usual overground feed tubes probably cools the feed tube - perhaps soil cannot. Any ideas?
2. Is my thinking regarding the insulative slab under the j-tube correct? And should I place any other insulation around the core (such as loose perlite), or can I simply put the soil directly in contact with the firebricks?

Thank you to everyone following my design

Some more pictures detailing the installation of the 5 minute riser. My plan is to put two layers of 1" ceramic wool blanket inside a 10" flue.
Apart from the mortar to cover the brick-metal connection, should I stabilize the riser in any other way?

Thomas Tipton wrote:If you were to make the top slab of your bench thicker, it won't make that much of a difference.

What thickness would you suggest to completely even out the heat distribution on the top, Thomas? (if I were to use the original design)

Ashley Cottonwood wrote:Hi Dimitiris,

I'm not sure if you are already aware of the tipi at Wheaton Labs with a rocket mass heater but there was a couple living in it over the winter, they might have some good hints as to what to plan ahead for: permies.com/t/29327

The rocket mass build for the tipi is included in the Better Wood Heat Videos: https://richsoil.com/wood-heat.jsp

I hope that helps and welcome to Permies! I'm very excited to watch your project!

Hey Ashley, and thank you for this advice! I am aware of this tipi - in fact, it's been my inspiration since I found out about it (if they can do it in Zone 5 Montana, it should be easy in Zone 8 Greece!). I also really enjoyed watching the rocket mass heater DVDs, as well as the rocket ovens DVD (another project of mine for this year). I'll definitely post all the updates to the RMH in this thread.