Rocket mass heater design for a tipi in Greece

Hello to this wonderful community. I've been lurking for a long time, and I've just been blown away by all the help people have been giving each other here. Well done everyone.

I'm in Larissa, Greece, USDA Zone 8. Super hot, dry summers, +40C, and super cold winters, reaching down to -10C at the worst (not as rainy as I'd like though). Just trying to make a 6" vanilla j-tube rocket mass heater for a tipi without too many mistakes, so I'm posting my sketchup model here for your scrutiny. You can hopefully see all the parts clearly.

My main concerns are:

- Are my dimensions correct (consider that the bricks are around 220mm * 110mm * 55mm)? I've had trouble figuring out the exact way that bricks should be placed. Is there any sketchup model or a precise drawing like the ones that Peter has for batchboxes up on his website, but for the classic j-tube? (Right now I have a 160mm * 160mm system with a P-channel at the entrance).

- I'd like some help calculating the ISA. Do I include the vertical barrel's surface, or just the half-barrel bell? Is it a problem if I have less or if I have more of it? And do the tables only work for batchboxes or for j-tubes as well?

- Is the (welded) manifold 45 degree connection between the vertical barrel and the horizontal half-barrels OK? I thought that by skipping a pipe at this point, I'd remove any constriction, making the flow more even.

- Is my layout of firebricks and insulating firebricks correct? Should I wrap the insulated firebricks of the heat riser in a ceramic blanket, or are they going to be fine just like this?

- Is the chimney too tall or too bendy? Could reach up to 5-6m if it exits from the top of the tipi...

- Would a 5-10cm perlite-clay base suffice for this build? Does it need a stone or gravel foundation (It's gonna sit on the ground, not a wooden floor)?

- How do I seal between metal and cob? How do I seal between metal and metal? And how do I seal lid on top of the barrel, which I'd like to keep openable (perhaps with a fireproof gasket - would love some pictures)?

Looking forward to your answers, and thank you for providing me with all the tools to make my dreams come true for all these years.

Hello Dimitris!

Welcome to Permies and please allow me to be the first to commend you on your excellent Sketchup skills!  Personally, I think you have an excellent plan for your RMH and I do believe that should work splendidly.  There are others here with much more experience than I have who will no doubt want to share their expertise, but for all practical purposes it looks like you have done your homework.
Well Done Sir.

Thomas Tipton wrote:Hello Dimitris!

Welcome to Permies and please allow me to be the first to commend you on your excellent Sketchup skills!  Personally, I think you have an excellent plan for your RMH and I do believe that should work splendidly.  There are others here with much more experience than I have who will no doubt want to share their expertise, but for all practical purposes it looks like you have done your homework.
Well Done Sir.

Thank you Thomas!!! It really feels good to know that I've got it. It's only been a couple of months now that I looked into RMHs seriously, but I've been absorbing all the info on permies and donkey32 that I came across. I tend to obsess over specific things each time and focus solely on the problem at hand (and talk to everyone about it incessantly...), and this was one of those times.

I've used SketchUp for quite a few years now, and I have to say that I prefer it compared to everything else in design. If anybody ever needs a model, hit me up with a private moossage, and I'll make one for you. Skills could improve but they're good enough for what I've been needing to do.

Also, here's the file in SketchUP 2013 for whoever is interested.

Now, just be sure you address any concerns about insulating the burn chamber from a wooden floor structure or otherwise just insulating the mass from the ground. (It acts as a heatsink).  

Are you familiar with the 5-minute riser?  You might find that innovation can improve your RMH performance right out of the box.  The square firebrick riser is a tried and true method, but the 5 minute riser is simpler and if done correctly, has proven to be an inexpensive and very durable, high performance riser.

Thomas Tipton wrote:Now, just be sure you address any concerns about insulating the burn chamber from a wooden floor structure or otherwise just insulating the mass from the ground. (It acts as a heatsink).

Will it suffice if I use a clay-perlite mix 10cm thick for the base of the rocket mass heater, with another 10cm of gravel underneath (sitting on a raised mound of earth)?

Thomas Tipton wrote:Are you familiar with the 5-minute riser?  You might find that innovation can improve your RMH performance right out of the box.  The square firebrick riser is a tried and true method, but the 5 minute riser is simpler and if done correctly, has proven to be an inexpensive and very durable, high performance riser.

I would love to have a circular riser, but I haven't figured out how to make the connection between the square and the circle. Do you have any threads to point me to?

You have a good looking design which I believe will work well. The barrel-to-barrel transitions look excellent. I would say that you should count all the barrels for ISA calculations; it appears that a 6" batch box is similar in power to an 8" J-tube for ISA calculation, so I would go lower for your 6" J-tube. Given the easy flow characteristics of your layout, I think around 10% less than a 6" batch box would be fine. You may be able to handle as much as the batch box - if you can test the operation and adjust barrel ISA accordingly before finalizing the bench, that would be ideal. A plunger tube at the chimney connection would be an easy way to do this. Run the chimney pipe down into the bell space to a few inches from the floor, with a moveable section at the bottom that can be raised or lowered. The higher the bottom of the exhaust, the smaller the effective ISA. Arrange a cleanout/access opening so you can reach in to adjust the plunger.

I think insulating as you have described sufficiently satisfies any safety concerns as per the floor.

When thinking about that 5 minute riser.  Consider that you are lining a stove pipe with ceramic fiber blanket.  As long as the round opening left in your stove pipe is larger than the riser hole, meaning that the ceramic fiber blanket completely covers the square opening of your riser opening, then the stove pipe can just sit in place.  You can always build up the base with some cob or furnace cement for good measure to make sure it stays put.

For instance, assuming you are doing a 6 inch system,  you would take a 8 inch, (or your metric equivalent) stove pipe, cut to the desired length, Then figure the length of ceramic blanket required to line the inside of the pipe so that the two ends meet with a friction fit that will keep the ceramic blanket liner in place.  This kind of liner, if done well, can reach high temps very quickly and last for years.  As long as care is taken not to allow gasses to infiltrate between the ceramic fiber and the stove pipe, there shouldn't be any issues.

If you are not familiar with the plunger that Glenn is talking about, just think of it as an adjustable downward extension on your exhaust flue that will allow you to tap the exhaust gases from a higher, or lower point in your hollow bench.  Essentially allowing you to fine tune your system performance in accordance with its particular CSA.

But I don't want to muddy your thinking with too many possibilities.  You have a great design there.  

The CSA of a 160mm batch-box must be around 6m² (remember that for a 150mm system it would be 5.3m², and for a 175mm system it would be 7.2 m²). I calculated it according to Peter's website (300 times the CSA, which in this case is 0.020m² * 300 = 6m²).

Removing 10% from the 6m² according to Glenn tells me that I need to have a maximum ISA of 5.4m²

My original bell was too large for that. I thought of lowering the total ISA by removing some length of run, as well as reducing the height of the barrels by filling their bottoms with insulation. Here's how: By shortening the half-barrel run a little, I approached that number a bit. Then, by adding a layer of perlite-clay 10cm thick INSIDE the bottom of the half-barrels, we can approach it even further. The final number I've managed is 5.89m² as can be seen in the second picture (but the barrels are actually slightly smaller in real life than that, so probably we are even closer to 5.4m²). Close enough?

I am a bit worried that the stratification chamber is not high enough now for stratification to occur, so it's instead going to function more like a flue and less like a stratification chamber (an oil drum's diameter is actually 57cm, a half-barrel's height is therefore 28.5cm, and now it's only got 18.5cm of height after adding the internal perlite-cob). I'm also not sure how high to place the exhaust at the end.

Regarding the plunger idea, I think it's very smart but needs a tad bit more involvement than I'm willing to have when using this heater. Thank you for that thought, I'll keep it in mind for the next project! And the cleanout access point acting as the place where one reaches in to adjust the plunger, also brilliant.

As can be seen from the pictures, the tipi's floor is made of 10cm of perlite or pumice or other lightweight insulative gravel fill (open to suggestions), and then 10cm of perlite-cob for insulation. On top of this sit the mass heater's core and the half-barrels. Inside the half-barrels is another extra 10cm of perlite-cob to lower their ISA.

Am I thinking of this correctly? Is there a simpler solution?

Thank you for guiding me through this with such precision!!

Dimitris.
I don't think your CSA is too large.  I think you are probably suffering from what we call "Analysis Paralysis".  There are other factors that will play into how well your stove works, such as water content of your wood, how well your flue draws, and other things.  Don't over think it too much.  What you need to understand about the plunger tube is that it will allow you to draw off gases from the top of the bell, that are still very warm, or gases that have cooled and settled to the bottom of your stratification chamber.  If you can arrange it so that your exit tube can be adjusted, you can try it from the very bottom of your bell.  If you find you are not getting enough draw, then you can take it out and shorten it so that you are drawing gases from a little higher up where the gases are hotter.  It can be a trial and error thing.  It doesn't have to be anything sophisticated and infinitely adjustable.  My guess is, one or two tries and you will have it where it works best for you.  Or you could just set it so it draws from halfway up the bell and call it good.  I would not remove any CSA from your proposed system.

With the plunger tube, you do not need to agonize over building the bell cavity just right; raising the plunger inlet effectively decreases the ISA with no further action, as the hot gases will more readily reach the exit. You would only touch the plunger as you were fine-tuning the system. Once it is running right, you never adjust the plunger again.

Dear Thomas and Glenn, I thank you for this advice. Indeed the plunger is much simpler than I thought at first. I originally thought that I would need to constantly adjust it, but now I realize that it can simply something that I fine-tune during installation and then don't mind it any more. So that's what I will do!

So I suppose that I will weld the tall vertical chimney at the end of the half-barrel (to make sure it's airtight), and then put a small piece of a slightly smaller diameter flue within its bottom end, which can be slid up and down, thus regulating the intake height? Any videos or threads about this that you are aware of?

If you search Permies for "plunger tube" you will find a lot of mentions and descriptions. This thread has pictures; although it talks about an adjustable bypass it is equally applicable as a plunger. https://permies.com/t/151891/Telescoping-stovepipe-adjustable-bypass-split

Glenn Herbert wrote:If you search Permies for "plunger tube" you will find a lot of mentions and descriptions. This thread has pictures; although it talks about an adjustable bypass it is equally applicable as a plunger. https://permies.com/t/151891/Telescoping-stovepipe-adjustable-bypass-split

Excellent thread Glenn. I feel much more secure in my design now.

Only question remaining in my mind is about how to seal the manifold brick-to-metal connection.

Manifold brick to metal connection.  How about soaking some canvas  or burlap  in a bit of fireplace cement, or clay for that matter, and slapping the pieces on like you were laying up Plaster-of-Paris for the cast on a broken bone.  

Glenn, what say you?

I expect something like that would work fine. Just packing around the duct with a few inches of cob would work too. Worst case, you need to put some stove door gasketing around the duct.

Dissimilar materials expand/contract differently which means this constant movement will always be wanting to shift the joint. To accommodate for this using a flexible material like stove gasket rope, ceramic ‘tape’, or even rock wool would help keep the joint properly sealed.

Greetings everyone. I just wanted to thank you for all you did for me, helping me out with all my questions. When spring comes along for good I want to begin building this system and experiencing the power of rocker mass heaters first-hand. I will keep you all posted!

Today I came upon Uncle Mud's video about his lightweight rocket mass heater: https://permies.com/t/151844/Uncle-Mud-Rocket-Heater-Bench

Of course, in Mud's case the half-barrels are actually full of cob, and the gases move through a 6" stove pipe inside the cob.

However, that gave me an idea to solve a problem that I'd been concerned about: Namely, the even distribution of heat on my own bench/bed. I've read posts by people having trouble with hot spots on the tops of the half-barrels, with the rest of the bench remaining colder. What if, like Mud, were I to turn the barrels upside down? It would require me to buy more steel and do much more welding (a learning goal for me this year), but the end result would probably be a much more even distribution of heat across the bed. I believe the SketchUp pictures here make this design change more clear.

What do you guys think?

EDIT: Forgot to mention that I plan to use rebar, tubes or other such reinforcement "beams" on top of the half-barrels, under the steel plates, to keep them from opening up and to hold the weight of the cob, mattress and people resting on top of the bench/bed.

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!

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.

Dimitris,

I think you are entering the phase where one begins to second guess everything.  Yes, having the barrels with the open sides down will lead to a concentration of heat on the slab along the lines where the cob, or whatever you choose to top your bench, is the thinnest.  This is also the simplest configuration for your design.  If you were to make the top slab of your bench thicker, it won't make that much of a difference.

I'm sure you are already aware that making the changes you put forward add a considerable amount of material and effort to your heater for what may only be a marginal improvement in how evenly the bench is heated.

That said.  It's your toy to play with as you please.

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)

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?

Dimitris Tsigopoulos wrote:

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)

Four inches.  Also consider that you can temper the amount of thermal gain by throttling back on how much you feed it, giving the mass time to equalize.

As long as your riser stovepipe sits nice and solid, a couple of dabs of mortar or refractory cement should be more than enough.

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

Dimitris,

Soil is a poor insulator and will act more as a heat sink than anything else.   On the other hand, if the feed tube is surrounded by sand, which is more of an insulator, then you may be right to have concerns.

As long as you build the feed tube along the lines as your drawing indicates, I don't see you having any problem with secondary chimney syndrome.  As long as you establish your fire and get the draft going you should be good to go.  I very much like your fine tuning of your design.

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.

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!

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.

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?

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.

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!

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

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.