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rocket mass heaters

8" J-tube with half barrel bench

Benjamin Dinkel

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posted 4 months ago

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I'll probably be building an 8" J-tube with half barrels soon.

My question now is, how many barrels can I use without running the risk of too much condensation and poor draft?
I have gathered that an 8" J is roughly like a 6" BBR, so that would mean a bell surface (CSA) of 5,3m^2.

With an entire barrel exposed on top of the riser (~2m2) that only leaves 3 more half barrels (0.85m2 each, plus two half lids).

In my first design idea I would need 4 half barrels. So maybe I have to partly cover the barrel over the riser?

Any input is welcome.

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thomas rubino

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posted 4 months ago

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Hey Ben, a bit of RMH history
Years ago, I asked Matt Walker how many half barrels an 8 " J can push, and his answer was 5 barrels.
Matt's 1/2-barrel idea was well before Batchboxes became mainstream. At that time, nobody knew the parameters between them.
He built his half-barrel J-Tube in a bar back east, stunning the fledgling RMH world!
Meanwhile, Lasse Holmes & friends were playing with fire on the west coast and invented the Batchbox design, which Peter then refined into an ultra-efficient powerhouse, and inserted it into a stratification chamber (Bell), which is known today as First Generation Batchboxes.
Matt was also the innovator who disassembled his 8" J-Tube with a piped mass and installed one of the new 6" Batchboxes into the 8" piped system.
This was where we learned that a 6" batch and an 8" J-Tube were comparable in performance output.

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Benjamin Dinkel

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posted 4 months ago

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Thank you Thomas for that update in RMHistory.

5 barrels would be more than enough

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Benjamin Dinkel

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posted 4 months ago

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Here's a first sketch:

The core and a small part of the bench are going to be in the kitchen.
Then the bench goes through the wall and a bigger bench is going to be located in the living room.
At the end of the bench it connects to the existing chimney (the little square on the right). Maybe the chimney will get a metal insert, maybe I'll use it as is. There's no option for a new chimney at another point. The roof was just renewed and we don't want to penetrate it anywhere else.

The biggest challenge I see at the moment is not having any bypass option. I'll be installing an opening where we connect to the old chimney to have an option to prime the chimney.
I could also install an extra exhaust pipe on the top of the bench, allowing the warmest air to exit in the beginning. That could act as kind of a bypass. Although the exhaust would still have to travel all the way to the end of the bench. Just not sink to the bottom.
Any opinions?

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Glenn Herbert

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posted 4 months ago

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Looks like a good plan. Just remember that when you join half barrels at 90 degrees, you will have to extend the top of one barrel over the joining one to cover the whole space. This means either shortening the run covered by the barrel, or adding a partial "triangle" of barrel to fill the gap. So for your plan, the best thing would be cutting filler parts from a third whole barrel rather than alter the shape of the layout.

Benjamin Dinkel

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posted 4 months ago

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Thanks Glenn. Yes, I was counting with another barrel for scrap cuts.

Does anyone have some insight on the (lack of) the bypass? Is a "top of bench" bypass better than nothing?
If I imagine the flow of the gases, I would say theoretically yes.

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Benjamin Dinkel

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posted 4 months ago

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So I'm still wondering about the usefulness of this "top of the bench" bypass.

I would only gain about 6" height difference between the regular opening into the existing chimney and the "top of bench" bypass opening.

I will go for a T (or other opening) anyways. For inspection and priming. But I'm wondering if the bypass will be useful.

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Gerry Parent

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posted 4 months ago

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Hey Benjamin, I personally don't think a bypass at the far end of the bench with only a 6" height difference will do a heck of a lot. Perhaps just a cleanout to access the base of the existing chimney where a small fire could be initiated if the draft is needing a boost would suffice?

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Benjamin Dinkel

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posted 4 months ago

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Hey Gerry,
thanks for the answer.

I don’t see any other way. But then there are lots of benches with the chimney at the far end.

When I go check the site on Sunday I’ll keep your idea in mind. I like it.

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Benjamin Dinkel

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posted 2 months ago

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This week I started preparing the build of the RMH.

Here's the main barrel before...

During...

And after sanding.

Sanding was tedious, but so far the best solution I found. I like the finish of the 40 grit sand paper.

Then I drystacked the core. It's my first 8" so I wanted to practice and make some cuts already. Also the client wants an ash cleanout.

The feed is 16" deep. So the length of the burn tunnel is 24" and the height of the riser 48".
The roof of the burn tunnel is only (24"-8"-8"=) 8" long. Is that enough to get a good mix of gases? Seems short to me.

Here's the manifold, with a little ledge screwed to the inside top that the other barrel will slide on to.

And the manifold in place with the cut out for the core

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thomas rubino

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posted 2 months ago

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Hi Ben, I always used a 10"-12" roof with an eight-inch J-tube.
I did try a 15" one time... not a good idea.
How did you decide on an 8" roof?

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Benjamin Dinkel

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posted 2 months ago

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Somehow I had the 1:1,5:3 rule for feed:burn tunnel length:riser height in my head… but it seemed short.
Thanks for the quick answer. I’ll probably go for 12 then.

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Henry Moore

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posted 2 months ago

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I built an 8" J-tube with half barrels. Used three half barrels and the riser barrel. Worked perfectly. Four might be pushing it. Draft was fine and condensation was minimal. I'd stick with three to be safe.

Glenn Herbert

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posted 2 months ago

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I used a 1:1.5:3 rule for my J-tube, with a 24" burn tunnel and thus an 8" roof. It works excellently and as far as I can tell gives plenty of mixing.

Benjamin Dinkel

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posted 1 month ago

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I haven't decided on whether to add in some more burn tunnel roof yet.
I guess I'll feel it out when mocking up the core at the site.

Here's the plan for now.

Still thinking about the thickness of the cob to cover the top of the barrel bench. I was aiming for roughly 4" but am very interested in your input. The climate is somewhat mediterranean, but because of the high elevation (3000' 1000m) it does get cold at night there.

The barrel bench will be covered in cob and local stones will be incorporated. Then I'll finish it with a 2 layer lime plaster and silicate mineral paint.

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Glenn Herbert

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posted 1 month ago

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"Mediterranean" and "cold at night" says to me that you don't want any thicker mass than absolutely necessary, so you don't have an evening fire heating the space in the middle of the next day. I might do the strip of insulation down the top of the barrels to minimize a hot spot, and consider pavers of some sort down the center of the bench to allow a thinner top than just cob could reliably do.

Benjamin Dinkel

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posted 1 month ago

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Hey Glenn,
thanks for your input.
The costumers do want thermal mass. They have a (poorly drafting) diy mass stove now and definitely like heat storage.
Do you think cob is not structural under 3" or why do you mention the pavers?

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Glenn Herbert

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posted 1 month ago

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I think cob (even well reinforced with straw) less than 3" thick between two much thicker masses will be a serious stress concentration and will tend to crack. Putting pavers over that thin spot will conceal the crack and prevent damage from jagged crack edges crumbling.

Benjamin Dinkel

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posted 1 month ago

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True, I hadn’t considered that.
The whole bench will be plastered over with lime plaster. And I’ll have plaster mesh with me. Maybe I’ll go for 3” of cob with more fiber on there and add a strip of the mesh to unify it all a bit.

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Benjamin Dinkel

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posted 1 month ago

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Since the site is all silt and no clay we needed to organize some clay for the cob bench.
And since I'll be building a guest house soon with light straw clay and clay plaster on the inside, I seized the opportunity and we ordered 6 tons of Alpañata.

That's the same material they use to restore the Alhambra here in Granada. Once on site I'll have to make some test pucks with the available sand.

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Benjamin Dinkel

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posted 1 month ago

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Yesterday I received the 3 revision openings and the ash door a friend of mine made for this Rocket.
I'm excited to start building at the end of the month.

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Benjamin Dinkel

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posted 3 weeks ago

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Today we finally started.
First day was entirely spent on "mining for urbanite".
Good thing is we've got a lot of bricks to use on the bench.
Bad news is the mystery copper pipe and the giant hole in the wall that I'll have to patch up.

Kitchen before, with barrel where the core will probably be.

Hallway stove, the mass stove that doesn't work and needs to be removed

Mystery pipe appears between hallway stove and living room

View from living room, later we will attach to the existing chimney in the corner.

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Benjamin Dinkel

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posted 3 weeks ago

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With the old heater and benches gone we could start laying out the new rocket and the half barrel bench.

Yesterdays urbanite:

Made a cob test brick with 1:1 mix of the red clay called "Alpañata" and "Pareja" which is the "couple" of sand and sharp gravel.
It felt quite "fat" (clay heavy) but 1:2 fell apart and the 1:1 with some straw survived the drop test and dried into a hard blob without cracks.

Spent some hours cutting barrels in half and removing top and bottom. Later on I realized I could have left 2 with the bottom/top intact... but nothing some good cob can't fix.

Mocked up the core:

Laid down the first row of bricks with 1:1 clay sand mortar:

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Benjamin Dinkel

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posted 3 weeks ago

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Today I felt like we actually entered natural building mode. We had 4 different masses at the site.
Clay slip for the refractory bricks for the core.
Clay mortar for the bricks 1:1 clay and fine sand.
Fine cob with 1:1:0,5 clay, fine sand and shiv fibre.
Rough cob 1:1:0,5 clay, gravel sand mix and long straw. We'll cut the straw shorter as it was almost impossible to mix in.

First off I put down 2" of expanded clay under the core:

Then went on to build the core:

Here's the core with insulation, ash cleanout and some of the outer cob already in place. The riser always goes up so fast and standing 48" plus the foundation tall is quite a sight. The ash clean out door took quite a bit of time to be properly anchored and placed but looks really neat:

Just before calling it a day I tried to solve the 3d puzzle of the manifold transitioning into barrel bench with revision opening... it'll have to wait for tomorrow:

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Benjamin Dinkel

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posted 3 weeks ago

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Today was spend closing one of the holes in the wall, attaching the new chimney pipes, starting to raise up the dead end bench, making openings for the registers in the barrels, adding more cob to the core and mostly on the manifold area.
At the end of the day we put down the barrel over the now insulated riser. It's starting to look like a real RMH.

Not so many fotos taken today, I was really busy jumping between tasks. The time lapse is looking great though.

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Benjamin Dinkel

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posted 3 weeks ago

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Huge progress with little fotos today.
Connected the barrels to the chimney, installed another revision door, connected the barrels to each other and started cobbing around and on top of the barrel bench.
By the end of the day we had it all airtight and started a test run.

Started smoothly, no smoke, good heat in the barrel.

We have a lift off for yet another rocket!

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Benjamin Dinkel

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posted 3 weeks ago

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Most of today was spent adding mass to the bench we completed yesterday.

Tomorrow we’re going to make it look pretty but most of the mass is now installed.
The amount of material that needs mixing and moving in any build still surprises me every time.
The Timelapse looks great on the camera already, I’m excited to stitch it all together.

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Benjamin Dinkel

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Today we finished leveling the cob of the bench, added a fun round window back rest, installed a chimney thermometer and covered the chimney pipe. The rest of the day was spent on the prep of a different project on site.

Yesterday evening the clients experienced a stalling burn after 15 min. Tonight we tried again and got a good hot burn going for at least 30 min before calling it a day.

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Benjamin Dinkel

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posted 2 weeks ago

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Day 8 we just did some minor touch ups and built a wooden structure for the sofa cushions.
We had a fire in the morning and evening to speed up the drying.
And the barrel stayed warm all day. And it started easily.

Tomorrow we'll plaster everything

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Benjamin Dinkel

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posted 2 weeks ago

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Day 9 (of continuos work) was a bit of a frenzy.
We applied a coat of clay and lime plaster with a mesh.

And then a last layer of lime plaster with whitish sand and rubbed it all into its final form.

Now the clients need to patiently wait for the plaster to dry before giving it all a coat of silicate paint

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Benjamin Dinkel

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posted 2 weeks ago

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This is what the heater looks like now, almost completely dried, before painting.

The clients will wait for it to completely dry and then paint it in a blue, I also like the white.
The colors of the barrel are very pretty to me.

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thomas rubino

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posted 2 weeks ago

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A thing of beauty, Ben! Great Job!
Looking forward to seeing it painted.

For all your Montana Masonry Heater parts (also known as) Rocket Mass heater parts.
Visit me at
dragontechrmh.com Once you go brick you will never go back!

Benjamin Dinkel

Rocket Scientist

Posts: 1124

Location: Province of Granada, Andalucía, Spain

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Thanks Thomas.
It was super fun to build and performs well so far.

I was surprised to see the barrel still warm the next morning after a 45 min fire. I'm assuming it's the full fire brick heat riser that stores a lot of heat and slowly gives it off to the top of the barrel.

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