6" J-tube RMH with half barrel stratification bench

Hey fellow RMH enthusiasts,

I got hired to build another RMH. The clients are pretty set on the "classic" J-tube, barrel and bench design and want to try to use a lot of local material while keeping the build on the simple and cheapish side.

So the first design challenge that I wanted some input on was on how to build the stratifaction chamber / bench.

I like the idea of using half barrels laying down and then adding cob on top. We'll have volunteers for mixing cob and will hopefully find suitable material on site.

How are your experiences with that style bench?
Would a cover of 4" of cob (on top of the highest point of the barrel) be enough?

The total volume of half barrels seems relatively small and the surface too.

Congrats, Ben, on another job!
I have not used the half barrels myself, but it is a tried and true system.
How many barrels in a row were you thinking?
Matt told me that with an 8", you could have up to five.
As far as how much cob on top, 4" at the high point might be more than you want, 4" at the top means 6"+ on the low points.
I would start with 2"+ on the high point and see what your heat transfer is like.
If you were up north and this needed to burn longer, a 4" cob layer would be perfect.

When you cut your barrels, leave an inch or so on the ends to maintain the barrel curve.

Thank you Thomas, and thank you for the reply. As always fast and on point with the info!

I think I can fit 3 in a row, roughly 9 ft. I'll add some sketches soon.
It'll be a 6" system.
I would have the barrel on top of the riser and then 3 half barrels in the bench as heat shedding surfaces.

The location is in the mountains here in souther Spain, relatively mild, USDA Zone 9a. So I'll plan with 2" and see whether it'll produce hot spots or not.

I'm going to turn this thread into the documentation of the design process and building of this new RMH.

Here's a photo of the living room it will be built in.


The bench will go where the sofa is.
The exhaust will be running up where the stove exhausts at the moment.

We discussed getting rid of the chimney/fireplace. It won't be fun but will enable me to come up with a functional J-tube with barrel design.

The clients asked me to brainstorm again whether it would be possible to keep the chimney structure, They like the looks of it.
So far I haven't come up with a solution.
The main problem for me is, that the barrel comes too far into the room, not leaving enough clearance to the door on the left.

Am I missing something? Do you have an idea of how to incorporate it into the design?

Benjamin, I remember your iteration of the cottage rocket, that looked great as a finished heater. Could you possibly use a similar design to save space, then begin your bench (not touching the existing stone)inside the chimney? That is some cool stonework and maybe you could preserve all or most of it

Hi Ben,  how about some measurements of the space?
I assume the current stone work is hollow to allow the current chimney access to the roof.

Off the  top of my head)  There is no rule that the core and bench must be 90 degrees; perhaps placing the core at a sharper angle to the bench, leaving room for the doorway.

thomas rubino wrote:Hi Ben,  how about some measurements of the space?
I assume the current stone work is hollow to allow the current chimney access to the roof.

Off the  top of my head)  There is no rule that the core and bench must be 90 degrees; perhaps placing the core at a sharper angle to the bench, leaving room for the doorway.

Ben, I am sure you will get some excellent input from the group, and with that added benefit, input from other sucessful units used in similar fashion. My first thought was when you mentioned a J tube, was don't rule out the possible iteration of the "shorty 6" Which has proved it can have a "side winder" right angle production of heat, that might blend super well to your corner situation.  Again, proven.

The only reason I bring this up, is the little extra work in building, can produce 1/2 the times to add wood, ( twice or better capacity )  at the same effciency. year after year, this will be even more appreciated.  A really big plus in my mind, is sitting in a chair, you can visually see ( if you have made a window in your door ) the status of the fire, NO getting up. It's just there for your observation. A wonderful plus.

These are just thoughts, or suggestions, I am sure you will get some good advice for your project!

Best of success
Scott

Thank you Rico, Thomas and Scott.
I'll upload the drawing of the space as soon as I finish it, probably tomorrow morning.

But the opening is roughly 35" high, roughly the same in width. Depth then a bit less, considering the triangle they span.

Building a type of bell in there seems complicated, but I had also thought about Shorty. Not sure how the build would work, not being able to access the core well from the sides.

The cottage rocket is very limited in wood intake and would also not really solve the barrel being big in the room if put in front of the existing chimney.

Can't tell the room layout, would the barrel/feed being  at the other end of the bench leave it more centered in the living space and leave you free to be creative around chimney?

Not a bad idea. But what about the barrel "kissing" the exhaust? And no bypass option I guess.

Attached a quick sketch of the room, measurements in cm. Also 1 square is 10cm.

Barrel kissing is to help with cold plugs. Part of the creativity around the exhaust could be some sort of bulge to collect the hottest gas with some sort of gate to create a bypass there. [edit: on reflection, there are likely more elegant solutions to a bypass over there.]

Something that can affect practical core location is how good the draft in the chimney is. If it draws by itself in weather where you might want a fire, you don't need to worry much about a bypass. If the draft usually needs help to start, you want a bypass.

This makes me think of a design feature I don't recall encountering discussion of yet- with a stratification chamber, what are the benefits/trade-offs (if any) of the exhaust pipe plunging down into the bell like a straw vs. the pipe exiting the side of the bell down low and running up alongside of it?

That setup is known as a "plunger tube" and used where it is not convenient to exit the side of the bell. It has an advantage that the height of the tube bottom can be raised or lowered to fine-tune the amount of heat extraction and the temperature of the chimney exhaust. The upper part of the tube needs to be insulated to reduce heat leakage from the top of the bell.

Thanks for all the valuable input.
I'll draw up 3 options for the costumer.

- classic J-tube with barrel left at the exhaust, taking down the existing chimney structure

- classic J-tube with the barrel on the right and exhaust on the left, conserving the chimney. problem: no bypass possible

- shorty core BBR integrated into the chimney

Benjamin - I would make it a priority (if you don't already know) to get the current chimney draft situation clarified, as that will affect how functional the different options are.

Benjamin Dinkel wrote:- classic J-tube with the barrel on the right and exhaust on the left, conserving the chimney. problem: no bypass possible

If there is an exhaust flue exiting low from the mass, couldn't there be a bypass right there, even at the opposite end from the firebox?