Peter van den Berg

Heather Arvensis wrote:There are windows and doors everywhere… a tree trunk in the middle of the room… and the exhaust is already installed so lots of variables to work around. What if we did something like this quick, not to scale sketch?

Possible, this design has the better papers.

Heather Arvensis wrote:Since we are building mostly with cob I can do a bench wider at gas receiving end and narrower at the other end. I would maybe need a baffle or wall between barrel area and exhaust since this design would have barrel in front of exhaust?

Provided the following is met: the entire bench the same width throughout. There will be a warm gas stream along the ceiling of the bench and a colder one along the floor in the opposite direction. The less friction (again!) the better this will work. The division between colder and warmer gases isn't at the same height all the time, this varies quite a bit. Most people think like you: trying to "steer" the gases in the right direction. However, this could also be done another way, by making use of the temperature difference of these same gases. The warmer ones are against the seat, as long the forward velocity of the gases is slow, shortcutting to the low positioned exit won't occur. Feeding into the bench should be as high as the bench internals, and width as large as you can manage. I.e. not by a single piece of stove pipe.
Details how to construct this later.

Heather Arvensis wrote:(I’m not sure on exact measurements of width yet as that can be adjusted to hit the desired ISA. I would also need to get creative with supporting such a wide bench top span. I do have rebar, angle iron and bricks for towers!)
I do like the backrest bench design you did! We are still pondering that idea.

Seeing the space where the heater should be placed: the French bench, couch, divan or whatever opens up other possibilities. Placing it in front of one of the windows looks downright silly to me, because of the high backrest.
But what do you think of this: the combustion core in the middle of the room and the bench itself at right angles with the wall?
The door side of the firebox could be placed in any of three directions this way.
Plus the possibilty of a ceramic glass window on top of the riser.
The only French couch to date was a single-sided one, what about a shorter but double-sided one?
Maybe even placed at another angle instead of 90 degrees?

Heather Arvensis wrote:Thanks for the tip on including the barrel in the ISA… does that apply to jtube builds as well?

Yes, heat is absorbed and radiated out by the barrel, so it's part of the heat absorbing surface area.

Cerbu Ulea wrote:nice , mine being a sidewinder affects somehow the position of the top lighting pattern , or the position of the air slits?

Both of those aspects are the same as in a normal straight Shorty core. This has been tried and tested thoroughly, already over a year ago. In general, the aerodynamics of this core are very specific, it will function even without a door at all. Even the reversed sidewinder showed exactly the same behaviour during the workshop in France this spring. Much to my surprise, I might add.

Coydon Wallham wrote:The exhaust pipe is constrained some on the sides around it. Would cutting a vertical slit in the pipe and bending it back to enlarge the opening on the side toward the firebox be enough to increase the draw ability, or will it be necessary to break the cob, cut and reposition the whole pipe?

If you are able to reach into the bench, cut the pipe and bend the ends open, it might help. Breaking cob and redoing the exhaust thing I would keep as a plan B.

Cerbu Ulea wrote:I'm splitting an oak and a horse chestnut tree I was given this January, would this piece be too big ?

To build a fire in a lukewarm or cold heater, in that case it's too big. When fed into a hot firebox with somewhat smaller pieces around it, it would be consumed in short order. The following is an old video, taken from a first generation batchrocket. Nevertheless, the Shorty design is equally capable of this.

Coydon Wallham wrote:The first firing went smooth, ran a couple of hours without incident. The following two days, it's been cold plugging severely. Even today with a few pieces of birch bark and some paper under the exit flue before starting, the firebox stalls out after ten minutes of a tepid burn. An hour or so of feeding the firebox a slight crack of air, as well as pointing a space heater directly on the base of the exit flue the whole time, is needed to get a draft going.

I'd say all of the wet cob (removing the cleanout covers after it plugs reveals moisture dripping from the roof of the bench inside) and ambient temperatures in the 60s F are clear causes for this and it is likely to be fine once the heating season starts, but am still not sure about placement of the bottom of the exit flue.

There's a lot of water in the cob and masonry, that's for sure. On top of that, even bone dry fuel will create water vapor, quite a lot of it actually. As a rule of thumb: half the weight of the fuel to begin with, is chemically converted to water vapor during the course of the burn. Adding all that to the water of the build, resulting in a very recalcitrant heater. During the first burn all seemed to go well, until more water was converted into vapor and condensed on any surface that was cold enough. Keep on going, that's all I can say, eventually the darn thing will dry out and come up to speed.

Coydon Wallham wrote:I created a ~2" depression in the stratification chamber bottom around where the flue would land, and set the bottom of the duct a little less than 1" above the rest of the floor of the chamber, so between 2-3" of space all around the opening to the 8" duct exit. Is using the inverse of the barrel top-riser clearance numbers a good guide for exit minimums?

This depression *could* work, depending on the size of it and where the exit is located.
Is this depression at least as wide as the bench' internals and shaped as a square, it could work. Provided the exhaust pipe is right in the center of the square.
Is the exit located at the very end? In that case, one quarter of the circumference is blocked by the end wall so no gases are able to enter from that side. When that's the case, the pipe should be higher above the floor in order to provide adequate space. I won't recommend this layout, I would regard it as much better when the floor is flat and the pipe ends about 4 inch from the floor.

Regarding your end question: yes it is, provided the calculation is done in a proper way. See the space above the riser as an imaginary ring. Think about it this way: the gases, rising out of the riser need to go through a 180 degrees bend in order to stream down.  When this ring's circumference, multiplied by its height is a figure that is twice as large as the riser's cross section area, then it's about the right size. Just by giving the gases ample space to round the bend without too much friction means a possible pinch point isn't there at all.
The same thing happens at the exit, the distance between the floor and the end of the pipe works the same way, although the change in direction is just about 90 degrees since most of the colder gases stream along the floor. So, in theory, the surface area of that imaginary ring should be at least 1.5 times as large as the exhaust pipe.

Calculating... the 8" pipe's csa is 50.26 sq. in. So, the imaginary exhaust ring's surface area should be at least 75.4 sq inch. Devided by the circumference gives the height of the ring. In your case, that would be 3" as the minimum distance from the floor, provided the exhaust pipe is free all around. As said before, the depression might be a negative factor.

Heather Arvensis wrote:I’ll include a rough layout photo for reference. I re-ran my numbers just now, twice. I’m getting 53.66 sq ft for the numbers in the photo. And 56.74 sq ft for the layout in original post. 🫣 Now, if my brick cutting today is any indication, I am totally bad at math… 😬 (I did subtract the floor… is that not ok??)

Apologies, I recalculated and it seems your numbers are correct. Almost sure that I forgot there are 12 inches in a foot... The floor shouldn't be in the calculation, that's true. In case you are using a barrel, this should be in the calculation, no doubt about.

Heather Arvensis wrote:As for the L shape- I had no idea that would pose a problem! I got away with it for my j-tube but maybe because it goes from barrel to transition area to into Strat right next to corner… shoot. What if I made the corner wider (less cob there) to help relieve friction?

It would help a great deal if you made the corner wider, yes. A great deal wider, I'd say, with rounded corners. Especially the inner corner rounded off in a quarter circle that's as large as the internals of the bench itself.

Heather Arvensis wrote:As for leading gases I was planning on going into barrel from riser and dump straight into bench with as wide and big a hole I can manage.
As for variant of core I’m not quite sure what you are asking… type of batch box? If so, I’m doing the basic, original design.

Straight into bench with a large dump and hole is good. Since you are using the first-generation batchrocket design, you already mention you would need a barrel. Keep in mind that the space above the riser (commonly know as top gap) should be as large as the system's diameter. Batchrockets are very picky about friction, that's why. It would be best to provide a straight trajectory and/or lots a space.
I spotted another potential friction spot, though. In your plan view, the barrel is in front of the bench. When the hot gases are dumped into the bench, they are forced around a 90 degree corner immediatly after that. It would be much better to place the barrel beside the bench, so that the gases just stream lengthwise into the bench, unhindered.

Just a remark: there's another possibility. Although the bench won't be as large and has a voluminous heated backrest. This has been done using the latest incarnation of the batchrocket concept, the Shorty. Doing away with the barrel and feeding the bench directly. Have a look: https://permies.com/t/281596/Mass-bench-fed-modified-sidewinder
Worked like a charm, for this workshop all bricks of the bell were on edge to speed up the build. Best to have all bricks on flat, mass would double like that.

Heather Arvensis wrote:I’m currently working on my second RMH build. Last year I did a j-tube and now I’m working on a 6 in batch box. The bench/strat chamber will be an L-shape with firebox on one end and exhaust on the other. In an effort to have the burn chamber where we want it to land, my internal measurements would be a bit small to stay within the 57 ft2. (Contrary to my lovely, sage teacher and mentor, I prefer not to install a bypass 😊). The internal measurements I would like to do are-
12 inches tall
13 inches wide
(Length is approximately 114” on one leg and 90” on other)
This would create a small but long chamber… does anyone know if there would be any negative aspects to this shape?

To be frank, it won't work at all. Just a quick calculation shows  that the Internal Surface Area would be close to 80 ft2. Far too large, according to the recommended values. The L-shape of the bench makes matters worse, lots of friction is expected, due to that sharp bend.
How do you plan to lead the gases from the combustion core into the bench, and what variant of core do you want to use?

Matt Todd wrote:@Peter van den Berg  
Do you think it would be alright to put the final exhaust opening on the BACK side of the riser instead of the front?

Yes, Glenn is right, it can be done. The lower riser part should be different, the pictures of the French bench build are quite clear how it should be done, in my opinion.

Cerbu Ulea wrote:but being hot I suppose a bread would cook on the extinct coals, i'll try , it is not a must

You are absolutely right on this. A friend of mine bakes his own bread, using the firebox of his mass heater to that purpose during winter. He's placing a stainless steel grid on legs over the coals, so the bread in its form is elevated above the coalbed itself.

Cerbu Ulea wrote:...than if you wont mind i'll ask what to do with the air intake after the fire is out , but coals still red, cooking inside the core I saw you wrote is a good option, maybe you have a testo diagram of the embers faze

In general, during the coaling stage lots of CO is formed and sent up the chimney. On the other hand, the heater is at its hottest so quite a portion of the heat is going to the great outdoors and heating the birds.
I am using this method: when the fire is in its coaling stage and no more flames are to be seen, I close the air inlet completely. The coals will die down due to lack of oxygen, rather quick in fact. A generous layer of coals is kept in the firebox, very good to start the next fire on. In Europe, this is called the Austrian method. It is assuming that there's no chimney damper, only an airtight door, otherwise carbon monoxide could get into the room. Without such a chimney damper the chimney is still pulling, so anything that's harmful will be exhausted in a harmless way.

And to round off, why is it good to start a new fire on top of a layer of charcoal? The self-ignition temperature of wood is about 320 ºC (608 ºF), while charcoals' is significantly lower. So, loading the firebox on top of coals *and* lit from the top will result in a good fire in a surprisingly short time span. Just because the coals will ignite easily, the fire creeps from the back to the front in short order. After the burn is done and the fire dies down, about the same layer of coals is left behind, to start the next fire on.