Batchbox and RMH projet

Hello friends of rocket mass heaters.
Firstly, I apologize if any part is incomprehensible, English is not my language and I use an automatic translator.
I am about to refurbish the old and inefficient chimney and build a massive heater. My experience with rocket fires is an RMH in a tool shed and a rocket furnace. Of them I can say that the first is completely underused because with mild winters, daytime use only and manual work in the field make it more of a whim than a necessity. I learned a lot of doing it and that allowed me to adapt the system to an oven that works today, and a lot, without any kind of problem. https://www.youtube.com/watch?v=RwxYjFLDXWY&t=13s and https://www.youtube.com/watch?v=ybctDCu8y4o in case anyone is interested.
My initial idea was to build a batchbox, for many good reasons. The only drawback that I see is that you need chopped firewood to measure for the firebox; Accustomed to using all kinds of wood formats in the J rocket fire, I don't like the idea of ​​chopping by obligation. I decided to also include a mass heater with a metal drum, perhaps it is excessive for the dimensions of the house, however in the literature on the matter the concept of oversizing is insisted on and that is what I do when doubling the burners.
I would like to expose my numbers and my reasoning to obtain the approval or the necessary corrections, from people with more knowledge and experience than me. It will be a major undertaking in labor and materials, and you will want to ensure ultimate success as much as possible.
The spreadsheet provided on the batchbox website tells me 2.3kw on two fires per day. A 150mm batchbox delivers 1.9kw.
Difficulty of replacement of Peter's channel superior, I lean towards the option of the channel at ground level of easier replacement.
The internal surface of the hood for batchbox in 150mm and 1.9kw is 5.3m2 and its volume (14cm wide brick) is 0.86m3.
As well as the batchbox I have specific data on power and dimensions, in the case of the RMH I do not have them and I make the following assumption. If a 175mm circular chimney batchbox delivers 2.9kw, it is not unreasonable to assume that a J fire with a square chimney (17.5x17.5cm) containing a 175mm circle delivers a similar power.
The idea is to alternate the fires in one system and the other, according to stocks of firewood and immediate or long-term heat needs, in principle one fire per day in each one. Working on the design, as you will see in the drawings, I was able to fit both ducts and enlarge the ascending diameters so that they even admit the flow of both systems on at the same time.
A bell bench for the rocket is an easy solution to carry out and with little cost, however the volume of mass obtained with a hollow bench is not close to the mass used for the batchbox, of lower power. To get closer to these parameters, the bench has to be solid.
The available space limits the dimensions of the bench for the J rocket; With the available measurements, a heat transfer surface of 5.74 m2 (tubes and drum) and a bank volume (discounted the tubes) of 1 m3 are obtained for a power (estimated from the bottom) of 2.9 kW. Thus, the 5.74m2 of absorbent surface and 1m3 of mass volume in the RMH are very close to the 5.3m2 and 0.86m3 of the batchbox; with a surplus of power, read high thermal differential in the gases, with respect to the final output, for a surplus thrust in the shot.
The 300cm2 area of ​​the rocket travels through three 15cm diameter tubes under the bench, with an overall area of ​​525cm2.
Adding the 300 cm2 of the rocket to the 176 cm2 of the batchbox, we obtain a maximum area of ​​476 cm2; I hope they will travel together (if necessary) through a final tube of 25cm in diameter with 490 cm2.
Stone house 65m2 without insulation, the dining room is 20m2, bench measures 0.8x0.4x2.9m, hood 0.86x0.76x1.65m
In the drawings you will see that the final tube goes down to a second level, for two reasons, one to attach another stove in the basement in the future and the other is to remove the cover that will close it from below and place it again with a simple burning paper a current is generated that sucks air from the system starting cold starts with ease. I learned this using the oven, it is outdoors and one particularly cold day, when I turned it on, I had trouble getting the flames to start their path horizontally, insisting on going up the feed tube. I found it interesting to implement the possibility of having an easy solution for stubborn power-up moments.
The entire project is pending completion, that is to say that what is appropriate for its correct operation will be varied, feel free to question the entire approach, if necessary.
Specific questions:
Will the smoke be equally distributed in all three tubes of the bank?
Has anyone changed an upper Peter's channel ? After how long of operation?
Any data on the duration of the lower channel?
For the calculation of the internal surface of the hood, I discard the upper cover because I intend to insulate it thermally so that it does not absorb heat. It seems more effective to me that it radiates heat horizontally than directly upwards, correct? For the surface calculation, I did not take it into account.
I separate the hood enough from the side walls to introduce insulation. I am not interested in losing heat to those walls that, on the other hand, are exterior, correct?
I think it's a good idea to isolate the thermal masses from the ground so as not to lose heat in this way. Correct?
In my design, the RMH tubes circulate inside the hood. Do you see any problem with this? Should I insulate them from heat that could be in the hood? Would a hot spot in these ducts affect the circulation of gases?
I have more questions, but its formulation will depend on whether the approach is correct or there are variations.
Thanks in advance for the answers and suggestions.

Hi Aurelio, you are building a whole story around assumptions, better to investigate whether or not these are viable.

Aurelio Ape wrote:
My initial idea was to build a batchbox, for many good reasons. The only drawback that I see is that you need chopped firewood to measure for the firebox; Accustomed to using all kinds of wood formats in the J rocket fire, I don't like the idea of ​​chopping by obligation.

Then there's good news for you! A batchrocket is able to burn larger pieces of firewood as compared to a J-tube of the same size.

Aurelio Ape wrote:I decided to also include a mass heater with a metal drum, perhaps it is excessive for the dimensions of the house, however in the literature on the matter the concept of oversizing is insisted on and that is what I do when doubling the burners.

I would regard this as unnecessary, since the batchrocket scales up very fast you could build a larger one instead.

Aurelio Ape wrote:As well as the batchbox I have specific data on power and dimensions, in the case of the RMH I do not have them and I make the following assumption. If a 175mm circular chimney batchbox delivers 2.9kw, it is not unreasonable to assume that a J fire with a square chimney (17.5x17.5cm) containing a 175mm circle delivers a similar power.

No, it doesn't. Common knowledge regard the batchrocket as twice as powerful in any given time frame as compared to a J-tube of the same system size. This is only logical, the J-tube burns at the bottom only, the batchrocket's fuel load burns altogether at some point during every burn.

Aurelio Ape wrote:The 300cm2 area of ​​the rocket travels through three 15cm diameter tubes under the bench, with an overall area of ​​525cm2.
Adding the 300 cm2 of the rocket to the 176 cm2 of the batchbox, we obtain a maximum area of ​​476 cm2; I hope they will travel together (if necessary) through a final tube of 25cm in diameter with 490 cm2.

The bench with three pipes in it won't work as intended, sorry. First the delivered power of the J-tube is much lower and the gases tend to take the shortest route to the exit. So at least one of the pipes won't transport hot gases, is my estimation.

Aurelio Ape wrote:Stone house 65m2 without insulation, the dining room is 20m2, bench measures 0.8x0.4x2.9m, hood 0.86x0.76x1.65m
In the drawings you will see that the final tube goes down to a second level, for two reasons, one to attach another stove in the basement in the future and the other is to remove the cover that will close it from below and place it again with a simple burning paper a current is generated that sucks air from the system starting cold starts with ease.

Maybe I don't understand this correctly, but another heater in the basement coupled to the same chimney higher up the house is a definite no-no. The heater downstairs will inevitably leak some air into the chimney and as such will influence the higher level heater. In my view, it would be better to incorporate a bypass, relatively easy done in a bell system.

Aurelio Ape wrote:Specific questions:
Will the smoke be equally distributed in all three tubes of the bank?

No, it almost certainly won't.

Aurelio Ape wrote:Has anyone changed an upper Peter's channel ? After how long of operation?

Never used one long enough, no experience enough to answer this question.

Aurelio Ape wrote:Any data on the duration of the lower channel?

Mine, one burn of about an hour a day and two while it's freezing, about 4 years.

Aurelio Ape wrote:For the calculation of the internal surface of the hood, I discard the upper cover because I intend to insulate it thermally so that it does not absorb heat. It seems more effective to me that it radiates heat horizontally than directly upwards, correct? For the surface calculation, I did not take it into account.

Better to have mass in the top of the bell and insulate on top of that. It's mass will be cooled gradually, giving up it's heat to the rest of the bell.

Aurelio Ape wrote:I separate the hood enough from the side walls to introduce insulation. I am not interested in losing heat to those walls that, on the other hand, are exterior, correct?

It would be best to separate the bell a distance of about 10 cm from the wall and mount a radiation shield against the wall on spacers, so there's a space of about one or two cm behind it. The shield should start about 20 cm above floor level, so air currents will form behind it. I am using this arrangement and it works wonders. Even in the case the rear side of the bell can't be touched longer than a couple of seconds anymore, the shield feels just handwarm.

Aurelio Ape wrote:I think it's a good idea to isolate the thermal masses from the ground so as not to lose heat in this way. Correct?

For a heater this is a good idea but on the other hand, a bell construction won't really have a warm floor as long as the firebox is lifted from the ground.

Aurelio Ape wrote:In my design, the RMH tubes circulate inside the hood. Do you see any problem with this? Should I insulate them from heat that could be in the hood? Would a hot spot in these ducts affect the circulation of gases?

Not sure what you mean by this last question.

Recommendations: build a single system, either J-tube or batchrocket. As I see it, place it in the middle of the spot you've drawn, with short bell benches left and right. Forget about having two burners and two different systems, never connect two systems to the same chimney. It's a nice space for a batchrocket of 18 or 20 cm system size, you'll never get cold again as long as you do at least a small fire a day. Have I mentioned that a batchrocket is equally efficient with half a load?

Peter: Thank You so much for your response and your continued participation on this forum!
Aurelio;  I looked at your thread earlier today.  I was overwhelmed by it and secretly hoped that Peter would reply!
My prayers have been answered as well as your questions!
Keep us posted as you begin your build!

Dear Peter, it is an honor to receive a reply from you. I take good note of everything indicated, and I thank you sincerely for your appreciations. I am going to remodel the whole project according to the guidelines received. Does the internal surface of a bench add to that of the bell to obtain the appropriate one for the diameter of the system?

Regarding my first failed design (it was to be expected; at the first time, things do not usually go well) and in order not to repeat mistakes, I am interested to know why if the fumes after leaving the rmh enter a small chamber, are stratified and being the tubes at the same level and same diameter, the flow rates are not distributed similarly.

On the other hand, I do not understand the interference involved in joining the up flows of both systems in a riser pipe of sufficient diameter to contain both. Nor do I see why not to include a basement fire, which evacuates through the same riser (always with a flow rate that added to the one already used by the fire above does not exceed its capacity) Thanks for your patience with my ignorance.

Thank you, Tomas, also, for your participation and good wishes.

I will make a new model and ask more questions about it.

Ultimately I think the system will be a batchbox, probably 200mm, a bricksidewinder, bell and bench. The configuration will be like the one made in Peter's workshop, in Mallorca. I'm struggling with the dimensions of it all.
At the moment the project is going in this direction. See drawing.
The bricksidewinder model has a shorter heat riser than other linear models of the same power. Is there a minimum and a maximum in height for each diameter and for each core model?
For the walls very close to the hood, the concept of heat shield has not been clear to me nor with what materials, a sketch would help me a lot. How far from the wall could I do without it?
The bricks in the core and the heat riser, refractory obviously. The rest of the bell and bench I suppose that common brick is enough. In Peter's construction mentioned above, are the bricks in the inner skin at the top of the bell normal or refractory?
In the bell, a gas inlet of the same size as the inside of the bank, and the outlet elongated in width and lowered in height, maintaining the minimum area of 200, to be as low as possible, and opening to connect with the chimney. Correct design?
What mortar do we recommend, refractory or clay and sand. In the case of both, in which places? My initial intention for the bench and the bell is to do it with exposed brick, with normal cement mortar. The bench tops are made of stone or cement slabs, depending on their value.
My intention is to cover the bench with cushions. The internal dimensions of the bench are 56 cm wide by 25 high plus 10 cm for the top slabs. I have thought of a cushion thickness of 15 cm, with which the total height of the bench is 55 cm, perhaps it is not well sized?
Covering with cushions limits the diffusion of heat. What experience do you have in this regard? What materials are the most appropriate? Should I expect a temperature around 60º on the bench?
I would like to isolate the work from the ground. The core will have a metal bench (a good idea or a bad idea?) that will raise it to isolate it so that the smoke can pass under it towards the chimney. What method and materials do you recommend? It is not so important to insulate the floor if it seems to be in contact with the less hot gases, however, what do you think about home insulation such as laying a cardboard/plaster sheet on the floor (material in Spain is known by a brand Pladur)?
My apologies for peppering you with questions, I promise to make good use of the information received and I thank you in advance.

Is there a specific reason why you want the bell in a corner of the room? May I suggest to place the bell in the middle of theat wall with a shorter bench left and right?
This room isn't overly large, maybe a 180 mm system would be large enough? Keep in mind, the batchrocket scales up very fast. A 200 mm system is only 33% larger as compared to a 150 mm system. But the volume of the firebox is quite another matter, this scales up by a factor of 1.33x1.33x1.33=2.35. Which means a firebox of the larger system could hold 2.35 times more fuel as compared to the smaller system. Some 6 kg per batch for the 150 mm size and 14 kg for the 200 mm size.
Just some remarks, nothing dramatic.

Aurelio Ape wrote:The bricksidewinder model has a shorter heat riser than other linear models of the same power. Is there a minimum and a maximum in height for each diameter and for each core model?

No, it doesn't, I am in the dark where you got that from. How to calculate the height of the heat riser is on the batchrocket site.https://batchrocket.eu/en/building#dimension
Dimensions are calculated by using an imaginary base figure, commonly expressed as a capital B. Height of the riser is normally 10B, this is the configuration giving the best results, higher doesn't add anything. However, aquired experience during the years indicate that a riser's height of 8B is the lower margin for this design.

Aurelio Ape wrote:For the walls very close to the hood, the concept of heat shield has not been clear to me nor with what materials, a sketch would help me a lot. How far from the wall could I do without it?

A heat shield is reflecting part of the radiation from the bell. When the shield is spaced from the wall by as much as 20 mm and starts well above the floor, an air current will start as soon as the shield is warmed. The wall behind it will be just handwarm, nothing more.
Of course, you could space the heater from the wall, I guess at above 50 cm distance the effect is much deminished.

Aurelio Ape wrote:The bricks in the core and the heat riser, refractory obviously. The rest of the bell and bench I suppose that common brick is enough. In Peter's construction mentioned above, are the bricks in the inner skin at the top of the bell normal or refractory?

The construction of the Mallorca build is done in such a way that the bricks above the riser's level are able to move quite freely. It might or might not be wise to use normal bricks there, I don't know. I prefer to stay on the safe side, construction-wise.

Aurelio Ape wrote:In the bell, a gas inlet of the same size as the inside of the bank, and the outlet elongated in width and lowered in height, maintaining the minimum area of 200, to be as low as possible, and opening to connect with the chimney. Correct design?

In principle, yes. By the way, area is not 200 cm², but 314 cm² instead. Ideally, the whole of the exhaust opening should be in the lower half of the bench' height.

Aurelio Ape wrote:What mortar do we recommend, refractory or clay and sand. In the case of both, in which places?
My initial intention for the bench and the bell is to do it with exposed brick, with normal cement mortar. The bench tops are made of stone or cement slabs, depending on their value.

Mortar for the bell and bench could be cement, lime and sand. The lime in there is to make the mortar a bit flexible.

Aurelio Ape wrote:My intention is to cover the bench with cushions. The internal dimensions of the bench are 56 cm wide by 25 high plus 10 cm for the top slabs. I have thought of a cushion thickness of 15 cm, with which the total height of the bench is 55 cm, perhaps it is not well sized?

I would recommend more height in the bench, 25 cm feels a bit cramped. Internal height of 35 cm, 10 cm for the slabs will make for a height of 45 cm, which is the conform a normal dining chair. A stone bench looks hard to sit on, and it is. Keep in mind however, it sits much more comfortable as soon it's warm. So a cushion of 5 cm or a plaid or whatever is already enough. Just my experience, subject to personal taste.

Aurelio Ape wrote:Covering with cushions limits the diffusion of heat. What experience do you have in this regard? What materials are the most appropriate? Should I expect a temperature around 60º on the bench?

Covering will limits diffusion, yes. Wool and cotton would be my preference. Depending on how hot you want it in your room, the temperature under the cushions could reach 60ºC, yes.

Aurelio Ape wrote:I would like to isolate the work from the ground. The core will have a metal bench (a good idea or a bad idea?) that will raise it to isolate it so that the smoke can pass under it towards the chimney. What method and materials do you recommend?

Good idea to raise the core. Better for you while loading the firebox, you will able to sit on a stool in front of it. A small table out of welded angle iron and the like would be sufficient. That's how I have it and I am very satisfied about it.

Aurelio Ape wrote:It is not so important to insulate the floor if it seems to be in contact with the less hot gases, however, what do you think about home insulation such as laying a cardboard/plaster sheet on the floor (material in Spain is known by a brand Pladur)?

I'd think you mean drywall sheet, that could be done. But is it really necessary?
During burning part of the fine dust carried through by the velocity in the riser will end in the bell, due to the very low forward velocity of it. Through the years, there's a blanket of very fine dust at the bottom of my bell. The first time I checked it was so fine I couldn't feel it while sticking my fingers in it. Sounds like proper insulation, don't you think?

Thanks for your answers, Peter. They are invaluable to me.

Part of my tribulations consists of fitting one of your bathboxes in the dining room. The distribution of window and doors openings does not allow me many options. In the chosen corner is the smoke outlet of the room and although this would not be a technical obstacle, it is for us on an aesthetic and functional level, we do not like the idea of ​​separating people on the sofa with a wall. If I had enough space in a corner, it seems obvious to me that two side benches is a very good technical, aesthetic and functional option. We are clear that we want a linear sofa of around 3m and I will adapt the project to that premise. The size of the bank sought also determines the size of the burner, if I can put a 180, I will not use a 200.

As for the height of the bench, another of my doubts. I understand that the 25cm offers little height for good stratification. 35cm is better and that's how I see it in your design. We would like 15 cm of cushion, a common standard in sofas; but plus the slabs, it turns out too high a seat. I will try to combine those 35cm of bench with thinner slabs and a higher density cushion with less height.

As the father of the creature, you are absolutely right, there are no discrepancies between models and heights of heat riser. I apologize for my stupidity. It's my confusion. For the drawing tests, I scaled from the 150 models on the web to achieve diameters of 180 and 200. For some reason the bricksidewinder's heat riser have lower heigth in the scaled than it should. I noticed that difference with his brothers and regarding the spreadsheet on the web, my question was in that direction. The 8B helps me in my design Thanks again.

The bricks in the double skin, refractory; Let's not take risks, then. Apparently they don't support a lot of load. Do you think they can be placed dry? In any case, it would not be difficult to ensure that the load exerted by the profiles is on the external skin.

Of course, the area for 200mm is 314cm2, I didn't express it well.

I have used lime mortar on occasion. It's certainly much more flexible. Do you find it inconvenient for me to use it (in authorized places due to the temperatures, of course) without adding cement?

To join the core's bricks, do you consider the best refractory mortar, slip, or clay and sand mortar?

Ruled out separating 50 cm from the wall, so I'll have to use a heat shield.

What material do I use to insulate the first course of bench and bell bricks?

What minimum separation do you consider prudent between the internal faces of the bell and the core?

Thank you very much for your answers, without them the success of my heater would be more the result of chance than of a well thought out project.

Aurelio Ape wrote:The bricks in the double skin, refractory; Let's not take risks, then. Apparently they don't support a lot of load. Do you think they can be placed dry? In any case, it would not be difficult to ensure that the load exerted by the profiles is on the external skin.

The design of the bell ceiling is that it rests on the inner skin. The steel will expand during heating up so this needs to be separated from any brick or other by supwerwool. By doing this, the steel is able to expand without pushing against bricks. The liner bricks can't be placed dry, especially not in case it's split fire brick.

Aurelio Ape wrote:Do you find it inconvenient for me to use it (in authorized places due to the temperatures, of course) without adding cement?

The mortar I've used in Mallorca and Brussels consisted of sand, lime and cement, the cement being the smallest part.

Aurelio Ape wrote:To join the core's bricks, do you consider the best refractory mortar, slip, or clay and sand mortar?

Refractory if available, clay and sand as second. Thin seams, 1 to 2 mm.

Aurelio Ape wrote:What material do I use to insulate the first course of bench and bell bricks?

I am not entirely sure what you mean. Insulate from what, could you elaborate?

Aurelio Ape wrote:What minimum separation do you consider prudent between the internal faces of the bell and the core?

The space between the bell and the core should be 5 times the riser's csa at least, preferably (much) more.

Thank you Peter, your clear and concise answers.

I was referring to the best way to insulate the first row of bench and bell bricks from the ground.

I was asking about the minimum separation between the core and the vertical walls of the bell.

Just now I have discovered your Double Shoebox Rocket, the web in Spanish does not contain it. I shouldn't have, now I don't know if this other design suits me better. A sea of doubts, I will try to navigate carefully.

Aurelio Ape wrote:I was referring to the best way to insulate the first row of bench and bell bricks from the ground.

To be honest, I don't insulate the bell and bench walls from the ground. Especially when the firebox is on a raised pedestal. Since a stratification chamber traps the hotter gases in the top, the lower ends of the walls won't get very warm. And the gases that are exhausted are extracted well above the floor so the first bricks won't get easily above handwarm.
My own heater is built on large tiles that are on a 7 cm cement slab. That slab is supported by a lot of insulation so one would think the floor around the heater would get very warm. But it doesn't, even in the case that the bell's wall higher up is too hot to touch for more than 5 seconds.
In short, don't worry about it. Did I tell you that the very fine dust will settle on the floor of the bell and forms a layer that's highly insulative?

Aurelio Ape wrote:I was asking about the minimum separation between the core and the vertical walls of the bell.

The minimum distance between core and  bell wall could be zero, provided the other side is wide enough to allow stratifying. Since the core is lifted the gases could stream underneath the core to the bench, no problem. And another provision: the core should be lifted at least up to the level of the bench' ceiling. And it would be wise to keep core and wall mechanically separated, i.e. by using some superwool or a gap. The core, being hotter, will expand a great deal more than the wall. So in case the distance is zero, you need to provide for a slip joint.
Let's see... that's about it.

Aurelio Ape wrote:Just now I have discovered your Double Shoebox Rocket, the web in Spanish does not contain it. I shouldn't have, now I don't know if this other design suits me better. A sea of doubts, I will try to navigate carefully.

The DSR isn't simpler to build as compared to a standard configuration, although a sidewinder can have its own issues. Mainly due to the fact a lot of people try to build that latter one with a port that's not centered into the riser and/or being flush with the rear wall of the firebox. In that case, the double vortex won't form properly and complete combustion is compromised. It would be wise to build a design that's been built multiple (one thousand, two thousand?) times before. The DSR2 design is a couple of years old now and just a month ago published on the English version of the website.

Thanks again, you are very kind to answer me. I take note of everything.

The project is already a challenge for me, we better not complicate it with a design that has not yet been certified. However, I'm sure it's on a level with your other designs

Hello partners.

I am designing the bricksidewider 200mm model with sketchup, with the actual measurements of the bricks. Until now I used the scaled web model and its bricks are not commercial measures.

I'm stuck on the secondary channel. If I understand it correctly, the calculation of the dimensions of the floor channel does not work the same as in the upper channel. For a 200mm (314cm2) section, the vertical section must have a section of 16.7cm2 (5.4%) and the horizontal section (8.25%) 25.9cm2, which is 1.5 times the vertical section.

I find a square tube of 45x45mm (internal 41x41) 16.8cm2 for the vertical and 80x40mm (internal 76x36) 27cm2. Is this sections setup ok?
I can't find how the height of the vertical section is calculated. Can I copy the 160 model from the web scaled to 200? Can I take the references of the scald model of its relative position with respect to the portal as valid?

What materials do you use for the lintel of the firebox?

It is of no importance and surely no one will do so; but I have detected that in the channel of the bricksidewinder model, the vertical section closes the horizontal one.

Thank you for your attention.

Aurelio Ape wrote:I'm stuck on the secondary channel. If I understand it correctly, the calculation of the dimensions of the floor channel does not work the same as in the upper channel. For a 200mm (314cm2) section, the vertical section must have a section of 16.7cm2 (5.4%) and the horizontal section (8.25%) 25.9cm2, which is 1.5 times the vertical section.

Yes, the overhead p-channel does the same thing as the floor channel, introducing secondary air. The way they do it differs, so that's why they aren't the same. In general, both channels sports an outlet of about 5% of the riser's cross section area. The floor channel's vertical part (named stub) is half the csa of the horizontal part (called feed). The larger feed is mentioned in a separate alinea in the article about these channels.
https://batchrocket.eu/en/designs#floorchannel

Aurelio Ape wrote:I find a square tube of 45x45mm (internal 41x41) 16.8cm2 for the vertical and 80x40mm (internal 76x36) 27cm2. Is this sections setup ok?
I can't find how the height of the vertical section is calculated.

The stub dimensions are OK, the feed could be done with two 45x45x2mm ducts welded side by side. The last 120mm at the end where the parts are connected could be opened up so they form one duct there. Somewhere I have pictures... digging...







These are rectangular ducts, pictures are from one of the many experiments. But this layout could be used for square ducts just the same. Of course you need to close the end of both ducts.

The height of the stub should be in such a way that the air is delivered in the top half of the port. So, the opening in the stub should start (measured from the floor) halfway in the height of the port.

Aurelio Ape wrote:What materials do you use for the lintel of the firebox?

I think you mean the lintel over the firebox, holding up the bell's wall. That could be refractory concrete, steel bar, a bog standard lintel meant for a house even. Mark that the space between the lintel and the top of the firebox should be filled with superwool in order to separate those items from each other.

Aurelio Ape wrote:It is of no importance and surely no one will do so; but I have detected that in the channel of the bricksidewinder model, the vertical section closes the horizontal one.

You are right, it's overcomplicated as drawn. See the pictures above, much simpler to do.

Thanks Peter.
All cleared up; for now
Great idea to join two tubes to get another profile.

hello aurelio,

i like your ideas and design of heating your room with different systems.

you can combine one or more heaters to one flue. no problem. only one thing..... you have to provide them with air tight dampers for each one of them.

you even can burn heaters at the same time, but only if second one is a closed door system. (no openfire like rocket or batchbox , but a normal woodstove with door with air intake regulation)

of course, everything depends on how well your flue is working. (stone, inox, isolated, more than 5 meters long, highest point on roof, ect, ect.)

it is very practical to have a cleanout in the cellar, or a flue warming up for cold start. see no problems there too.


also the bench with 3 pipes will work! but not all 3 at same time. you will need direct one of them at the time with 3  (not so air tight) valves. so you can experiment with the best settings.
but im not sure if the extra build complexity will bring you more efficiency or joy for that matter.

i have experience for years with batchbox, bell, mass benches.

there is simply no 1 perfect solution in one system.

at the moment i use my rocket the most, because of my (cheap) wood supply. i don't like sawing wood. so i use long/ small sticks. (even 2 meters long)
another reason is because it burns better at long burning times with less heat. (with sticks and cheap wood findings alongside the road)

in the morning i need quick convected air warmth from the steel rocket barrel (before going to work).
in the evening a slow, more stone mass heating fire is required (for the night.)
for less cold days, of course only one fire is required. (cold days i will use 6kg wood. 25m2 kitchen, not well isolated)

the batch box system or normal stove i like to use when i'm not around to fiddle with fire. but uses expensive wood. (in money or workout)

a bell is nice, quick build, but also loses its warmth easily to the room.
the mass of a bench is more "warmth durable". you can isolate it on top with wool cushions and regulate in that way your warmth release to the room. (in night you will need less air to heat.)

i came up with a bench which is a combination of a thin wall bell front and mass back.
the front is a kind of bell/ syphon system, the backside of the bench is with a normal flue.
short fire, bell does it's work.
it will warm up to 60 or even 80 degrees (celcius) and helps convection (of the barrel) warmth for one day.
the back takes a longer fire to warm up completely.
the back will be 40-50 degrees max. and is convenient to keep the room at 15 degrees 24 hour round.


so in designing them you need to stay open to experiment and learn from your mistakes. in other words, if you want "one build", a near perfect system in terms of guarantee, please stick to the plan of known working systems, like peter advices you.

Hello Joa.

Thanks for your contribution to the thread. I agree with you in your aversion to “expensive” wood (time and/or money); For this reason, try to combine both systems to use them at my convenience according to fuel availability. In the rocket oven that I made, we have burned pruning sticks, pallets, chips and all kinds of formats, we even experimented with doping the fire with used cooking oil with very good results in calorific power and duration of the wood; finally we abandoned it because more temperature was not necessary and there is a great availability of "cheap" firewood

After reading the posts, I understand that a project like this, is an experiment and I cannot risk the result. Your advice and Peter's are gladly accepted and so the project will take the form of a batchbox with bell and bench.

I'm drawing all this and take the opportunity to ask another question.

I try to find the best balance between the accuracy of the dimensions, according to the spreadsheet, the correct interlocking of the bricks (vertical joints no aligned) and the fewest possible cuts, with the measurements of the bricks. From this will result larger . As much as 3cm more in depth and 1 or 2cm more in height in the firebox. The heat riser and the portal have the recommendation dimensions. I hope that's not critical.

Is it necessary to take into account the thickness of the mortar to get the final dimensions? Although minimal, in successive bricks, they can add centimeters.

Thanks for your time.

Aurelio Ape wrote:I try to find the best balance between the accuracy of the dimensions, according to the spreadsheet, the correct interlocking of the bricks (vertical joints no aligned) and the fewest possible cuts, with the measurements of the bricks. From this will result larger . As much as 3cm more in depth and 1 or 2cm more in height in the firebox. The heat riser and the portal have the recommendation dimensions. I hope that's not critical.

You are right, the extra cm's you are mentioning aren't critical. Let's say this is well within reasonable tolerances.
Just two dimensions turned out to allow a larger tolerance, namely the depth of the firebox and the height of the riser. Depth could be enlarged upto 25% extra or 5B, the B being the base figure. The riser could be lowered down to 8B instead of the 10B of the spreadsheet. Below 8B the thing seems to get a bit unstable, above 10B there's nothing to be gained.

Your answer makes me very happy, because I suspect that I will also be able to modify the following detail.

On the lintel of the portal, to achieve the proper dimension, I should put a piece of 13mm, ridiculously small. I think I would be able to pull it out of a brick; but that little thickness (in the drawing, in reality, with the growth of the mortar, it will be less); may not be suitable for the temperatures of that place.

The obvious thing is that the lintel is formed by the upper brick, in this case without any notch it would be very close to the recommended distance. Are 2cm more in the height of the portal within the tolerance?

On the other hand, I managed to respect all the dimensions, except for the depth of the firebox, which is 3cm longer in the plan, slightly more than 4cm in reality.

It's good that this is a build and not carpentry to the millimeter.

You are building a 200 mm system, that extra 20 mm is within reasonable tolerances.
Keep in mind though, the floor channel is right in front of the port and blocks off 45 mm of it. Some people tend to forget this, and the port will end up too low as a consequence.

In short: the port's height should be measured from the top of the floor channel's horizontal part.

Thank you very much Peter.

Your instructions here and on your website are clear on that issue. The stub tube does not obstruct the portal, I was careful about it.

I will publish the drawing when it is finished to receive your approval.

Once everything is basted, I will do a test of the core mounted in the outside to make sure that the heart beats at a good rate, the rest if it is well calculated and well done should not fail.

Aspirant builders who are drawing their own design, brick by brick.
As the following step me, validating the design so it could be built without uncertainties.
Or sending it back with comments until it's correct. Feels like being a school teacher, checking pupils' homework.
Would be an ideal situation, lots of published designs being the result.
Sounds like fun to me, I'm in.

All drawings in SketchUp, saved in version 17 format please, bring 'em on!
At no charge, your design will be open source from then on.

Dear Peter, I hope I have not abused trust with so many questions. I am not sure (because of the translator) if you are upset or willingly accept the role of teacher besieged by questions from students.

I want to understand that as the proud father of the invention, he loves that it is multiplied throughout the world.

I appreciate your efforts in developing and refining your heaters and making them available to the permies. It is very commendable.

I think it's great and an honor that you accept my design to share it with others and create a database of designs to share. I am dedicating many hours (I lie, many, many, many) to this and knowing that it can save others work, means a lot to me. It is a way to return the favor obtained with all the information that I receive here.

Sketchup 17, the latest free of course.

Sharing knowledge advances humanity. Collaboration is better than competition.

You read it correctly, I was expressing myself a bit too much tongue-in-cheek for the translator, I guess.

I'd think it's a good idea to extend the open source idea a bit further, validating drawings is light work. It'll give me something to do while I am at home. As a result, the drawings could build up to a library, I have to think about a way to make such a collection accessible in a simple and straightforward way. In order for this to work, the drawings should adhere to a certain standard, drawn out brick by brick.

I can afford to do correction and validating work without payment, I live a satifying and fulfilling life and don't have a desire for more income.
I am retired and I'll turn 77 years old in three months, deo volente.

Peter van den Berg wrote:
I am retired and I'll turn 77 years old in three months, deo volente.

Long live then, Master.

"Vale, Petrus"

With ears down and tail between legs (I hope it makes sense in English ), I expose my bricksidewinder batchbox model 200 mm in diameter, adjusted for commercial refractory bricks of 22x11x5cm.

The height of the heat riser is 8B. In the drawing there is a volume that symbolizes the exact dimensions of the firebox, recommended in the calculation sheet for a 200 model.

If Peter gives the approval, I will be delighted that he becomes part of the library of models adapted to different formats of bricks, to share with all those interested. It is a long and tedious task to draw it, if you find it done it is a great advance.

Browsing in a materials store, I have seen aerated concrete plates, 62.5x50x7 that seem ideal to cover the upper part of the firebox. The question is, is it a suitable material for that location?

Greetings to all.

Aurelio Ape wrote:Browsing in a materials store, I have seen aerated concrete plates, 62.5x50x7 that seem ideal to cover the upper part of the firebox. The question is, is it a suitable material for that location?

No, it isn't, not at all. The proper name for it is Air Entrained Concrete or AEC. The binder is ordinary portland cement, so it won't last more than three burns. It's marvellous material for demonstrations, though.

I've had a close look at the drawing, there are some issues with it. In order to get a proper running bond it's best not to use pieces that are shorter than half a brick. Which means combinations of 1/2, 3/4 bricks and full bricks are in order.  Also, full bond between firebox and riser would be good. Please don't use strips between layers of full bricks, those are weak points in the construction.

In order to show how it could be done properly, I amended the drawing with a complete core. This one doesn't have a double layer firebrick floor, no strips in the walls, no smaller pieces than 1/2 bricks, the chamfered bricks left and right are changed in such a way the 100x50x3 mm feed tube of the floor channel will be covered by those, the placement and height of the stub is corrected, the top slab of the firebox has a slot cut out so the riser is standing independently. And I added a threshold on top of the channel to keep the ashes in. Plus the bricks in the subsequent layers are grouped together so each layer can be moved around at will.

Of course, there are more ways to skin a cat, there ought to be at least one other method to adhere to the above mentioned rules. In order to distinguish this amended drawing from your original, I changed the title.

Dear Peter, I am very grateful to you.

Hats off to how quickly the one I sent you improves. It is noted that there are kilometers traveled on this road.

The refractory base is not included, why is it not necessary or economy of drawing? If it has to be maintained, would the aforementioned blocks of AEC be worth it?

I've put the baffles back on the riser, aren't they necessary?

Your measurement distribution is better than mine, of course. I agree with your comments on small bricks, I was miserably trying to save material, only ten bricks less than in your design.

However, I have allowed myself to move some of the bricks so that the vertical joints do not match, this has forced me to slightly resize some of the others, but everything seems to fit. I am forwarding the file to you so that you can consider whether the changes are appropriate.

I have also entertained myself to free some components that were interlocked, so that the drawing is all movable, I think I have achieved it.

The ash retainer plate, I understand, prevents the primary air from passing through the lower part of the firewood directly and going to the upper part first. A wanted effect, I suppose.

If I'm not mistaken, the chimney door must be separated from the box enough to leave a space equivalent to that of the primary air, and that the feed is fed by this same air, in the style of the Mallorcan workshop.

Aurelio Ape wrote:The refractory base is not included, why is it not necessary or economy of drawing? If it has to be maintained, would the aforementioned blocks of AEC be worth it?

Refractory base isn't worth the cost and effort. The refractory floor of the firebox is protected by the slanted bricks and the ash layer in the middle of it. So this floor doesn't get really hot, no need to install another refractory layer undermeath this. You could use the AEC to use as the base, yes.

Aurelio Ape wrote:I've put the baffles back on the riser, aren't they necessary?

Those angular pieces in the rear wall corners of the riser aren't really necessary. On top of that, due to the high temperatures in that spot the bricks are moving quite a bit due to thermal expansion and contraction. In my experience, those pieces tend to brake loose after some years.

Aurelio Ape wrote:However, I have allowed myself to move some of the bricks so that the vertical joints do not match, this has forced me to slightly resize some of the others, but everything seems to fit. I am forwarding the file to you so that you can consider whether the changes are appropriate.

It's your heater, as long as you adhere to good masonry practises you will be fine.

Aurelio Ape wrote:The ash retainer plate, I understand, prevents the primary air from passing through the lower part of the firewood directly and going to the upper part first. A wanted effect, I suppose.

Yes, that's true. This particular feature is there to hold the ash in but in addition to that, it promotes a more smoothly burn overall. Less of a drag racer, so to speak.

Aurelio Ape wrote:If I'm not mistaken, the chimney door must be separated from the box enough to leave a space equivalent to that of the primary air, and that the feed is fed by this same air, in the style of the Mallorcan workshop.

It might be that you interpreted this detail the wrong way, between the bricks and the steel door frame there isn't an air gap. This space of about 10 mm needs to be filled with strips of superwool. Have a look at the video of the Mallorcan build, there's superwool between the firebricks of the core and the red bricks of the bell. The door frame is separated with the same kind of strips from the bricks as well. The door frame is fixed to the bell with steel wire around the whole core, fastened through small holes at the back of the door frame.
In general: all steel parts are separated from the (hot) bricks by the same wool.

Peter, I will never be grateful enough to you.

All understood, thank you.

Reviewing the entire thread, extremely surprised by your response, I have understood why. The truth is that your brick fitting is very correct, in fact it is practically the same thing that I ended up doing. I'm confused, I don't know how it could have happened; but in one of those cut and paste, do and undo components, I got the result of the drawing that you see, and that was what I based it on. Without being an expert in the matter, it can be seen that stacking the bricks like this is not correct at all. I assumed it was a minor bug due to speed and corrected it. I went to bed thinking that it was not possible for you to stack bricks like that. Looking at your file you can see that the work is perfect. I have also seen the interlocking elements that were actually loose. I did useless work, then.

Newbies with sketchup, yuck.

Hello partners.

Although late, I wish you Happy Holidays and a Happy New Year

After much drawing and erasing, he arrived at a final design, except for any objections you may present.

I have tried to separate the hood as much as possible from the walls. The bench makes a small jog to fit the wall.

The interlocking of the bricks to interlock the different directions of the walls will probably not be the best design; but that's what occurred to me. I accept all possible improvements.

If you have not made a mistake in the calculations, the area of the hood and bench add up to 9.35 m2.

I have several specific questions.

The lintels that give way to the smoke on the bench and the exit to the chimney are supported by pillars instead of metal beams. Appropriate option?

For the lintel of the hood over the firebox, he used a 30x30x3mm L-profile and to support the first layer of the hood roof, a 30x30x3mm T-profile. Are the dimensions and thickness adequate?

The door is an adaptation of the Mallorcan design to the dimensions of 200mm. The air inlet in the door is 7,182mm2 and the gap between the door and the anti-ash cover to feed the primary inlet is 7,700mm2. Is this setting correct?

The back of the sofa and the armrests are designed from aerated concrete blocks. Suitable option? The exhaust manifold cover is designed with the same material. Appropriate option?

Thanks for your patience.

Aurelio Ape wrote:The lintels that give way to the smoke on the bench and the exit to the chimney are supported by pillars instead of metal beams. Appropriate option?

It could work this way. But... it looks like you shifted the whole of the main bell forwards in order to be able to open up a third opening to the bench.  Plus a joggle at the left hand front in order to make the connection to the bench doable.
I'd recommend to shift the bell back to about 100 mm from the (external) wall. To solve the pillar versus lintel problem, you could use a simple steel column in the middle of the opening. The troublesome 45 degree angles in the bench aren't then required anymore. In order to simplify the exhaust system, you could build a joggle at the bell's left hand rear corner in order to house the bare chimney pipe.

Apart from all that, I don't see how you will be able to sweep the chimney.

Aurelio Ape wrote:For the lintel of the hood over the firebox, he used a 30x30x3mm L-profile and to support the first layer of the hood roof, a 30x30x3mm T-profile. Are the dimensions and thickness adequate?

The 30x30x3 mm T-profile is definitely too small, use 40x40x4 mm minimum. You'll need some small gap between the bricks and the steel to allow thermal expansion anyway. Every other encounter between steel and brick should be separated by 12 mm superwool. The bricks in the top of the bell could be turned 90 degrees with the steel T's spaced further apart. Steel directly above the riser should be avoided if possible.

Aurelio Ape wrote:The door is an adaptation of the Mallorcan design to the dimensions of 200mm. The air inlet in the door is 7,182mm2 and the gap between the door and the anti-ash cover to feed the primary inlet is 7,700mm2. Is this setting correct?

The air inlet opening turns out to be 7182 mm² according to my measuring. Standard opening  for a combined primary and secondary air inlet is 25% of the system's csa. So this would be 7850 mm², while starting up you need more anyway, so opening the door a crack is required then. The anti-ash cover or threshold is about as high as the slanted bricks, that's correct. The area of the threshold is largely irrelevant but the distance is not. The shortest distance between the steel of the door and the threshold in your drawing is 25 mm, that could be a bit more, say 33 mm. Just place the threshold a little (8 or 10 mil) to the back and you are there.

Aurelio Ape wrote:The back of the sofa and the armrests are designed from aerated concrete blocks. Suitable option? The exhaust manifold cover is designed with the same material. Appropriate option?

No problem, but it's fragile so the back rest need to be protected by plaster or tiles.

Thanks Peter for your answers.

The reason for the intricate design was my desire to separate the hood as much as possible from the walls, without interfering with the passage of the door or the window. In my ignorance I thought it would be for the best. If you think it's completely unnecessary, I'll redesign it. t was a little hell drawing the turns of the walls and their links and I'm sure it would be no less to do it in reality.

Chimney sweeping is planned from the lower level and can ultimately be accessed from the roof terrace, Dick van Dyke style.

I did not think of any record opening at the bank. maybe one at the beginning of the bank and one at the end? What opening surface is suitable? Does the hood have to have a register?

“The standard opening for a combined primary and secondary air intake is 25% of the csa of the system” That data is crucial, I didn't know it, is it on the web? I will redesign the door with that premise.

The common brick joints are designed to be 5 mm thick and the refractory joint is 2 mm. Are they appropriate?

Greetings to all.

Hello partners.

I attach the last drawing of the heater and the bench. There are new things that I submit to examination.

Metal profiles. Metal and heat do not combine well, I made a design in which the lintels rest on pillars, it was well within the dimensions of the web; but it is undeniable that they restrict the passage of gases. I made this other one, with beams for the openings. As the joint is 5mm I have opted for a 50x50x5mm L profile. The wings are inserted in place of the joint and the brick rests without mortar on the beam. The expansion of the thickness will be minimal, but not in the length, for which I will leave a 3mm tolerance on each side, I think it will be enough. I use two facing L's to hug the bricks. They are located on the lintels leading to the bench, at the exit to the chimney and on the batchbox. The latter will be wrapped in superwool, the others I have not contemplated. Give me your opinion on all of this.

The internal area of the bell is 5.41m2, that of the bench is 3.53m2; their sum is 8.94m2. It is below the 9.4m2 of the web. To get closer to that figure I have three options. First, raise a brick plus the bell (11cm), it has a height of 2.09m so it would be 2.10m; Keep in mind that the ceiling is lower than usual, it is only 2.38m. Second, lengthen a brick plus the bank (24cm). Both report 0.3 m2, so we are very close to the recommendation. Third, leave it like this to ensure a slight power excess and good draw in all circumstances. Which option do you consider more correct?

I was worried that the fumes that go down the right and rear side of the batchbox would find the exit to the outside before the passage to the bank. I placed some unglued bricks over the lid of the box to cover that gap, is it useful or a waste of time?

I placed three 10cm diameter round logs, please tell me if the number and location are correct.

On the door, I sized the air inlet to 7850mm2, I found that the profile that supports the glass is the one that restricts the passage of primary air upwards, to eliminate this possibility I have chosen to reduce the lower part of the glass by increasing the metallic part, with which we leave enough space for the primary air. Tell me if it seems correct to you.

For the bench I found some cement slabs with 100x100x4 cm, a bit big to handle; I can always cut them in half. I have to ask if they have metal mesh inside, I imagine that this is not advisable for mixing with heat.

Thanks in advance for your cooperation, Peter.

Aurelio Ape wrote:Metal profiles. Metal and heat do not combine well, I made a design in which the lintels rest on pillars, it was well within the dimensions of the web; but it is undeniable that they restrict the passage of gases. I made this other one, with beams for the openings. As the joint is 5mm I have opted for a 50x50x5mm L profile. The wings are inserted in place of the joint and the brick rests without mortar on the beam. The expansion of the thickness will be minimal, but not in the length, for which I will leave a 3mm tolerance on each side, I think it will be enough. I use two facing L's to hug the bricks. They are located on the lintels leading to the bench, at the exit to the chimney and on the batchbox. The latter will be wrapped in superwool, the others I have not contemplated. Give me your opinion on all of this.

There are some things I would do differently.
First: the steel lintel between bell and bench could be left out altogether. Start with the surroundings of the bench and bell, up to the seat height. Next the core and after that the closing of the bench. The bench' seat could be extending to under the bell's wall so that wall would be supported by the seat. In case you think that won't be sturdy enough, best to support the seat temporarily while the bell is built. After hardening of the mortar it's a monolith and won't press down as with loosely stacked bricks.

The lintel above the door need to be separated completely from the bricks by superwool, otherwise the steel will expand and crack the bricks apart. I would use a steel strip completely encapsulated by superwool, no steel exposed at the inside or outside.

Aurelio Ape wrote:The internal area of the bell is 5.41m2, that of the bench is 3.53m2; their sum is 8.94m2. It is below the 9.4m2 of the web. To get closer to that figure I have three options. First, raise a brick plus the bell (11cm), it has a height of 2.09m so it would be 2.10m; Keep in mind that the ceiling is lower than usual, it is only 2.38m. Second, lengthen a brick plus the bank (24cm). Both report 0.3 m2, so we are very close to the recommendation. Third, leave it like this to ensure a slight power excess and good draw in all circumstances. Which option do you consider more correct?

I would leave it as it is. The recommended values are considered to be the maximum.

Aurelio Ape wrote:I was worried that the fumes that go down the right and rear side of the batchbox would find the exit to the outside before the passage to the bank. I placed some unglued bricks over the lid of the box to cover that gap, is it useful or a waste of time?

What most people need to learn, myself included, is trusting the laws physics. You don't need to cover that gap and besides, when the temperature difference of the gases that are in the bell and the ones provided by the core is small, the gas current tend to bypass the bench. As such, it'll help with startup. As soon as the temp difference is greater, the stratification effect will kick in.

Aurelio Ape wrote:I placed three 10cm diameter round logs, please tell me if the number and location are correct.

In case you mean the clean-out ports, they seems OK to me.

Aurelio Ape wrote:On the door, I sized the air inlet to 7850mm2, I found that the profile that supports the glass is the one that restricts the passage of primary air upwards, to eliminate this possibility I have chosen to reduce the lower part of the glass by increasing the metallic part, with which we leave enough space for the primary air. Tell me if it seems correct to you.

The air inlet won't need that extra metal part. In fact, the design is done in such a way the primary air is limited by its path and the secondary air is
favored by it. During most of the burn, primary air isn't that important so don't worry, it's all tried and tested.

Aurelio Ape wrote:For the bench I found some cement slabs with 100x100x4 cm, a bit big to handle; I can always cut them in half. I have to ask if they have metal mesh inside, I imagine that this is not advisable for mixing with heat.

Not a problem for most of the bench, the part closest to the bell will be warmer though.

Thank you, Peter
What a good idea that one of the slabs of the bench is the lintel.
All the rest, I have understood.
I just want to ensure one detail, I think I understand that the bench slabs CAN have an internal metal mesh and that the heat they will receive will not cause the expansion of the mesh to crack the cement.
I will redo the drawing with your instructions.

Hello everyone
I have made the corrections indicated by Peter. I think the design is finished. Peter give me your thumbs up if so.
I have two more questions left, I need you to confirm that the slabs on the bench can have an internal metal mesh and that metal expansion does not cause problems. What material is the right one to close the top of the batchbox?
Thanks in advance.

Aurelio Ape wrote:Hello everyone
I have made the corrections indicated by Peter. I think the design is finished. Peter give me your thumbs up if so.

Two small niggles: with supporting the wall of the bell with the seat of the bench I meant both slabs, not just one. The other niggle is a bit more complicated so I'll explain. As a rule of thumb, I calculate the height of the opening to the chimney as half the internal height of the bench. In your drawing, the bench is 8 bricks high and the exhaust openings only two. I'd recommend to have that 3 bricks high, at least. Reason: over time there will be a layer of fine ash at the bottom of the bench, potentially blocking part of the exhaust openings.

Aurelio Ape wrote:I have two more questions left, I need you to confirm that the slabs on the bench can have an internal metal mesh and that metal expansion does not cause problems.

As I wrote before, for most of the bench it won't be a problem, although close to the bell it might. Just a suggestion: lower the bench internals by one brick and add an extra slab on top. By doing so, the seat will contain much more mass which lowers the seat's temperature a bit, much more comfortable to sit on.

Aurelio Ape wrote:What material is the right one to close the top of the batchbox?

A reactory slab, the same material as the firebricks. Or you'll need to cast a slab out of refractory castable. Not refractory cement, that's a different material.

To conclude: the closing of the bell is best done with bricks that doesn't overlap the bell's outer skin. Keep everything inside that skin so there won't be cracks in view.

Thanks Peter
I will gladly make the indicated modifications. However, to expand my knowledge, doesn't raising the output threshold make us get less heat capture? If the problem is soot clogging, in my design I see it as feasible, using the clean-out port and a vacuum cleaner, to clean it. I thought an exit from the hood to the chimney was sufficient, with an obvious minimum equal to the diameter of the riser, in this case something less than double. Do I increase the height by one brick and keep the widths?
Thank you, professor.

Try to imagine this: the hot gasses entering the bench from above, rounding the corner at a low pace because of the wider space. Due to being hotter than the air that's in there already, it'll cling to the top. At the same time a colder stream in the lower half of the bench is moving in the opposite direction. The lower stream is moving at the same pace as the top one, although the lower one is at a lower temperature. The hot stream is moving inside the bench right to the end, sinks down to the bottom, becoming the one in the opposite direction. There will be a slight difference in the volume of both streams overall, since the bottom stream is cooler but still giving off heat to the sides of the bench. You will feel for yourself that the seat will be hottest all the time, even when the fire is gone and the heater closed.

In theory, the lower stream would take up less space but at the same time even the smallest restriction will hamper the stream. One could argue the openings together are larger than system size but there's already some friction at the T-piece so in my mind it's important not to add more potential friction points. Give the gasses plenty of room, much more than one would expect to be sufficient, that's all.

Crystal clear, thanks Peter.