My first Batchbox build, on top of Barcelona's mountain.

Hello permies, I'm Sara from Barcelona!
I'm happy to finally introduce myself, after some months of lurking around and absorbing every bit of knowledge all you nice people so kindly share!

I heard for the first time about rocket stoves and batch boxes this February. I was doing some research on masonry heaters to replace a useless pellet stove and finally solve
the heating of my home, and ended up in this forum. I tell you, for a couple of months all I could do was to read and watch every bit of info available on the subject!
As a person who's been almost my entire life living with fire stoves, my mind was blown up by the idea that most of the wood I've burnt in my life has been thrown away up the chimney.

Since then, I've been in a journey most of you people have already shared in so many useful threads. I've learned a lot, read about the pros and cons in many different options available, considered my needs and capabilities, and I'm finally ready to start planning my first build (*spoiler alert, it won't be the last!!).

Now I came to all of you with my ideas and plans and any question, contribution or correction you like to share will be much appreciated!

My needs

    - I live on top of Collserola, the mountain at the back of Barcelona, facing the sea at 411m/1.348 feet above sea level.
      Barcelona has really mild winters, average temp. 10ºC/50ºF, and what we can call winter goes from November to March if we are lucky.
     
    -My home is an open space plan building 92 m2 with 3m ceiling ( 301 square feet / 9'8 feet)
     My exhaust pipe runs through the old chimney 2,45m outside the building, straight from the ground, only the last 30cm are exposed pipe ( 8ft. / 12in.)
     Old building with 40cm. thick masonry walls (15'7 inch.) and the whole north wall is below ground level.
     Most of the ceiling is insulated by 6cm/2'36in. extruded cork slabs, and the dorm has a 10cm/3'93in. insulated sandwich panel roof.
     Only 3 old sized windows with double glass in the whole space.
     Last year the house was empty and cold, and inside the temp hasn't dropped below 14ºC/57ºF.
     Ideally would love to be around 22ºC/71'6ºF when living there.

   -I spend my day on my workshop next door, so my idea is to build a system that allows me to light one fire a day and forget about it until the next morning.

Considering all of the above, I believe I can manage with a 15cm/6"in. batchbox, but would love to hear the input of the rocket scientist
community, cause all I have are ideas : )

For the building, I've enrolled my father, who's a skilled craftsman and an experienced builder, I've learned a lot being around him, and
I'm a skilled craftswoman myself. I'm quite positive we can manage to do a great job with our skills and your guidance on stove making, and we already
plan to build another system at his home further on. In fact, if this works as I'm sure it will, I plan to build more for family members who already have expressed
interest : )

So far, I have made several decisions on materials and design that I would love to share and get your ideas on.

- Batchbox right sidewinder core to adapt to the shape of the space I have, with a single skin bell, a mass bench bell on the right and a hollow small bench to the left, to store
wood under and link the different levels of the space.

-Secondary air chanel through the floor of the firebox with RA330 metal tube.

-Heavy firebricks for the firebox and riser (VITCAS 60% Alumina), and IBF to insulate it (VITCAS Grade 26). Heavy firebricks for the top of the bell and DPB (Dry pressed bricks) for the base of the bell and the bench. Vermiculite board to top the bench.

-Gray concrete blocks to make the base of all the build and make a 20cm/7'8in. open air chamber to insulate from the floor and have an air flow from below. On top,
a layer of mortar with some insulation as the foundation for the whole system.

-Clay and sand render for the finish layer.

-Bypass to help start the fire when the stove is cold or when the temp outside is not that low, but you still want to heat the inside of the house (these days we're having below 22ºC/71'6ºF at noon but at night it can drop to 8º/46'4ºF).

My idea is to make a build that won't need repairs in a foreseeable future, I'll plan to be busy making the next one for my father, probably a small one for my workshop later.
An eventually replace all the wood stoves me and my family have around. No more throwing heat up the chimney!

Ok, I believe that's all for my first post, I can't wait to hear your inputs, corrections and thoughts, and I'll come by with new questions and dilemmas.

Best wishes to all of you wonderful people around the globe!






 
   

P.S I forgot to mention that we have a wet saw!
     And that to bind and seal all those bricks we'll use clay and sand, refractory clay for the high temp. areas and regular on the rest!

Hi Sara;
Welcome to the wonderful world of rocket science!
Oh, and Welcome to Permies!

It sounds like you have a good understanding of how to approach a new build.
An overall good plan, of course, there will be details that will need clarification.

I do suggest buying an RA253ma stub for your secondary tube, rather than the RA330.
I have used both and sold both, but the RA253 has proven itself to be as impervious to the internal batchbox temperatures as the RA330.
I have stopped stocking the RA330 and now only stock the RA253, as it is that good.

With a heavy brick core and riser, it will take a bit to reach working temperature, but it will retain that heat far longer.
All my cores are heavy brick, but for the riser, I use Morgan Superwool and create five-minute risers for them.

Especially with your climate, a good bypass is essential for enjoyable operation.  

thomas rubino wrote:
It sounds like you have a good understanding of how to approach a new build.
An overall good plan, of course, there will be details that will need clarification.

Hi Thomas! so nice to finally talk directly with you, after so many hours spent reading all the knowledge and experience you so kindly share : )
I'm here for the details, as they are the key to the success of any project.  With so many possibilities and variations to adapt to every specific project, I have a storm of ideas in my head, and I need to start making decisions.

thomas rubino wrote:
I do suggest buying an RA253ma stub for your secondary tube, rather than the RA330.
I have used both and sold both, but the RA253 has proven itself to be as impervious to the internal batchbox temperatures as the RA330.
I have stopped stocking the RA330 and now only stock the RA253, as it is that good.

This will be my first detail to settle : ) The idea of getting the RA330 came from this encyclopedic and enlightening thread on donkies
https://donkey32.proboards.com/thread/3909/firebricks-refractories-fail-silica-flux  
Doing my simple math ( I'm an arts girl myself; my science teachers would be over the moon to see me devouring chemistry and physics threads in my forties!) I concluded that if burning metal accelerates the degradation of the materials inside my building, I should get the metal that degrades the least in contact with the fire.
This way, I'll have a longer lifespan for my build, right?

thomas rubino wrote:
With a heavy brick core and riser, it will take a bit to reach working temperature, but it will retain that heat far longer.

When you say it will take a bit, considering I'll be burning mostly hardwood bought from  cleaning forests, kindling will come from the garden where I have an endless supply of soft and hardwood, does this mean the stove will reach working temperature within the first load? Or maybe when the stove is cold, I will have to make a couple of loads in a row to get it going?
I don't mind if this is the case, when you start the heating in a home, it takes a while after the place gets warm. I work next door so if necessary, I can give more attention to the stove for a while, I've been attending two unefective stoves (home and workshop) for a while now, still sounds like heaven.

thomas rubino wrote:
All my cores are heavy brick, but for the riser, I use Morgan Superwool and create five-minute risers for them.

I've considered the use of ceramic fiber, but after reading another useful thread on donkies, I'll try to keep it only in unavoidable places like gaskets and expansion joints, and away from fumes as much as possible.
When this dream come true eventually requires some maintenance, I would like to be able to work with as little precaution as possible handling the repairs. I recognise it is a super useful material but I'll go for a slower rocket that goes far away ; )

I've find a supplier for this manufacturer Izospar in Spain,  and I'm wondering if the ring B150/100 could make a good riser, and maybe a second layer of some of the other bricks (I have still to figure out the math) for good insulation.

thomas rubino wrote:
Especially with your climate, a good bypass is essential for enjoyable operation.  

On that point, first of all, I wonder if experience with lightning fires in stoves and open fires does help in this subject of starting a rocket stove? Because I can set a fire going in any condition, I've been warming my home like this since I can recall. I don't know if this gives me any advantage?

Anyway, another detail I still have to figure out, is where exactly to locate the bypass. I want to keep a straight exit draft to take advantage of the 5m/16ft. chimney I have, and with my floor plan that's on the back corner of the stove.
I have to find a way to make the bypass operable from the front of the bell.

As you can see, I still have a lot of things to figure out, but every question brings me closer : )

Good night from Barcelona!

Hi Sara;
Nice to meet an aspiring rocket scientist like yourself.
You certainly have the right attitude to accomplish this!
I'm looking forward to seeing your future builds.

So, I have two Batchboxes in un-insulated buildings.
This is northern Montana, we get cold here...
These stoves burn back to back batches all day long.
Much more than any indoor RMH would.
I call them extreme burning batchboxes.  If something is going to fail, this style of burning will quickly identify it.
Carbon steel stubs lasted 6-10 weeks in my extreme batches before they began to spall badly.
RA330 was introduced to the RMH world by Matt Walker, using it on his cook stoves.
I bought and tried the RA 330; it works as stated—no spalling after two years of hard burning.
I became a dealer for rolled alloy and sold dozens of RA330 stubs at $90 each; they were not inexpensive.
I researched other Rolled Alloy high-heat products and discovered RA253ma, which is listed as only 100°F less than RA330.
I gambled and purchased enough to offer them for sale at $60 each.
Meanwhile, I removed my RA330 stubs and installed RA253ma in its place.
Multiple years later, I can confidently state that the RA253ma is a lifetime purchase; they have proven themselves able to withstand extreme burning with no wear.

One load in a batch is enough to heat the core and riser to operating temperatures.
However, except for the steel door area, you will not feel the heat for several hours.
If you burn two loads, you may find it quite warm and start opening windows.
Heating with bricks is quite different than heating a steel box...
As I say at Dragon Tech, once you go brick, you'll never go back!

Morgan Super Wool.
It is sold as a non-ceramic, fully bio-soluble (Safe) alternative to ceramic blanket.
After heating, it becomes hazardous, similar to fiberglass insulation, and can cause life-threatening problems.
The thing is, you have to breathe in the dust.
For me, it is relatively easy to wear a mask, lift a used riser off the core, and place it safely out of the way.
Treat a used riser with care, and avoid sticking your face over it, makes common sense.

Insulated firebricks (IFB) make the best risers, usually wrapped with rockwool.

You want and need a bypass.
If you have a bell, there will be a way.
A smoke-filled house does not make for happy spouses!

Hi Sara,
the plans for your RMH sound very good. What an exciting project. Also with the outlook of building more for your friends and family.

I will be in the area probably in July and then again in November to build a J-tube RMH for a costumer close to the french border.
I would be very interested to see your rocket.
If it works timing wise, I also would be happy to help in the build. I haven't built a BBR yet and it would be a good opportunity for some first hand experience.

Hi Thomas,

I'm glad to be a freshman in the Rocket science academy so far, hoping to graduate before next winter! I can't wait to see how it's going to turn out myself ; )
I have some ideas for the finishings too, more details to settle!

Certainly, at the rate you burn your fires I can get an idea of what mine can stand in decades : ) RA253ma sounds like exactly what I'm looking for.

I'm going to have a bypass for sure, just have to figure out how's it going to look like. Could it work to add a T section on the tube, right before exiting the bell?
We could open the short side of the T, allowing hot smoke to get out right after exiting the riser.
Would it affect the function of the bell to have a thin metal bar up there? It would cross from the front of the bell to the back corner, where it would open or close the lid of the T. And can be coated with something like VITCAS Zircon Paint Coating to protect it from the heat?
Also, how tight has to be this bypass? Is it important that it can be shut airtight or can be a metal disc slightly smaller than the tube it has to block?

As for the cold start, I'm comfortable with the idea that it will take a bit to warm up the beast. Once the season is on, it won't happen often that it stops for many days in a row. And when it eventually happens, I know for a fact the thermal inertia of the house will play in my favor.

Morgan Super wool, the fact is my concern is not just about the moment to handle it when doing repairs, but how to dispose of it afterward.
We're a small territory highly polluted due to industrial activity, and this type of residue is a nightmare to get rid of.
Unless it's completely unavoidable, I'll go for other options, and I'm willing to handle the inconvenience these can carry.
If I have a stove that will take a bit to light and warm up, I don't mind tending to it a little bit when necessary.

On this subject, does any sense to make a IFB riser, and make an outside layer of IFB also to insulate the riser? Am I making any sense? How can a riser be well insulated if not using Super wool?

I'm hoping to have some time this week to start some drawing, see if I can come up with something that can show you more what I have in mind!

Best,
Sara

Hi Benjamin!

It is as exciting as the night before Christmas : )
I don't have a clear calendar yet as we have to coordinate with my father and my work, but for sure you're welcome to see whatever there is at the time you're around.
And if timing is right, any help, especially of someone with more experience than us in rockets, will be much appreciated.
I'll keep you posted!

A riser built from one layer of IFB will give enough insulation, you don't need more. You would want to wrap the riser with wire to be sure it doesn't split over time, and a layer of rock wool (mineral wool) under the wire wouldn't hurt. This is much more durable and heat-resistant than fiberglass.

Hi Glenn, nice to meet you too!

All you frequent flyers have been my night reading for the past months, so nice to finally be digging in : )

So if the riser is made of IFB, it will have enough insulation on its own? That's good to hear! The wire to secure the riser should be coated to expand its life inside the bell?
Strips of mineral wool are a downsizing from a full sleeve for the riser to dispose when doing maintenance, so I'm getting closer to what I want.

I have found another manufacturer closer to home, from the Basque Country, that offers a wide range of IFB and other materials.
They're called LABELAN https://www.labelan.net/en/products/bricks and now I have too many options. My goal is to make a stove I won't have to worry about in a foreseeable future, which of these bricks would work better to build the riser and insulate the firebox?

-"Firebrick AL80
Dimensions: 230 x 114 x 64 mm
Maximum temperature: 1550ºC
Alumina: 79-82%
Format: Possibility of other formats
Characteristics:
High purity.
Low iron content.
Good mechanical resistance.
Excellent resistance to thermal shock.
Intended use:
Coating of industrial forging furnaces, heat treatment furnaces, boilers... especially screeds.

-"Insulating firebrick G-26
Dimensions: 230 x 114 x 64 mm
Maximum temperature: 1430ºC
Alumina: 56%
Characteristics:
Excellent insulating qualities.
Mechanical and temperature resistance superior to group 23 bricks.
High purity.
Thermal shock resistance.
Intended use:
First layer lining (direct exposure to heat source).
Second layer lining behind dense materials in forge furnaces, aluminum melters, etc."

They make also slabs, I'm wondering if these would work as the top of the fire box and the bell:

-"Refractory insulating slab G-23
Dimensions: 610 x 230 x 64 mm
Maximum temperature: 1250ºC
Alumina: 42%
Possibility of mechanisation.
Characteristics:
Excellent insulating qualities.
Low iron content.
High purity.
Intended use:
First layer lining (direct exposure to heat source) for heat treatment furnaces.
Second layer lining behind dense materials in high temperature furnaces (forging, etc)."

And if this will make a good ceiling for the bench?

-"BIO fibre plate 1000ºC 5-50 mm
Dimensions: 1200 x 1000 x 5/10/20/40/50 mm thickness
Maximum temperature: 1000ºC
Characteristics:
High insulation capacity.
Easy handling and cutting to make adjustments.
BIO product - Does not contain products harmful to health.
Intended use:
All types of insulation, both industrial (insulation layer against sheet metal), and for use in rocket mass heaters, home ovens, etc. In treatment or tempering furnaces they can be placed as a working layer."

For the lower rows of the bell and the bench I'm planning to use Compressed Stabilised Earth Block, made close from home. I'm trying to locate a translated technical sheet to share, but I understand they're made from pressed clay and sand and the stabilizer is less than 0'5% cement. Dry pressed.  

My idea for the whole build is to finish it with clay, sand and straw to seal it and aesthetics too. For the siting area on the bench, I would love to finish with ceramic tiles on top to have a resistant surface. Because it's going to be the favorite spot of my dogs besides mine, and those paws are like grinders.
This are the tiles: https://ceramicaferres.com/en/products/catalana/
And this is what the manufacturer says about them:
“  The base used is a mixture of clays from the area, giving them strength and unique properties
for extrusion and single firing. The wide range of colors is derived from the
clays themselves and from oxides applied together with a varnish unique for
its transparency and smoothness, which gives the surface an appearance and
texture similar to glass. The transparency of the surface and the unchanging
quality achieved by the colors at a temperature above 1000°"
I assume they're alright to place on top of the insulating slabs and the clay and sand finish.

I understand every layer of material is going to add thermal mass, but also delay the heat transfer. Is there a limit to how thick the walls can be? I'm not intending to over do with the clay and sand, but for what I know different layers have to be applied with different composition in order to achieve the effect I'm looking for.

Here you go, another round of questions, and I have many more to come!

Best from Barcelona to you all!

It looks like the G-26 IFB would be fine for the riser, while the AL80 firebrick would stand up to wear in the firebox and port.

I think the slabs would work for firebox and bell ceilings, though you don't really want insulation on the bell or bench. I would be concerned about the insulating slabs on the bench without clear indication that they are very strong. Simple concrete pavers (available in the US up to 24" x 24") work excellently as bench top structure, and transmit heat well. Any structural ceramic pavers should work.

Cob outer layers will work nicely for giving organic form to the heater and another layer of sealing assurance. I would not go more than about 10 cm thickness on bench and bell sides, maybe 15 cm total thickness including the CEB. I have a layer of bricks on their sides ( 2 1/2" or 6-7 cm) plus 6" or so of cob (15-17 cm) in my bell walls, and it takes hours for the heat to start to come through. If I had it to do again, I would make the cob an inch or two thinner. Corners give much more exterior surface for a given amount of interior surface, and even if rounded on the outside will be much cooler than flat areas. My bell corners never feel warm unless I am burning hard for several hours. I would make exterior corners with a radius at least equal to the total wall thickness.

The G23 sounds like it would work for the riser too, and sounds less "special" i.e. probably more affordable.
They don't have any prices on their website, please share the price if you get a quote.
So far I've only encountered big companies that don't sell to individuals around here.

And in general I found the insulating firebrick quite expensive. So far I've built a riser from dense firebrick with Rockwool insulation around and one "5-Minute"-Riser with SuperWool.

As a potter, I have occasionally come across old electric kilns that do not work, but are made of insulating firebrick. Each of these has around 50 to 100 firebricks which can be salvaged and used to make risers and other RMH parts with a little work.

Glenn Herbert wrote:It looks like the G-26 IFB would be fine for the riser, while the AL80 firebrick would stand up to wear in the firebox and port.

That's good news! I'm sure this IFB will be more expensive than the regular ones I've been seeing on oven and barbecue suppliers, but for this first stove I have a certain budget. Considering any other alternative to replace the innefective appliance I have now would be super expensive, and that there won't be labor cost because we'll be doing the job, I have room to invest in materials.
Let's say my first experiment on this one is going to be how to make an everlasting stove (considering the weather it's going to be up against).

I'm waiting for the quote from them, but on the website they specify they sell to craftsmen and individuals, and ship throughout Spain and Europe. They also sell this mortar,
I wonder if depending on the cost, it's something worth the money?

Mortar MH-25 Base raw material: Chamottes, clays and silica
Maximum temperature: 1450ºC
Alumina: 20-22% (wet) - 25-28% (dry)
Packaging: 25kg pots
High thermal resistance.
Fast hardening.
Supplied in a semi-liquid state.
Intended use:
Assembly of bricks, wedges and refractory slabs.
Approx. 200kg per 1000 bricks


So far:
Riser - G-26 IFB
Firebox and port - AL80
Firebox ceiling - slab G-23
And this way the firebox and riser are already well insulated and don't need a second layer of insulation? Just some wire to reinforce the riser.

For the bench ceiling, some slab with enough strength that is not insulating, but rather conducting.

Super helpful reference on the cob, as it's probably the area where we have less experience. My father did a cob oven some twenty years ago, and it's still standing and working. Considering it's completely exposed outside, it's a good beginning, yet it's always been rough in the corners. But I have my mind set on not just making a safe and efficient stove, but also a beautiful object that's going to be the center and heart of the home.
A cracked surface will be something I want to avoid, and if need be, repair, so any advice on this matter will be much appreciated. I'm much more confident in our brick laying abilities : )
I take note on the corners, it's super logical, but before you mentioned it, I didn't even think about it.

Also, repurposing oven bricks it's super helpful advice for further projects!

I'll come back with more, thank you for all your inputs!

Hi Sara,

I built a batch in Barcelona two years ago. The original design included benches but I found lots of barrels and opted for a simpler version consisting of three barrels and I can retrofit it and incorporate the benches in the future.
You are wellcome to come by and have a look if it helps you anyhow.

Did you complete your build?

Hello fellow rocketeers,

I'm back on track! After last we talked, life got on the way of my plans and the stove came to a second plane.
The cork insulation was finished, learnt to fix the pellets stove, and I was getting really comfy in my house after years of not being able to live in.
But it hasn't lasted long, last christmas what I thought to be water leakage from heavy rains flooded the house through the chimney hole.
After a week of rain we discovered that actually it was a pipe that burst that nobody knew it was there (old house's charm, right?).
Conclusion, my house is a wreak once again, and it's a clear sign this rocket has to be built.
I had to throw away the current floor, and with it part of the old one came out too, so now I don't have to bother for the mess that comes with a build.

So I'm knee deep again with the rocket.
I have a first sketch, for sure full of errors, inaccuracies and miscalculations, any input will be welcome!

My first disappointment is that with the plan as it is I'm already at 5m2 ISA, so no space left for a bench.

First questions raised by these drawings:

-Can the depth of the portal be 6.4cm? And the height of the riser 105cm instead of 108? 6.4 is the width of my firebricks.

-Between the back of the firebox and the back wall of the stove I have left 21cm( x 61cm width). Is that enough?

-Would it make sense to draw the riser on the left, place the exit pipe on the right corner and have a much shorter lever for the bypass?

-Is 3 or 2 cm space enough to leave between the pipe and the internal walls? The exit pipe will have an angle as it is because of the thickness of the wall of the stove,    so I may well place it wherever I want.

-If I rise the core with a metal structure, would a vitcas paint coating 1750º/3180º or similar be ok to protect it and the wires to reinforce the riser and core?

Ok, that's all for now! I hope the drawings are clear enough. Thanks for your time and I hope you're all doing well!

P.S. Hola David! As you can see, nothing done yet. I would love to see your build and hear your experience with it, are you far away from Barcelona?

sara ventura wrote:First questions raised by these drawings:

-Can the depth of the portal be 6.4cm? And the height of the riser 105cm instead of 108? 6.4 is the width of my firebricks.

Yes, the port could be deeper than suggested, so 6.4 cm won't be a problem. The riser could be anything between 10B and 8B. Shorter isn't recommended. Top gap, above the riser, AT LEAST the chimney's diameter. More is better.

sara ventura wrote:-Between the back of the firebox and the back wall of the stove I have left 21cm( x 61cm width). Is that enough?

This should be enough by itself, but there's another opening, at the right side of the firebox. Together they provide plenty of space. It is important that the gases are able to stream under the firebox, and from there to the chimney.

sara ventura wrote:-Would it make sense to draw the riser on the left, place the exit pipe on the right corner and have a much shorter lever for the bypass?

It would make sense, although not for the bypass. The door is further away from the wall as a left-hand sidewinder, I would see that as an advantage. Thought: you are building a 1st generation batchrocket. Provided there is an adequate chimney, this model doesn't need a bypass. So, what will the chimney be like?

sara ventura wrote:-Is 3 or 2 cm space enough to leave between the pipe and the internal walls? The exit pipe will have an angle as it is because of the thickness of the wall of the stove,    so I may well place it wherever I want.

This internal chimney pipe should be insulated somehow, otherwise the heater's exhaust gases will be higher and efficiency would be quite a bit lower. Insulating of the pipe could be done with 1" superwool around it, fixed to the pipe with chicken wire.
Another thought: since your chimney pipe is in a corner, about half of the opening won't be used by the downward streaming gases that need to go through a 180 degree change of direction. Stream profile should be about twice the chimney's cross section area, in order to avoid friction spots. Calculating... that internal pipe should be at least 15 cm from the floor, assuming afor-mentioned pipe is shoved in the corner of the bell.

sara ventura wrote:-If I rise the core with a metal structure, would a vitcas paint coating 1750º/3180º or similar be ok to protect it and the wires to reinforce the riser and core?

Of course you could use that Vitcas coating. But I doubt it will be necessary, because during the running-in period the heater will produce quite some soot. All the cold steel inside will be quitely coated with a layer of wood tar, no corrosion expected. It escapes me what you mean with wires to reinforce the heater core?

Yet another thought: the top of the bell should be double-walled (bricks on edge), with thin (1/2") superwool between it. See the Mallorca or the Brussels build. The reason for this is that the top of the bell will get hottest during the burn and the bricks will expand because of that. The inner skin will be able to expand in the above construction, without pushing the outer skin apart.

Hello Peter, and thanks for your fast and helpful answer!

Yes, the port could be deeper than suggested, so 6.4 cm won't be a problem. The riser could be anything between 10B and 8B. Shorter isn't recommended. Top gap, above the riser, AT LEAST the chimney's diameter. More is better.  

Noted, both portal and riser will be ok then. And as it is now I'm leaving 30 cm top gap, I understand I could lower to 20cm and still be into optimal functionality.

That leads me to the next answer:

It would make sense, although not for the bypass. The door is further away from the wall as a left-hand sidewinder, I would see that as an advantage. Thought: you are building a 1st generation batchrocket. Provided there is an adequate chimney, this model doesn't need a bypass. So, what will the chimney be like?

I was hoping to be able to have a hot bench next to the bell. Making some twitches to my current design I'm around 4.7 ISA.
I've read somewhere that by adding a bypass you can increase the ISA, how much are we talking about?
Is it feasible to have a small operating bench like the picture I attach below?
I'm almost certain it's an IA picture, but it's the one I've found that looks closer to what I have in mind for my build (Minus the oven).  In case this was possible, would all the face next to the bell be all open wide inside to allow the gases to flow freely?

Another thought: since your chimney pipe is in a corner, about half of the opening won't be used by the downward streaming gases that need to go through a 180 degree change of direction. Stream profile should be about twice the chimney's cross section area, in order to avoid friction spots. Calculating... that internal pipe should be at least 15 cm from the floor, assuming afor-mentioned pipe is shoved in the corner of the bell.

The exit pipe from the stove is currently 2cm and 15cm from the corner, so I have to move it anyway. If it's centered at the back of the bell, gases can flow from 3 diferent directions? Left, right and from under the core, is that correct? And the section of the pipe can be parallel to the floor?
Being at 15 cm from the floor, is it convenient to rise the core a bit more than that to not have them both at the same level?

This internal chimney pipe should be insulated somehow, otherwise the heater's exhaust gases will be higher and efficiency would be quite a bit lower. Insulating of the pipe could be done with 1" superwool around it, fixed to the pipe with chicken wire.  

I was planning to buy double wall insulated pipe for the masonry chimney to avoid condensation, which has always been an issue as it can be seen in the pictures of my corner. Can this same pipe be used inside the bell? and the section inside the house be simple wall?

It escapes me what you mean with wires to reinforce the heater core?  

Sorry, I got lost in translation, I meant rods. As seen in the pictures of the Thomas Rubino's new build, I meant the internal frame that braces the core, and the wires to reinforce the riser.

All the cold steel inside will be quitely coated with a layer of wood tar, no corrosion expected.

Then, probably just coating the wires from the riser that face the highest temps, and the rest of the metal inside the bell will be protected by the wood tar coating?
If I go with the double wall tube, will this be protected enough? or giving a layer of Vitcas on the upper part won't hurt?

Yet another thought: the top of the bell should be double-walled (bricks on edge), with thin (1/2") superwool between it. See the Mallorca or the Brussels build. The reason for this is that the top of the bell will get hottest during the burn and the bricks will expand because of that. The inner skin will be able to expand in the above construction, without pushing the outer skin apart.

My idea to crown the bell was to make double-skin only the last row and the top, as in the sketch below*. Should this apply to all the FB rows? Even with cob finish instead of double skin bell?
*I’ve realized just after posting that the dried press bricks are completely out of proportion. For some reason
I was working on the misconception that the profile was almost square…

Thanks again for all your inputs and time, I'm getting closer with each answer!
I'm a detail oriented person, and every input helps me figure out the matter in my head further.

sara ventura wrote:I was hoping to be able to have a hot bench next to the bell. Making some twitches to my current design I'm around 4.7 ISA.
I've read somewhere that by adding a bypass you can increase the ISA, how much are we talking about?
Is it feasible to have a small operating bench like the picture I attach below?

Good question... based on what I know about these things, I'd say you might go up to 6m² ISA. Mind you, when fired just half a load per day, the bypass need to be open a crack all the time. Having said that, it looks like a small bench is a possibility.

sara ventura wrote:I'm almost certain it's an IA picture, but it's the one I've found that looks closer to what I have in mind for my build (Minus the oven).  In case this was possible, would all the face next to the bell be all open wide inside to allow the gases to flow freely?

I'd think you mean the opening between the bell and the bench need to be as high and wide as the bench internal, the answer is yes. However see the remark about a bench lower down.

sara ventura wrote:The exit pipe from the stove is currently 2cm and 15cm from the corner, so I have to move it anyway. If it's centered at the back of the bell, gases can flow from 3 diferent directions?

That's correct, so the pipe's position, relative to the floor, could be lower.

sara ventura wrote:Left, right and from under the core, is that correct?

Yes.

sara ventura wrote:And the section of the pipe can be parallel to the floor?

Yes again.

sara ventura wrote:Being at 15 cm from the floor, is it convenient to rise the core a bit more than that to not have them both at the same level?

The pipe could be lower than the 15 cm, and even lower again when you are going for the following recommendation. The stream profile into the chimney could be improved by mounting a 18 to 15 cm reducer at the bottom end of the pipe. In order to ensure free flowing of hot gases in and out of the bench, I highly recommend that the core would be lifted to the level of the bench' seat.

sara ventura wrote:I was planning to buy double wall insulated pipe for the masonry chimney to avoid condensation, which has always been an issue as it can be seen in the pictures of my corner. Can this same pipe be used inside the bell? and the section inside the house be simple wall?

Yes, the same pipe could be used inside the bell. A short piece of single wall pipe inside the house isn't doing any good, better to continue with the double wall pipe.

sara ventura wrote:

It escapes me what you mean with wires to reinforce the heater core?  

Sorry, I got lost in translation, I meant rods. As seen in the pictures of the Thomas Rubino's new build, I meant the internal frame that braces the core, and the wires to reinforce the riser.

Hmmm... so you want to use the same construction as Thomas', a compression frame and no mortar? Can be done, if you want to.

sara ventura wrote:Then, probably just coating the wires from the riser that face the highest temps, and the rest of the metal inside the bell will be protected by the wood tar coating?

In my mind, it would be sufficient to use some mortar for the riser and have insulation around it, held together by the wire.  The wire is out of the highest zone and the wire is coated automagically.

sara ventura wrote:If I go with the double wall tube, will this be protected enough? or giving a layer of Vitcas on the upper part won't hurt?

It won't hurt, but the double wal insulated pipe is probably stainless steel anyway.

sara ventura wrote:My idea to crown the bell was to make double-skin only the last row and the top, as in the sketch below*. Should this apply to all the FB rows?
Even with cob finish instead of double skin bell?

Yes, and also every piece of wall that's above the riser's end, not just one row. The cob finish won't  work the same as a double skin bell. It is what is called, a finish, not the same as a construction. Please, don't under estimate the thermodynamic forces that are in play here.
The firebrick liner inside the higher portions of the bell could be split bricks, probably 3 cm thickness. This would help to simplify the build. Position of the bypass: just below the riser's top end, to keep it out of harm's way.

The bell's ceiling in the sketch doesn't need to be done like this, better lay the bricks dry, a layer of superwool and another layer of dry laid bricks on top, for ballast. The heater will be high enough to have the top out of sight anyway, so a cob finish is some overkill there.

With all the above, and the talk I've had today with my father, next week I'll make new sketches ; )

Definitively going for a bypass as I want to squeeze a bench for my build as much as it's possible.
Also reading through the post again, Thomas Rubino recommended it to start the fire on this not so rare mild days we enjoy in Barcelona during winter, early spring.
I want to start the fire on the days that inside the house is colder than outdoors.

I'd think you mean the opening between the bell and the bench need to be as high and wide as the bench internal, the answer is yes.

Yes. So the transition from the bench to the bell needs to be as open as possible. Let the gases settle freely, noted.

In order to ensure free flowing of hot gases in and out of the bench, I highly recommend that the core would be lifted to the level of the bench' seat.

The floor of the core aligned with the top of the bench? So the temp of the core doesn't mess with the cool gases going down and find the exit, yes?

But that will rise my top of the bell too, thus increasing my ISA again, no? 1m ISA would be a square bench of 50 cm square right? I'm having trouble imagining any other proportions that would work as a relatively comfortable seat for one person/dog.

The stream profile into the chimney could be improved by mounting a 18 to 15 cm reducer at the bottom end of the pipe.

Something like this, better in inox? And then the double wall pipe up to the exterior. Total length exit flue 5m.

you want to use the same construction as Thomas', a compression frame and no mortar

Not really, I didn't realize that was what it was. I thought it was just meant as a reinforcement  of the core and a way to secure the door anchorage.

Actually for the core, today my father suggested the possibility to do tongue and groove construction. Is there a reason I haven't seen any so far in the forum, other than the obvious extra work of carving the bricks? We have a wet saw and an skilled brick layer here.

In my mind, it would be sufficient to use some mortar for the riser and have insulation around it, held together by the wire.  The wire is out of the highest zone and the wire is coated automagically.

The firebrick we'll be using for the riser will be AL 79-82% 1550ºC,  with wire and a band of superwool for movement allowance and protection I'll be good to go?

Please, don't underestimate the thermodynamic forces that are in play here.
The firebrick liner inside the higher portions of the bell could be split bricks, probably 3 cm thickness.

About that, we've opened a can of worms with my dad, will be making new sketches next week with some (probably) crazy ideas, you'all'll let me know!

And to recapitulate, which are the highest temps expected at this points:

- Inside Firebox -
-Inside Riser -
-Inside Top bell -
-Inside Top Bench -
-Exit pipe -
- Optimal fumes temp at exit? -

Thank you Peter, rocketeers, have a great weekend!

sara ventura wrote:I want to start the fire on the days that inside the house is colder than outdoors.

That's asking for trouble, especially without a bypass. In order to have updraft, the temperature outside need to be LOWER than inside.

sara ventura wrote:The floor of the core aligned with the top of the bench? So the temp of the core doesn't mess with the cool gases going down and find the exit, yes?

Partly correct. I was refering to the movement of the gases, not so much the temp of the core. The gases are streaming under the core and into the bench quite freely, coming back along the floor and to the exhaust pipe, provided the core is at the level of the bench.

sara ventura wrote:But that will rise my top of the bell too, thus increasing my ISA again, no? 1m ISA would be a square bench of 50 cm square right?

A square with sides of 50 cm is 2500 cm², equal to 0.25 m², as opposed to 1 m². The seat plus the three closed sides of the bench should be in the calculation, all internal. Make a drawing and calculate the different surfaces, excluding the floor in its entirety.

sara ventura wrote:I'm having trouble imagining any other proportions that would work as a relatively comfortable seat for one person/dog.

I'm assuming you are talking about the outside measurements of the bench, please stick to inside, as long as ISA is in play.

sara ventura wrote:Actually for the core, today my father suggested the possibility to do tongue and groove construction. Is there a reason I haven't seen any so far in the forum, other than the obvious extra work of carving the bricks? We have a wet saw and an skilled brick layer here.

You haven't seen any of this tongue-and-groove in the forum, because it's a redicously amount of work without much benefits, plus you'll need more of the expensive firebricks.

sara ventura wrote:The firebrick we'll be using for the riser will be AL 79-82% 1550ºC,  with wire and a band of superwool for movement allowance and protection I'll be good to go?

I said: some mortar, not without mortar. Otherwise, keeping all bricks dry stacked and applying superwool and wire, you'll probably end up with something that feels like a bucket full of eel, every brick wants to slither somewhere else.

sara ventura wrote:About that, we've opened a can of worms with my dad, will be making new sketches next week with some (probably) crazy ideas, you'all'll let me know!

Sara dear, you speak about your dad as a masonry heater builder? In that case, he on the one hand and me on the other, seems to be involved in a competition. I know how that works, it won't, been there, done that. Better having your dad advising and overseeing the build then.

sara ventura wrote:And to recapitulate, which are the highest temps expected at this points:

- Inside Firebox -
-Inside Riser -
-Inside Top bell -
-Inside Top Bench -
-Exit pipe -
- Optimal fumes temp at exit? -

I've measured some of the above, about others I can do an educated guess, hereafter given as estimated.

- Inside Firebox - 900 ºC
-Inside Riser - 1150 ºC, tops, exit riser 900 ºC.
-Inside Top bell - estimated 500 ºC.
-Inside Top Bench - estimated 200 ºC, tops.
-Exit pipe - Something between 75 and 150 ºC.
- Optimal fumes temp at exit? - 80 ºC, depending on the temperature of the mass to begin with.

Thanks again for your time, Peter. I was trying to give fast and concise answers, but I'm afraid my English skills are not enough to accomplish both.

Sara dear, you speak about your dad as a masonry heater builder? In that case, he on the one hand and me on the other, seems to be involved in a competition. I know how that works, it won't, been there, done that. Better having your dad advising and overseeing the build then.  

My dad has no previous experience in masonry heaters. He has experience in masonry, brick and mortar mostly, wood craft and welding. He also has a tendency to overkill.
I was showing him this thread : https://permies.com/t/360535/Build-Generation-Batchbox-Double-Brick
From here he raised two questions I wasn't able to answer, one about the groove-and-tongue build on the core, which you already cleared for me.

The other is about the possibility to finish the top of the bell with an arch instead of a flat roof. Leaving aside the technical difficulty of making a vault, would this construction offer advantages on long term durability and performance?

The firebrick we'll be using for the riser will be AL 79-82% 1550ºC, with wire and a band of superwool for movement allowance and protection I'll be good to go?

What I meant to say is, the core will be bonded with just refractory slip, the wire in the riser will be for extra stability.

I'm assuming you are talking about the outside measurements of the bench, please stick to inside, as long as ISA is in play.

You're right, I was forgetting the extra thickness the outside will have with the DPB and the render.

Another idea that rose from the conversation with my dad, design wise, was to place the core in the center of the corner. That has brought me back to the Mallorca build plan, which was my original thought for the space I have.
I discarded it originally because I wasn't keen on the idea to leave an empty space inaccessible at the back of the stove, but seeing the real options I have I'm keen to compromise on this.

Having said that, I would make some adjustments on the plan to fit in to my space, as long as those won't affect the functioning of the engine.

-I would like to make the bench 40cm shorter on lenght.

-With the ISA I gain from shortening the bench, I could make the bell a bit higher. That would allow me to rise the core a bit further from the floor, not as much as to align with the ceiling of the bench, but closer.

- Having a higher bell, I want to keep it as slim as possible. In order to do that, would it be possible to leave part of the core outside the bell? How does it work, construction wise? Is there any thread around here where this has been done?

-And the last twist in design would be to forget about the cob render and clad the whole build once finished with glazed tiles. The house is poor in natural light and I have to finish the build in light colors. Would refractory mortar be necessary for bonding? This way I could have a render 5cm thinner than with the cob.
What are the contraindications with tiling, besides cracking?

I'll make up time this week to draw all this and keep moving forward!

Once more, thanks for your time and patience with an eager apprentice with the head full of clouds : )

sara ventura wrote:The other is about the possibility to finish the top of the bell with an arch instead of a flat roof. Leaving aside the technical difficulty of making a vault, would this construction offer advantages on long term durability and performance?

A vault isn't better in long time durability, sorry about that. The short story: every vault is bound to have a horizontal pressure force, most of the time countered by a ring of steel and/or large pieces of wall on both sides. To make matters complicated, applying heat will cause the bricks to expand, so the horizontal force will be increased and decreased following the burning cycle. Not good for a build that is meant to be as stable as a rock, bricks may start to wander.

sara ventura wrote:-With the ISA I gain from shortening the bench, I could make the bell a bit higher. That would allow me to rise the core a bit further from the floor, not as much as to align with the ceiling of the bench, but closer.

Hmmm... What I feel now we are coming closer to a newer design, the Shorty core. This will allow to have a lower bell, higher raised core and aligning the seat with the bottom of the core.

sara ventura wrote:- Having a higher bell, I want to keep it as slim as possible. In order to do that, would it be possible to leave part of the core outside the bell? How does it work, construction wise? Is there any thread around here where this has been done?

It has been done before, not very succesful I have to say. The core itself will get awfully hot, so you have to build something around and over it. Even then, there will be a difference in temperature between the protuding core and the bell's wall. I won't recommend this construction.

sara ventura wrote:-And the last twist in design would be to forget about the cob render and clad the whole build once finished with glazed tiles. The house is poor in natural light and I have to finish the build in light colors. Would refractory mortar be necessary for bonding? This way I could have a render 5cm thinner than with the cob.
What are the contraindications with tiling, besides cracking?

A cob finish of 5 cm (2") or more looks like quite a bit. When clay/sand plaster is used, the layer would be much thinner. Applied in two layers, a shrink netting embedded in the first. A finish layer in a light color and you have your white heater.
As for tiles, of course you could do that. But you'll need high-temperature silicon caulk for that, otherwise the expansion of the bricks will cause them to fall off. The brickwork underneath the tiles should be very flat and straight, maybe even plastered thinly.

Did you see the heater of the guy from a valley in nortern Italy? That would be a good one to follow, not the shape as such but the core and the build-up.

A vault isn't better in long time durability, sorry about that.

In Catalan architecture vaults are a thing, so I'm used to thinking that it's a shape that offers long time durability. But it's true Gaudi did never build masonry stoves, it makes sense that with high temps and expansion we're talking of a totally different game.

What I feel now we are coming closer to a newer design, the Shorty core. This will allow to have a lower bell, higher raised core and aligning the seat with the bottom of the core.

Deep diving right now into everything I can find about the shorty core.
Last year, when I came across the batchbox design, the Shorty seemed it was still a work in progress. I have the intention to build something that has long been proven and tested, that built to specs will offer optimal results and won't need modifications ever, and long time maintenance. With this in mind I set course for the high riser.
But I'm seeing now there's been some developed in this past months, Thomas has two burning seasons to vouch for and all are good news.
I can find clear measurements and plans to develop the core, and the idea of a shorter bell is actually a deal breaker.
With the dimensions of my bricks and the constraints of my space, the bell with the tall riser inside would take up more space than I'd like.

Did you see the heater of the guy from a valley in northern Italy?

I hadn't, thanks for the lead, really helpful! Cedric has made a great build and an excellent thread documenting it.

I have found the sketckup files from Amayuelas and the DSR2+doubbleB, and definitively would love to make something similar.
Adapting to my bricks measures and space dimensions, I'll go for the best footprint I can squeeze for my crooked corner. Hoping to have time tomorrow to draw something.

Two first questions:

-Why is there a second door on top of the firebox on both designs?

-In this design, the top of the bell and front wall are both lined with IBF to protect from direct heat, right?

Thanks, can wait to have something drawn to move forward!

sara ventura wrote:In Catalan architecture vaults are a thing, so I'm used to thinking that it's a shape that offers long time durability. But it's true Gaudi did never build masonry stoves, it makes sense that with high temps and expansion we're talking of a totally different game.

Catalan vaults are awsome, I've a book in my library (ahem!) about those.
What I recommend is exhaustively tested and proven in daily use. When I am unsure about some aspect, I'll tell you that.

sara ventura wrote:Two first questions:
-Why is there a second door on top of the firebox on both designs?

Both the DSR2 and DSR3 could have a second door above the firebox. Behind that second door is the afterburner space, in the Shorty core this same function is in the short riser, without the possibility of a door.

sara ventura wrote:-In this design, the top of the bell and front wall are both lined with IBF to protect from direct heat, right?

Thomas did go a long way to build his single skin heater containing a Shorty core, out of mostly normal red bricks. Since then, I designed a large 200 mm system heater in The Sculptor's Shop, located in my home city. And I designed a small(ish) heater lined with concrete paving bricks inside. Both heaters only have a firebrick shield at the side where the highest heat is expected, directly in front of the core's exhaust. No insulating firebricks, at all. And as a completely different heater: the French Bench, built as a proof of concept in France.

The Sculptor's Shop build is remarkable, in the sense it is fired all day long, without overheating or excessive chimney temperatures. No problems so far, the same goes for the smallish 900 kg Pepper Shaker in my workshop, which is fired every day once or twice the past four months or so. There's a series production on its way of the latter one.

Both the DSR2 and DSR3 could have a second door above the firebox. Behind that second door is the afterburner space, in the Shorty core this same function is in the short riser, without the possibility of a door.

Ok, now I see the differences between the DSR2 and 3 and the Shorty. I downloaded the files under the impression that both were shorty designs.

For the purpose of clarification, If I wanted an oven with a shorty core, it would have to be placed above riser's exit port? Thus getting back to a much taller bell.

I'm drawing what will be my floor plan. Considering space constrains and materials thickness, I have two options:

-Exit flue outside the bell: Would be in the same wall as the door's firebox, cramped, and too exposed to casual contact.

-Exit flue through the roof of the bell: That leads me back to double wall insulated pipe → bigger area at the back of the core to allow gas flow. The pipe placed near    the wall that transitions to the bench, thus allowing flow from 3 different directions.

Leaning for the second option, which would be the minimum space needed at the back of the core to ensure optimal operation?

When clay/sand plaster is used, the layer would be much thinner. Applied in two layers, a shrink netting embedded in the first.

Getting back to the idea to keep tiling just for the outside top bench for longevity. How thick would be the finishing layer?


I'm an endless pit of questions, thanks for the patience!

Sara;
On the Shorty core, the riser exit does not have to face forward; it can be reversed and point to the rear.
A White oven could be built over the core.
I believe a build is underway that is adding that feature.

Peter may offer more info on this.

sara ventura wrote:For the purpose of clarification, If I wanted an oven with a shorty core, it would have to be placed above riser's exit port? Thus getting back to a much taller bell.

In the Shorty design, the firebox is doubling as an oven. A friend of mine bakes his own bread, the firebox of his heater is used to that purpose during winter.

sara ventura wrote:-Exit flue outside the bell: Would be in the same wall as the door's firebox, cramped, and too exposed to casual contact.
-Exit flue through the roof of the bell: That leads me back to double wall insulated pipe → bigger area at the back of the core to allow gas flow. The pipe placed near    the wall that transitions to the bench, thus allowing flow from 3 different directions.

Leaning for the second option, which would be the minimum space needed at the back of the core to ensure optimal operation?

Hard to say from such a distance. Let's say the core could be leaning against the back wall or nearly that. But there need to be ample space in other places of the least 5 times the chimney's cross section area, as I posted in the thread about the Pepper Shaker.

sara ventura wrote:Getting back to the idea to keep tiling just for the outside top bench for longevity. How thick would be the finishing layer?

Not sure what you mean. In general, the top of the bench should be 10 cm as the bare minimum. That could be done in different ways, concrete slabs, steel T-profiles and bricks or even a bunch of small pillars and bricks to close the bench. Keep in mind, temperature of the bench' seat is lower than higher up in the bell. Best to make a proper drawing and post that. Then, I will be able to evaluate it and try to shoot the proverbial holes in it.

In the Shorty design, the firebox is doubling as an oven. A friend of mine bakes his own bread, the firebox of his heater is used to that purpose during winter.  

That's a very interesting alternative, without any extra work. Since it's after the burnt is finished, it could be seen as a white oven?
I wouldn't be using an oven that much so for occasional bakes sounds perfect!

But there need to be ample space in other places of the least 5 times the chimney's cross section area, as I posted in the thread about the Pepper Shaker.

That's a really helpful reference! I've seen it on the other thread, but I thought best to keep the answer in this one to avoid missing information here.


Nothing more to say, will be posting cross section drawings later today, I'm starting to see the light at the end of the design process : )

In the Shorty design, the firebox is doubling as an oven. A friend of mine bakes his own bread, the firebox of his heater is used to that purpose during winter.  

sara ventura wrote:That's a very interesting alternative, without any extra work. Since it's after the burnt is finished, it could be seen as a white oven? I wouldn't be using an oven that much so for occasional bakes sounds perfect!

The firebox is like a black oven, the glowing charcoal is still there the moment the bread goes in. He's using a simple stainless steel grid with legs, to elevate the grid above the charcoal. And the bottom of a frying pan bolted to the grid, otherwise each loaf would be pitch black at the side that's facing the glow bed.

Ok, I believe I'm into something now!

With this plan my ISA is at approx 4.73m2 for a 15cm system, so there's room to wiggle if need be.
Probably bench measures will be polished during the building in terms of depth and height, to make sure it will end up with the right balance to give a comfortable rest.
If need be I could add a bit of width or depth to the bell, but if possible I wouldn't want it to be taller.

I'll extend the floor slab along the facade of the bell, this allows me to keep the bell walls at 90º and serve as a little mantel.

Questions that raised during the drawing:

- I've drawn a square pillar to support the bell wall over the bench span. This leaves two openings to the bench of 35 x 40 cm. Is this room enough for optimal flow?

- Exit flue position -->  The back wall of the build has the shape of the old fireplace, and in order to keep the exit flue always in line with the wall the pipe must exit from the center of the build. Is this the best position for optimal flow? What if it was at the left back corner?

- Bypass --> Considering the mild climate of Barcelona, high humidity all winter long, do I need one to ensure optimal starts when the system is cold?
--> If so, could the handle exit through the roof too?  This way I just need a T section on the pipe and a butterfly door. And the back seat of the bench won't have a metal handle in the middle of the way.

- Optimal distance from the end of the pipe to the floor?

Notes on the drawings:

-Firebox and riser measures are from the "shorty150 sidewinder core.skp" from https://batchrocket.eu/ontwerpen plus the thickness of my IFB that is 6.4 cm.

-3 cm between core and side walls.

-To calculate ISA I extracted the area of the firebox from the front wall, is that correct?

-5 cm layer over the whole slab the build will be in, mortar + vermiculite to insulate. Same between corner walls and house wall but 3 cm thickness, will it be enough?

-Building details: if I have a double layer of red bricks lying flat for the roof, the total thickness will be 9 cm plus the fiber blanket thickness. On top of this I plan to finish with render, shrink netting and 1.5cm glazed tiles, will this be enough mass?
Walls will be 13 cm thick red brick and clay / sand render, the top of the bench will have also glazed tiles. IFB in front of the riser's exit port.

Ok, it feels like I'm moving in the right direction! Have I miscalculated or missed something obvious?

Thanks for your patience and inputs, have a nice weekend!

sara ventura wrote:- I've drawn a square pillar to support the bell wall over the bench span. This leaves two openings to the bench of 35 x 40 cm. Is this room enough for optimal flow?

Looks OK to me.

sara ventura wrote:- Exit flue position -->  The back wall of the build has the shape of the old fireplace, and in order to keep the exit flue always in line with the wall the pipe must exit from the center of the build. Is this the best position for optimal flow? What if it was at the left back corner?

The center is the better position, definitely.

sara ventura wrote:- Bypass --> Considering the mild climate of Barcelona, high humidity all winter long, do I need one to ensure optimal starts when the system is cold?

Yes, a bypass is indispensable for starting a cold Shorty, let alone while the heater is still wet.

sara ventura wrote:--> If so, could the handle exit through the roof too?  This way I just need a T section on the pipe and a butterfly door. And the back seat of the bench won't have a metal handle in the middle of the way.

Yes, the handle on top would be a correct construction assuming a low(ish) bell. The T-piece inside the bell is a proven concept.

sara ventura wrote:- Optimal distance from the end of the pipe to the floor?

Minimum distance would be 8.5 cm from the floor in this configuration. The scheduled bench is 40 cm high, I would go for 12 cm as optimum.

sara ventura wrote:-Firebox and riser measures are from the "shorty150 sidewinder core.skp" from https://batchrocket.eu/ontwerpen plus the thickness of my IFB that is 6.4 cm.

IFB is insulating firebricks, yes? For the firebox you'll need hard firebricks, no escaping from that. The riser/afterburner could be built out of hard firebricks as well. If you want to use IFB's, those should be the same size as the hard firebricks.

sara ventura wrote:-3 cm between core and side walls.

Sufficient, no problems.

sara ventura wrote:-To calculate ISA I extracted the area of the firebox from the front wall, is that correct?

Even when the core is insulated, it will take up some heat from the outside, at first. What I normally do is ignoring the core entirely and calculate as if the core isn't there at all. Worked best so far.

sara ventura wrote:-5 cm layer over the whole slab the build will be in, mortar + vermiculite to insulate. Same between corner walls and house wall but 3 cm thickness, will it be enough?

The heat will go through the insulation, conduction will be slowed down, that's all. So some heat will go into the walls around the heater, which isn't a bad thing at all, especially when those walls are internal in the house.

sara ventura wrote:-Building details: if I have a double layer of red bricks lying flat for the roof, the total thickness will be 9 cm plus the fiber blanket thickness. On top of this I plan to finish with render, shrink netting and 1.5cm glazed tiles, will this be enough mass?

If you use the red bricks on edge, there will be more mass on the hot face. The superwool blanket will insulate a bit and separates the first layer from the second. The second layer should be done in clay/sand mortar in your case, and on flat. The tiles aren's adding a whole lot of mass, but those will be absolutely fine.

sara ventura wrote:Walls will be 13 cm thick red brick and clay / sand render, the top of the bench will have also glazed tiles. IFB in front of the riser's exit port.

IFB's are OK for the heat shield.

It feels like I've got a building plan, Peter, thank you so much for your time and patience!
Thanks permies for all the knowledge and kindness poured here!

Minimum distance would be 8.5 cm from the floor in this configuration. The scheduled bench is 40 cm high, I would go for 12 cm as optimum.

Would it make sense to help gases flow through the exit pipe to cut the last section of the funnel? Like in the picture below

IFB is insulating firebricks, yes? For the firebox you'll need hard firebricks, no escaping from that. The riser/afterburner could be built out of hard firebricks as well. If you want to use IFB's, those should be the same size as the hard firebricks.

Yes, I have located Hard firebricks for the firebox and IBF's for the riser and heat shield.

The heat will go through the insulation, conduction will be slowed down, that's all. So some heat will go into the walls around the heater, which isn't a bad thing at all, especially when those walls are internal in the house.

That's good to hear. The left wall it's underground but it must be well drained and insulated because it has no moisture issues at all, the back wall is open to the kitchen and the floor it's a slab on top of the lower floor, wild guess it's probably more than a meter of concrete foundation underneath.

If you use the red bricks on edge, there will be more mass on the hot face.

Given the size and height the top of the bell will have, I'm considering making it also sitting place like the bench.
Would it make sense to extend the heat shield of IFB to the first layer of the top? If it's two layers of red brick, it will get hotter than IBF and red brick on top?

Now that I can move forward, a whole new range of questions arise : )

Haven't touched my basic technical drawing skills since high school, but it helps me to go through details and figure out how to proceed. It's helpful to imagine better how it will fit into the space. And can wait to see it!

sara ventura wrote:Would it make sense to help gases flow through the exit pipe to cut the last section of the funnel? Like in the picture below

No, not at all, definitely.

sara ventura wrote:The left wall it's underground but it must be well drained and insulated because it has no moisture issues at all, the back wall is open to the kitchen and the floor it's a slab on top of the lower floor, wild guess it's probably more than a meter of concrete foundation underneath.

So the left wall is really, really massive. It's going to act as a huge thermal buffer, evening out temperatures and close to ideal! No insulation at all between bell and wall then.

sara ventura wrote:Given the size and height the top of the bell will have, I'm considering making it also sitting place like the bench.
Would it make sense to extend the heat shield of IFB to the first layer of the top? If it's two layers of red brick, it will get hotter than IBF and red brick on top?

I've been thinking about that IFB as heat shield, it's not very sensible there. Better to use hard firebricks for the shield. The idea is that the firebricks are better to pass the heat on to the bell wall itself. If you want to make the top of the bell as a seat as well, best to have one layer of bricks extra, separated by a layer of 12 mm superwool. It works wonders like that with my red bell heater, the top didn't get as hot as it was before, with insulation only.

Duly noted, thanks for the clarifications!

On to the next phase, I'm gathering suppliers and sources for the build materials. I've resumed what I have so far, see below.

Supplier for the high temps stuff is https://www.labelan.net/en/
They're open to give advice on the optimal material from their catalogue for every need. I want to go the extra mile to ensure longevity of the build, but I don't want to over spend either.

Red clay brick https://lacoma.com/
Red clay brick is easy to source, I just want tot make sure it's local production.

Questions:

- I've found this product, would it be suitable for tiling top bench and bell? https://www.unistara.com/en/product-/
I'd prefer to avoid high temp silicones as I imagine in case of repairs it's more difficult to get rid of.

-The bell wall facing the riser's port exit - It will also be the face of the exterior bell that will serve as back support when sitting on the bench.
With a double skin of firebrick and red brick, what can be expected temperatures outside? Too much to rest your back on?

Most grateful for your time and inputs, thanks!

Side note about house cork insulation

When this house had a ceiling made of  4 cm High density poliurethane panel 80 kg/m3 and MDF panels, I was consuming 75 kg / 165 lb. of pellet a week.
That was a compromise between getting cold or broke... The house was chilly, and the rides to load pellet bags with the car, a constant hustle.

Previous to that the house had no ceiling insulation and a cast iron fireplace insert, without the fireplace around. That was a cold pit, didn't matter the hours you burnt the stove, you could feel the heat of the stove going up through the roof. I used to wrap myself up warm WHEN I entered the house...

Last spring, the ceiling was covered by a single layer of 6 cm / 2.36 in. expanded cork.
Now, the past week with average temp. outside of 15ºC / 59ºF day and 8ºC / 46ºF night, the house has kept steady between 18 - 19ºC / 64 - 66ºF.
No heat input, except opening windows 15 min to ventilate during sun hours on the south facade.

This gives me high hopes of real comfort when the Shorty is finished and the floor installed again 😊

Plus, now I have a material that's oblivious to moisture, mold and critters, breathable and 100% biodegradable!

Go cork!!

PS. I was planing to paint it white, but in the end I felt in love with the dark color and thanks to the height of the house it doesn't weights down the ceiling.

sara ventura wrote:- I've found this product, would it be suitable for tiling top bench and bell? https://www.unistara.com/en/product-/
I'd prefer to avoid high temp silicones as I imagine in case of repairs it's more difficult to get rid of.

I'd say it is a suitable mortar for this application, yes.

sara ventura wrote:-The bell wall facing the riser's port exit - It will also be the face of the exterior bell that will serve as back support when sitting on the bench.
With a double skin of firebrick and red brick, what can be expected temperatures outside? Too much to rest your back on?

It greatly depends on how often the heater is fired and how many refills are done. As a guess: one fire (batch) every day could result in 30-35 ºC and two fires a day in 50 ºC. It might be a good idea to shape the seat's back wall with a generous layer of plaster, this will make the transfer and the top temperatures flatten out somewhat.

sara ventura wrote:This gives me high hopes of real comfort when the Shorty is finished and the floor installed again 😊

Even people who tend to know what the effect of a mass heater would be, are surprised what it can do in the long term. The Mallorca build was one of those, it turned out the house wasn't just warm, but also dry. You have an awful lot of mass there, in the house itself. which means you probably start having a fire once in two days, later one per day. In about two weeks, the buffer of the house itself will get saturated and the indoor environment will stabilize.

In short: it may be so that at the start of the heating season the heat seem to disappear, until all the walls and part of the mountain that is behind one of the walls is finally warmed up.
I've seen it happen, a long time ago, high in the French alps in the Vallée de la Clarée.

In short: it may be so that at the start of the heating season the heat seem to disappear, until all the walls and part of the mountain that is behind one of the walls is finally warmed up.

I can imagine 😊
Something similar happens in summer, when all this mass keeps the heat outside for a while, until it doesn't.  
Winter gives me an easy way to regulate inside, the heat in summer is becoming dreadful...

It might be a good idea to shape the seat's back wall with a generous layer of plaster, this will make the transfer and the top temperatures flatten out somewhat.

Can the thickness be done with clay / sand render or better to shape it with cob and render on top?

It might be a good idea to shape the seat's back wall with a generous layer of plaster, this will make the transfer and the top temperatures flatten out somewhat.

sara ventura wrote:Can the thickness be done with clay / sand render or better to shape it with cob and render on top?

Best to shape it with cob and lots of straw. Th render is just for looks then.