Counsel for new Shorty Core + statification bell Rocket Mass Heater

Hello everyone, this is Leonardo from North Italy.
First of all, I'm thankful for this forum, where I've found SO MUCH information about building Rocket mass heaters.
Let me first introduce myself. I'm 27 years old, and with my precious wife and 4 little children we live in North Italy, close to Venice. We are part of a small christian community, which is a great blessing from God for us!  We thank God for that! We homestead, we try to eat healthy and we heat with wood.
I first found out about Rocket Stoves quite some time ago, but only now I get the chance of building one. Actually, if the first build goes well, several families of our community would want one as well. So we might end up building at least 4 RMH.

I'm writing this topic to ask for your counsel on my first RMH project.
I will attach a picture of a rough house plan, to give you an idea about the situation. The house is a two-story house, pretty long and narrow (20m x 5m,  65 ft x 16ft). We would like to build a RMH in the living room, which should be able to heat the livingroom and the entry room (they are pretty much an open-space) .
I would like to build a Batch Box, specifically, the Shorty core variation.
I have no constraints on the chimney size, because it must be rebuilt. I thought of a steel pipe going through the second floor and then out the roof, or out the side of the house.
The walls are 30cm (1 foot) thick, made of full bricks, insulated on the inside with approximately 4cm (2") of polistirene. The windows are double glazed.
The climate is pretty mild here. In the winter the temperatures drop below 0°C (32° F) only for some days a year. I would say that -5°C (23° F) is the minimum we would reach on most years.
I used the spreadsheet available on Peter van Den Berg's website to calculate the heat loss of the environment. Based on that calculation, I would have to build a 180mm (7") core.
For the bell I thought about a stratification chamber made of concrete clinkers lined with firebricks in the upper part. I also thought of a double layered bell so that it would retain the heat longer.
The bell's ISA should be 7.6 m2 (75,35 sqft), which is 15% smaller than the ISA of the bell for a 7” standard batch box.
I’m not sure if I will keep the flue pipe inside or outside of the bell. And, of course, I will install a bypass higher in the bell.

Does this project sound good to you all? Is there anything that I'm not considering, or considering wrong? Do you think I could go on calculating and drawing a detailed design? Would such a heater heat the two rooms? Would it somehow heat a bit also the second floor?

Do you have any suggestions on the materials to use, or building techniques, or anything else you would share with a complete novice? For example: is it worth spending extra money for better quality firebricks, or would basic hard firebricks be equally good for the job? Are there any planning, or building, “tricks” that would make the heater last longer, or perform better?

Thank you VERY MUCH for any advice you have for me! And thank you in advance if anyone of you would take this project at heart, and would like to support me in this journey!

Hi Leonardo. And welcome to permies!
I’m sure you’ll get some answers soon.

Leonardo Bevilacqua wrote: is it worth spending extra money for better quality firebricks, or would basic hard firebricks be equally good for the job?

We built one two seasons ago in Portugal using basic firebricks bought off the shelf. They work well BUT two seasons in a couple of them have cracked.

Your room is 45 m2. Fi 175 mm in your climate should do the job. My climate is similar and calculations that I did for a 100 m2 house resulted in fi 170 mm.

Please remember that for the shorty you will need to fabricate the door, for regular BBR you can purchase door and adapt it for the heater.

I would get at least 40% Al2O3 bricks, the higher the better. Wood ash is fluxing the bricks lowering their melting temperature and high alumina is countering it. Because of that wood is a fuel that demands more from the material.
For the riser insulating firebricks IFB 26 (1400 C rating, but 1300 C will also do it).

Regarding long lifespan. Masonry heater builder master that I know recommends to build all parts of the firebox from bricks - smaller elements handle expansion better than large brick slabs or refractory concrete. If you also encase them in a tension frame it will serve for at least 30 years or more, because you live in a climate that will not require intense firing schedule.

If your ceiling is solid masonry then I would not count that any heat would percolate to the upper level.

Leonardo Bevilacqua wrote: Hello everyone, this is Leonardo from North Italy.

Welcome! You will find many helpful people here! I'm also still constructing my first RMH, so experts may correct me or chime in.

Leonardo Bevilacqua wrote:
For the bell I thought about a stratification chamber made of concrete clinkers lined with firebricks in the upper part. I also thought of a double layered bell so that it would retain the heat longer.

Peter has proven this is possible with the pottery shop build which you probably have seen. I think it's a good idea. At least where I live, many people renovating their driveway give away these clinkers for free. You could copy where Peter applied his firebricks for some peace of mind. If you don't object the look, having the chimney outside the bell is more simple and more robust to execute. That being said, I will incorporate it into the bell myself, because I find it more aesthetically pleasing.

Leonardo Bevilacqua wrote:
Does this project sound good to you all? Is there anything that I'm not considering, or considering wrong? Do you think I could go on calculating and drawing a detailed design? Would such a heater heat the two rooms? Would it somehow heat a bit also the second floor?

Have you thought of putting it here:
In my opinion this is a better (more central) location of the house + putting the heater in a corner, facing exterior walls on 2/4 sides, is a bit of a shame I believe. You could use those corrugated sheeting which I believe Peter has behind the red heater in his house, inbetween the heater and the wall to transfer the radiant heat to convective heat, or at least pull it away from one exterior wall.
Drawing it out will never hurt in my opinion, I'd start with a top cross-section. The heater would also contribute to heating the upper rooms, in the way that heat always rises, but not in an active way. If you want that, you would have to continue the bell upstairs or make a second bell there. Personally, I would not invest in high-end firebricks for the bell (second-hand or cheap will be sufficient). But for the firebox, just buy new, decent dense firebricks and lay them with refractory mortar or with a tension frame and ceramic gasket (watch your lungs!)
Make the heater perform better: follow Peter's design meticulously. Allow for enough room for the gases to pass the core while cooling down. Insulate the chimney.

Good luck!

Hi Leonardo, I received your PM about a month ago, and I was convinced I answered it. As it turns out, I didn't, sorry for that. Sometimes I miss one, this time of year is always the busiest.

A 180 mm system would heat the space. A double skin will hold the heat longer, that's true, although the external temperature will be somewhat lower as compared to a single skin item. Start with a drawing of the bell and how the core is positioned in there. Having the stove pipe at the side of the heater and the connection plus bypass like the heater in The Hague will result in less friction and a heater that will start cold and wet without problems.
The upper part of the bell in refractory bricks isn't necessary for a Shorty core, just the wall where the core exhaust is pointing at will be sufficient.

Regarding your question about coming to Italy and help with the build, I have to decline. I will turn 80 years of age in March, ice and weather permitting. Whether or not I will be able to travel to Italy and act as a build director remains to be seen. But, let's don't get ahead of time, start with planning first.

I fully agree with Julian, his suggestion for placement is much better.

Thank you all for your thoughts. This helps A LOT!

To you, Peter: you did answer to my PM. Thank you for your super fast answer! I just didn't get to write back to you. But, here we go! I totally understand your choice not to travel! Thank you for all the counsel you are willing to give us, even at a distance!

As far as I understand from the different comments, my idea sounds feasible. So, I will sketch a design that I will share with you later, so you can help me. As you suggested, I will sketch a top view first.

Thank you Julian (and Peter) for your idea for the better location for the heater. It makes a lot of sense to put it where you marked it.

Just a question before I start designing. When building the core, I need to stick faithfully to Peter’s dimensions, otherwise it won't work. But what about the bell? I get the point of not exceeding the ISA determined by the size of the core. But, are there any other proportions that need to be respected for the bell? What about the relation between the size/placement of the core and the size/shape of the bell?
Would it work if I make a bell which is long an tall but wide just enough to fit a core inside? Or should there be a minimum space between the core and the bell?

I hope I was clear enough with my question. Thank you very much for your thoughts

Leonardo Bevilacqua wrote:I get the point of not exceeding the ISA determined by the size of the core. But, are there any other proportions that need to be respected for the bell? What about the relation between the size/placement of the core and the size/shape of the bell?

The core could be placed almost anywhere in the bell. Even with one side against the bell's wall will work. The shape of the bell is largely irrelevant, a flat ceiling is better than one with steps, the gases will stream more freely like that.

Leonardo Bevilacqua wrote:Would it work if I make a bell which is long an tall but wide just enough to fit a core inside? Or should there be a minimum space between the core and the bell?

The bell could be deep and tall, no problem. Down to just wide enough to house the core, although in that case there should be at least a space at the back. Size of that space: at the bare minimum, 5 times the cross section area of the stove pipe, more being better. That space is there to lead the gases down to the bell exhaust.

Just a tip: place the core higher in the bell, very convenient for loading. At the same time there will be an empty space below the core, where all the fine dust will settle. And as a bonus: there will be much more options available for the exhaust pipe's position.
The core shouldn't touch the walls of the bell, though, one layer of 12 mm superwool would be absolutely adequate.

Peter van den Berg wrote:
The bell could be deep and tall, no problem. Down to just wide enough to house the core, although in that case there should be at least a space at the back. Size of that space: at the bare minimum, 5 times the cross section area of the stove pipe, more being better. That space is there to lead the gases down to the bell exhaust.

Peter, I have been thinking about your requirement to have the section around the core where the gases pass at least 5 times riser CSA. You gave me the advice last year not to count narrow slits. Since then, I've discovered the 'hydraulic diameter', which I believe, covers the load better, when we are talking about friction through a section. Are you aware of this? If so, what is the reason you chose not to use it?
I have made a small comparison between hydraulic diameter and your expression in function of riser CSA.

For those wanting to skip to the conclusion: it seems encasing a core in a drum, produces a (roughly calculated) free section of 6.8 times the riser CSA, but only results in a hydraulic diameter of 175 mm. If we take this same hydraulic diameter, and assume the passageway is a rectangle behind the core (green), it would only need to be 340 x 117 mm2, which is a mere 2.27 times the riser CSA.

Curious to hear your or anyone elses take on this.

Calculating the hydraulic diameter is probably beyond the knowledge of all but a few. A simple number is much easier for most people to understand, even if it is only partially accurate, as long as errors don't fall in a nonfunctional range.

Hi Glenn, thanks for your input. Personally, I would be suprised if you are able to calculate the free flowing CSA, but not the circumference, especially since most of these areas are likely to be rectangular or a sum of rectangles. I do agree with you that it is good to have the 5 times CSA as a simple rule of thumb. At the same time, maybe it could kill a project for someone because of space restrictions, while there is a setup which could be acceptable in terms of friction, but without following the 5 times riser CSA rule.

Julian Adam wrote:

Peter, I have been thinking about your requirement to have the section around the core where the gases pass at least 5 times riser CSA. You gave me the advice last year not to count narrow slits. Since then, I've discovered the 'hydraulic diameter', which I believe, covers the load better, when we are talking about friction through a section. Are you aware of this? If so, what is the reason you chose not to use it?

Julian, a long time ago, an emeritus professor in hydrodynamics explained what the hydraulic diameter was all about. He even offered some examples to illustrate his point. I never bothered to learn how to calculate such a hydraulic diameter correctly, too complicated for DIY people and autodidacts like myself.

Bar one example from an entirely other source, a chimney sweep, in fact. He used a simple formula to calculate what a rectangular chimney cross section would be when compared to a circular one. This goes as follows: twice the width multiplied by the depth of the rectangle, devided by width plus depth will give the comparable diameter. This would look like this: 2xWxD : (W+D) = comparable diameter. As you may be aware, this is how I came about to compare a square riser to a circular one.

To keep it simple enough for the ordinary layman, I recommend at least 5 times cross section area of the chimney pipe. Without taking a lot of small slits into account, and the more space the better. This will work in the vast majority of cases, a certain president we all know about would have been mighty pleased with such an election outcome. Although my own nearly 11 years old heater sports roughly 14 times the pipe csa around the core. Being a good example of bigger is better, as a well-known motorbike manufacturer used to advertise.

My original take on this was derived from the work of Heikki Hyytiäinen. His Finnish contraflow worked with two 70 mm wide slits, each slightly larger than the chimney csa. One could say, roughly 2.25 times the masonry chimney csa in total. Later on, I realised this could lead to far too much friction or even a non-functional bell-type heater, so I switched to 5 times, just to be sure.

Making building these bell heaters more complicated means there will be more mistakes. As such, the short explanation of Glenn Herbert is wording it real nice:
Calculating the hydraulic diameter is probably beyond the knowledge of all but a few. A simple number is much easier for most people to understand, even if it is only partially accurate, as long as errors don't fall in a nonfunctional range.
Thanks, Glenn.

Peter, thanks for your clarification!

Peter van den Berg wrote:
Bar one example from an entirely other source, a chimney sweep, in fact. He used a simple formula to calculate what a rectangular chimney cross section would be when compared to a circular one. This goes as follows: twice the width multiplied by the depth of the rectangle, devided by width plus depth will give the comparable diameter. This would look like this: 2xWxD : (W+D) = comparable diameter.

It's nice to see this coming from such a practical implementation, because this is indeed exactly the formula for hydraulic diameter applied to a rectangle!

Peter van den Berg wrote:
To keep it simple enough for the ordinary layman, I recommend at least 5 times cross section area of the chimney pipe. Without taking a lot of small slits into account, and the more space the better.  

Clear! Anyway, should anyone be reading this while doubting if the gases will be able to flow freely enough, I can calculate hydraulic diameter for you if you just give me a cross-section of the top view, where the gases go past the core, with all dimensions on it of the channel.

Peter van den Berg wrote:
My original take on this was derived from the work of Heikki Hyytiäinen. His Finnish contraflow worked with two 70 mm wide slits, each slightly larger than the chimney csa. One could say, roughly 2.25 times the masonry chimney csa in total. Later on, I realised this could lead to far too much friction or even a non-functional bell-type heater, so I switched to 5 times, just to be sure.

Interesting Peter, I had no previous knowledge about counterflow heaters, but now that you mention it you can indeed clearly see that the downdraft channels are quite small compared to your advice for the bbr bell.

Thanks for taking the time to reply!