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.