sara ventura

How much below the exit riser's port should it be placed? Should the opening of the bypass be facing the floor or the wall?

I used the riser's exit port as reference as I imagine the bypass is best placed close to it, so the hot gases find an exit before going down to the floor and bench.
I know the bypass is placed in the exit pipe, and the shape is a T section, in my case I'll use a butterfly flap that will be operated through the bell's top.

   sara ventura wrote:
   I can squash 20 cm from the top, leave the core aligned with the bench top and lower my ISA to 5'2.
   Will I get away with that ISA and the bypass?


Hard to tell, as long as the chimney is of good quality, it might, since the ISA oversize is now roughly 15%.

From the lowest point of the exit flue to the exterior top: 5 meters with 1 35º turn exiting the bell and another to enter the masonry chimney that the pipe will go through up to  the exterior. Pipe will be double walled.

I am unsure whether or not I mentioned this, but there shouldn't be a wall between the main bell and the bench at all, just a few columns. Try to view the whole of it as a strangely shaped bell.

You mentioned it here earlier, but it's a good thing to keep in mind.
I imagine the inside of the built like the inside of a big cavern in the dark, a large open floor plan and a ceiling at different levels.
In my case there will be a masonry column to divide the 90cm span of the bench and support the bell's wall, and 5 thin metal legs to rise the core.

The accepted bell size for a 150 mm first generation batchrocket is 5.3 m². However, the top combustion rate of a Shorty core is somewhat tuned down, in order to create a much more reliable core. So, for a 150 mm Shorty, that ISA should be 15% less, being 4.5 m².
However... you could have an oversized bell ISA although a bypass is a must in that case.

The bybass was already in my to do list. How much below the exit riser's port should it be placed? Should the opening of the bypass be facing the floor or the wall?

   sara ventura wrote:
   -Space between riser's top and bell's top  ---> I have now 30 cm, could it be reduced to 20cm?


Top gap could be zero, but for safeties' sake, let's say 10 cm or 4" would be sufficient.

That's good news! I can squash 20 cm from the top, leave the core aligned with the bench top and lower my ISA to 5'2.
Will I get away with that ISA and the bypass?

I also like the idea of the overhang of the bench slab, it gives the bench a more comfortable seat if you can bring your heels at a closed angle than 90 degrees from the knees.

So I can move ahead with my plans, will have something drawn this week and keep breaking the building details of the bell as I come across new doubts.

Thank you!

Thanks Peter for the drawing! I'm crunching numbers for the bell again since I realized I was leaving out the top of the bell in my previous calculations...

As it is now, I'm at 6m2 ISA and I need to reduce it a little. Quick questions about internal dimensions :

-Space between riser's top and bell's top  ---> I have now 30 cm, could it be reduced to 20cm?

-Lowering the core 5cm ---> Core 5 cm lower than the bench's top, but still 25 cm above the floor, is it feasible? Would it compromise the gases flow too much?

-Bench height and width ---> Height is now at 30 cm, I can't go lower. Width is at 60 cm, could I make it 50 cm and add this in mass to the exterior to keep 60 cm depth for the bench seat? It could be done either with brick or cob this extra thickness outside? I assume this would lower also the temp of the wall outside, but the idea of keeping the calfs fresher than the rest of the body sounds good to me.

If I can combine these three changes I could lower the ISA to 5'56.

   Peter van den Berg wrote:
   This core type can do very nicely without insulation at all.


Maybe I should explain a bit what happens inside this Shorty core.
In order to obtain complete combustion or close to this state of affairs, the three T's are important. In no particular order: temperature, turbulence and time. Some mainstream knowledge say smoke should disappear as soon as a temperature of 850 ºC (1560 ºF) is reached. There's even a German group by the name of 850 Grad, which is promoting this idea. Of course, nobody specified where that temperature should be reached and accompanied by which amount of turbulence and during which amount of time. Since I've seen numerous times smoke disappearing within 5 to 10 minutes into a burn after a cold start, I started wondering. The numbers doesn't look like fixed figures, it could be that the figures are at least partly interchangeable. Maybe, with an awful lot of turbulence induced both the other two T's, time and temperature, could be smaller.

I get lost with the specifics of the science, but I get the general idea.
So I could go for a core made all from firebrick, without covering it with ceramic blanket to keep the maximum heat in, and still have a clean combustion that guarantees the optimal functioning of the heater.

I used what I thought was the best "insulating fire brick" I could get my hands on in the states. And after a few years these have started to break down, while the hard fire brick has NOT SHOWN any problems. (same stove)

Thanks Scott for your input. It's actually helpful because the main reason I was going with IFB was longevity, the not needing extra insulation outside was a bonus (for me).
And considering IFB are the most expensive type of brick from the three I was sourcing for the build, it's a great experience to hear.

I keep drawing the bell, let's see if I can come up with a functioning bench in the end.

Thanks for all your time and experience!

That's good news, indeed! IFB are 1€ plus per piece than Firebrick😅

I am unsure what type of insulating firebricks you have there. Better to use just hard firebricks for the entire core. The riser is the part that's heat stressed most of all, especially the liner in the lower half. Thin strips of insulating firebrick, I feel scary about its durability.

I have located a high temp materials factory that offers:

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.

Firebrick AL40-
Dimensions: 230 x 114 x 64 / 76 / 20 / 30 / 40 mm
Maximum temperature: 1420ºC
Alumina: 39-41%
Characteristics:
High purity.
Low iron content.
Good mechanical resistance.
Excellent resistance to thermal shock.

I'm trying to avoid ceramic wool as much as possible, I was going with the insulated firebrick for the riser, so I don't have to add extra insulation outside. Keeping the ceramic blanket at minimum when directly exposed to gases.

I"ve had a look at your drawing of the core, and question arose. The depth of the firebox, is that a fixed figure because of the bell depth?

I've drawn 50 cm because the depth is the lenght that has more tolerance, and I'm thinking in the type of wood I'll be using.
Most of the times the horitzontal cuts of the wood are at an angle, length is always irregular. So extra depth to make sure I can fit any kind of cuts in the box load.

Hello, it's not the same but it has reminded me of something called in spain Churrasquera.

I can't find info in english, only this https://www.gastronomiaycia.com/que-es-una-churrasquera-y-como-se-utiliza-en-la-cocina-domestica/

In short, it's a stove barbecue, we use it to cook on open flame indoors. It's placed straight on top of the flame, no heat diffuser.
Maybe you could source a round inox grid from these for your oven, they always have the hole in the center.

Or if you ever come across a churrasquera, it's a great cooking tool to have.

What I usually do with this size (150 mm) of core is this: I'll take the width of the bricks, in your case 114 mm, and use that as the Base figure.

Don't worry too much about it, I'll draw you a core that is the right size and proportions, based on the bricks that are available to you.  

Thanks Peter, that's so generous! and life-saving, I've tried to do the math with Base figure, but my brain switches off at some point and I get lost. 😅
No rush at all. From my drawing, I can do an overall estimation of the bricks I need already.

I've read somewhere that a 20% increase is the usual to add to the amount needed of bricks, is that correct?

Also, I've realized while drawing I forgot there are 2 types of brick, not sure that I can lay both between them as I've done? Different expansion rates, perhaps?

Yes, this is right direction and yes, there are some crucial mistakes.
Namely: the liner opposite the port shouldn't be there, the top half of the riser box should be square, sides equal to 2B.

No wonder, I was trying to copy from the .skp Sidewinder shorty from your site combined with the dimensions from the shorty chart, but doing the riser I felt a bit lost. Never used sketchup before.


I've drawn the top's firebox using the refractory slab G-23, which leaves three joints on the top. Would it be better to have a single piece instead?

And the single piece that doubles as top exit riser's port and base of the riser's top, should be from the same material as the rest of the riser? Can I use a cut from the G-23 slab?

thanks for your guidance!

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

That's convenient because we can use this extra thickness to try and make the bench a bit more comfortable. A 90º angle seat it's not good for the back.


I've finally managed to draw something that makes sense with sketchup.

Firebrick - 230x114x64 mm

IFB - 230x114x64 & 230x114x30 mm

I'm I going in the right direction? Are there any crucial mistakes?  

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?

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.