Shorty Core & Bell under construction - some questions

hello all
I'm excited to be making contact with the inspiring people whose posts I've been reading for a long time

I am proceeding slowly with a build, having spent a lot of time circulating around the RMH world, reading, watching videos and learning.
It was worth the wait to evolve my aspirations from my first encounters with steel barrel RMH with horizontal flue pipe & thermal mass and from there to Walker cookstoves, but finally I'm thrilled to have found the Shorty core, got my hands on the SKP files, and have all the materials ready.

I'm attaching a few pictures of progress so far and hoping that some pair of experienced eyes will glance at this.

I have a 150mm flue to use as the chimney, and the core will be surrounded by a red clay brick and stone bell with a bench on one side. A bypass will be included in the bell.

For the box & riser I have dense firebricks - 230mm x 114mm x 76mm (9" x 4.5" x 3")

I've made the firebox 232mm wide (by using a complete brick as the floor) which is slightly more than the 216mm required but in all other aspects I plan to keep to the recommended dimensions:

Box:
width 232 / height 324 / depth ~430

Port:
width 54 / height 216 / depth 120

Lower riser:
150 x 150

Upper riser:
216 x 216

Question - does my wider firebox (232 instead of 216) cause a problem or require adjustment of the above?
I'm hoping it makes no difference but if necessary I can break down the existing couple of built layers and reduce the firebox width by cutting

Should I keep the riser exit opening at 216mm wide to match the riser or at 232 to match the box?

The firebox is built on an elevated base with AAC (autoclaved aerated concrete) thermally insulating "Aircrete" blocks immediately under, and below those standard soolid concrete blocks. The floor of the room is reinforced concrete.
The Aircrete blocks are suitable for exposure to max temp of about 500 Celcius. Is there a risk that temp could be exceeded on the external face of the firebricks under the firebox, sitting directly on top of the Aircrete blocks. There is no heat dissipation via air channels between firebricks and Aircrete blocks.

Hi Alan, your plan sounds solid, and it sure looks good so far!
I do recommend bringing your box width to the standard size.
Yes, on having the horizontal exit match the riser, even if you go with the larger-size box.

I'm guessing, but you might be ok with the aircrete blocks under the floor.
Perhaps use an inch of clay mortar between them.

Hi Alan,

I am assuming you are geared to building a 150 mm system. You are going to run into trouble with extending one dimension and trying to keep all the others (or most of it) within the specs of a proper 150 mm system.

There's a simple solution to this, namely enlarging the base value slightly. In fact, in a proper 150 mm system, the base number is the width of a brick. This isn't a happy accident, it came out this way during development of the first generation batch box rocket. In order to keep all the measurements within specifications, with slightly larger bricks a lot of cutting need to be done.

By enlarging the base value a bit, the brick pattern will be a much better fit and the slightly larger core will run on a 150 mm dia chimney just as well. So, the base value for a standard 150 mm system is 108.51 mm. By upping this to half a brick, it will be 114 or 115 mm, fitting for a chimney that is 159 mm dia, according to specs. But the enlargement is well within reasonable tolerances, so the core built according to this specification will run without any problem on the 150 mm chimney.

The bell could be done the same way, take the cross section area of the theoretical larger chimney in meters, and multiply that by 255. The result will be the internal surface area of the bell, excluding the floor. Since the bell do need a bypass anyway the whole thing will run beautifully.

Now you'll need to break down the beginnings of the core, make a complete floor with the walls on top of it. This way, you'll need three bricks on edge for the height of the firebox. The height of the port of 2.1B will be very close to two firebricks on edge, also.

I also would like to suggest that you elevate the core a bit more, by using a layer of firebricks extra on top of the AEC blocks. They tend to crumble over time when repeatedly being close to their maximum temperature. Keep in mind, the temperature inside the firebox could be up to 900 ºC, the underside of the firebricks could be dangerously close to the maximum specs for the AEC blocks.

Just vent your questions here, I will keep an eye on your progress.

Lots of succes!

P.S.
The depth of your firebox looks a bit on the short side. As it looks like, it's just 1.5 bricks deep, this should be at least two bricks as the standard depth. Eventually, this could be even 25% more without repercussions.

Regarding calculation of the bell, think of it as a completely closed box, without the core at all. When the calculation is applied to this, you'll get the best efficiency out of it.

Thank you so much Thomas and Peter, you are so helpful to the worldwide community of RMH builders.
I will restart the build & proceed as you suggest and post a few more photos as I go along, hopefully others in my position will find this useful too.

So here are a set of photos with dry-stacked bricks and blocks (so far uncut) based on advice above.
I have doubled the layer of firebricks under the firebox as Peter suggested, and made the firebox longer.
My base value is 115 as suggested.

Where you currently see gaps in this dry stacked model, they will be filled in the build. When I start cutting...

As I want to have a large bench to sit or lie on - up to 2000mm long and 600mm deep (front to back) - I have made changes to reduce the bell ISA.
I want to keep the system size at a 150mm chimney.

As the firebox is about 300mm off the floor of the room, the entire floor of the bell and bench have been elevated by 200mm off the floor using the lightweight AAC blocks (aerated concrete, very good insulators also) in order to keep the ISA figures lower.

The bell has been redesigned to fit more closely around the core, again to reduce ISA.

The opening from bell to bench is relatively large, but I plan to build the bench with thick walls so that the inside of the bench will measure only 300mm high and 300mm wide, and around 2000mm long. Therefore ISA for the bench will be approx 1.8sqm, leaving only 3 sqm approx for the bell.

I'm not sure if is this long, narrow dead-end bench will allow hot gases to flow through to the end, but it is not critical if the bench doesn't warm up so much,  I am more concerned with keeping the ISA within limits.

However, if anyone thinks this design may be faulty and the operation of a front-exiting Shorty core will be compromised by the parameters above, please tell me.

Apart from that, the layout looks pretty standard I think?
It will be a double-skin bell probably, I'd like to have lots of mass to store heat.

Thanks!

and the other pics

I'm curious where the exhaust pipe draws from. If it opens to the near side of the core, gases from the bench area would have to (at least mostly) rise back up over the core to get to the exit, making flow to the bench anemic. If the exhaust goes under the core, then hot gases will hardly get to the near side, reducing the effective ISA as if that section of wall was floor.

Having a good gap around the back or under the core would allow the whole bell to act as a single unit. I think 2" would be the minimum, and 3" would be better.


Looking closer at the pictures, I see the pipe ending just at the wall, which would really reduce the effectiveness of the bench, and the core actually touching the brick bell wall. I think, given the footprint already established, your best move would be to elevate the core a couple of bricks higher and leave a large space for cool gases to travel under it freely.

I second that notion Glenn.
I was wondering if that gap between core and wall wasn't a bit tight.

Also, is that flexible corrugated stove pipe in the background of one of the fotos? I'm sure you would get much better draw from a straight, smooth metal pipe.

As I see it, the gases that ought to stream to the exhaust of the bell aren't able to reach it. Remember, the stream is drifting into the bench, goes down at the end and streams along the floor to the exhaust. Much more space at the back corners of the core and channels under core would help.

Keep in mind that the floor of the entire bell, bench included, should be one level and smooth overall. So no bumps, obstructions and holes. I see what you are trying to do with rising the whole of the heater, but the bench will also get quite high like this. The internal size of 30x30 cm for the bench isn't exceptional, the exhaust should be at least in the lower half.

Moving the chimney pipe to the other side of the bell would be a good idea, lots of space there. By minimizing the space left of the core and maximizing the other side would be another possibility. In the bell around the core, there should be at least a horizontal space of 5 times the cross section area of the chimney, all connected by spaciously passageways. Having that space all at the side where the bench is, means gases are able to enter the bench more freely. Having a low and wide opening leading to the exhaust that's twice the size of the chimney pipe would help a lot as well.

When done correctly, the whole of the bell and bench will get warm, right to the end. I did the Mallorca build in 2017 this way and it worked exactly what it was meant to do.

Thanks Glenn, Benjamin. Peter.

Of course, it makes perfect and obvious sense (now that you have pointed it out) that the exhaust is blocked and has no clear pathway for gases from the bench, and a very restricted path from the bell.

And yes the bell walls will be a few inches from the core at a minimum, the dry stacking of bell walls was very approximate.

I will look at the two options of moving the exhaust to the other side of the core, or a large opening under the core

Regarding the bench height I am happy with approx 500 to 600mm mm off the floor of the room, which is a little higher than a sofa, but we have long legs

The flexible stove pipe has provided very good draw for a conventional wood burning stove to date, we will see how it goes

thanks again

A wood stove has a much higher exhaust temperature and thus much more draw due to the chimney than a RMH will. You want the chimney to be as smooth as possible to minimize drag. I can't say if the corrugated pipe will or won't work here, but I would be cautious.

Alan, maybe my description wasn't quite clear. That's what you get when I am getting ideas while typing.
What I meant was this:
When all walls of the bell are staying in place and the core is shifted to the left, *against* or nearly to the bell wall, that part of the bell wall won't be counted in the ISA calculation. At the same time, all things being equal, the space at the right side of the core will be larger, creating more space for the gases to come down.

It might be that the core should be shifted somewhat forward, enlarging the bell a bit to the front. The chimney pipe would fit beautifully in the corner at the right, where the bell and wall meet each other.

Just ideas, it's up to you what and how to implement.

Glenn, thanks for the word of caution on the chimney, noted.

Thanks Peter, as you were typing a few hours ago I was taking the pictures below - and I think your advice is the best way, nice and simple, aesthetically pleasing also.

The core has been moved forward by one brick (and to the left a little)

This allows for a proper chimney exhaust box right behind the bell, and I will be able to add a clean-out port at the chimney base accessible from the left side.
The stove pipe is just to illustrate positioning, I'll build a brick box there external to main bell.

Great to know that where the core is *against* (or nearly) to the bell wall, to the left in my layout, that part of the bell wall won't be counted in the ISA calculation - gives me more scope on the other side where the bench is.

Would 50mm be about the minimum gap between bell wall and core?

I think I can make the walls of the bell thicker in certain areas, eg at the front right, between core and bench, to reduce ISA and keep it at ~ 5qsm.

This already is looking much better, the placement of the exhaust pipe in particular.

Extending on the left side of the core and bell: it would be best to have a layer of 1/2" superwool between the brick and firebrick. This will take the gap between those down to roughly 12 mm.

I don't know if you calculated the core also. If so, best to insulate the core all around, so it is out of the calculation in its entirety.

On the idea of doubling one or more walls: the front wall above the firebox do need a shield consisting of firebricks. This paricular spot will be the hottest area of the entire bell/bench combination.

A bit of shifting here and there, and the situation is completely different. This highly creative process is called "designing as you go".

hi again

I have dry stacked core, bell, bench and chimney "base box" into a rough idea of the planned layout, please excuse the crude-looking effort

a few notes to explain the pictures:

as before, the objective is a Shorty core with front-facing exhaust on the riser, in a bell with a reduced ISA to allow for a long bench.
Ignore the fact that the firebox has no floor layer of bricks or refractory slab in in this layout, I am only illustrating the core outline here

The bell and bench share an elevated floor, to reduce ISA, on this solid floor the core also is built

2 pieces of wood show the bench top and the top of the external chimney exhaust box

The gap from core to bell wall is 100mm at the back and the right side, but on the left there is no gap (just 12mm superwool) - to reduce ISA
The opening from bell to bench is 300x300mm
The bench internal dimensions are 300x300mm
The opening to the chimney exhaust box is 200x200mm - 100mm lower than the opening to bench

The floor of the bell and bench continues into this external box, all at same height throughout, level & smooth.

Mindful that the space between core and bell walls should be at least 5 times cross section area of the chimney pipe, I think this has been achieved with the gaps at the back and one side of the core
And particularly that the 100mm gap allows gases to sink down between core and bell walls and pass from bell to bench adequately

I hope its clear and thanks for any comments - or questions please

One thing I'm unsure about is the tendency for gases to bypass the bench in this arrangement and escape to the chimney easily
As stated above, the path to the chimney is only 100mm lower than the opening into the bench

That looks like a good functional layout. One thing that I have questions about is the raised floor. That plus the internal bench height plus thickness of the bench top make the bench a bit high for comfortable sitting (unless you have tall people ). You could keep the floor one brick thick and lower the bench and exhaust by one brick to have the same ISA. For that matter, since the rule of thumb is that walls below the exhaust height do not count as ISA due to hot gases not going lower than that, you could lower the floor while keeping everything else the same if the bench is at the height you really want. That might allow a bit more freedom of flow in the bench which might help heat distribution. You would have to ask Peter if that would actually work - I don't have experimental evidence there.