Peter van den Berg

Costi Rado wrote:I would like to know your experiences burning incompletely dry wood in batch heaters . Does mixing dry wood with some greener wood work? How does it affect the burn cycle?

First of all: start a batchrocket with dry fuel. Once the firebox and afterburner are thoroughly hot, the system doesn't care as much whether the fuel isn't dry or not. It does affect the burncycle, though. The moisture need to be driven out before the fuel is properly off-gassing. This process will drag down the efficiency of the burn greatly, even as far as half the energy contents of the wood.

Costi Rado wrote:Are there some heater cores more forgiving in this respect?

Yes, it seems that the latest Shorty batchrocket core is quite good at that. Also, the larger the heater, the less it is hampered down by fuel that contains 25% or even more of its weight as water.

There's also a peculiar phenomenon I've seen and heared with moist fuel in particular: the afterburner "rockety" sound seems to be much louder. And through that roar at the same time a very low, irregular rumbling, that makes one think about a large diesel engine that's running outside. Hard to describe, this effect is stronger in a first-generation batchrocket, one without a door of any kind.

I think Glenn is right here, having the batchrocket core in the annex and a bell in the living room is asking for problems.  
May I suggest this: a hole in the wall between living and annex, big enough to have the entire core sticking through, walls, floor and deck of the firebox. Plus some space extra to be filled with superwool, acting as an expansion joint. Depending on the thickness of the wall, this core could be a reversed Shorty or a sidewinder. A straight Shorty that is exhausting in the normal way back to the wall isn't advisable, almost certainly too cramped.
The bell on the other side of the deviding wall can be of any shape, as long as the end of the core fits in it. The only real requirement for building such a core is the specific door frame which is also acting as an air duct.

A Shorty is the most user-friendly of the whole batchrocket family. It's a casual burner, easy to refill and no fiddling with the air controls is required.

Alan Leahy wrote:heres a few pictures of the dry-stacked lower riser / liner with the thicker bricks...
Although this looks like a simple approach so far, I have a suspicion that it could become a trickier construction versus the design in the SKP file - the 2.1B port being one of the factors, as I am missing 11mm from the port height at the moment (currently its just 2B) so I would have to "notch out" a channel of approx 10mm from the brick spanning across the roof of the port.
Then again it may be doable by adding 5mm to the top & bottom clay joints of the vertical bricks either side of the port

Again, and please, just use the pattern I supplied in the core drawing. By trying to save on firebricks, you might find a couple of bricks in the riser are starting to wander due to the repeatedly expanding and contraction cycles. Better to have a sturdy construction with a solid running bond and a separate liner that is taking the heat cycle.

Alan Leahy wrote:I don't yet have a ceiling slab for the firebox so I'm stalled here for a while anyway regarding the core, I can make progress on the bell and bench for the moment
I will play around with cutting some firebricks and see where I go from here.

You do whatever you like, it's your project. But I can't advise on a build when one is trying to invent the wheel.

Alan Leahy wrote:The bench floor has been lowered and the bench's interior void now measures 300 wide x 350mm high, and the opening from bell to bench will be this size also
This allows for the chimney exhaust box opening to be mostly in the lower half of that void (measuring 200x200)

Normally, the opening to the exhaust box should be something like two times the cross section area of the chimney. Following that, with a width of 200 mm, the height of the opening could be 177 mm. Or just 175 mm, exactly half of the bench' vertical internals.

Alan Leahy wrote:The chimney exhaust box will be external to the bell

Bear in mind that the chimney pipe shouldn't be resting on the bricks but on a steel plate with a hole in it instead.

Alan Leahy wrote:The cooling gases should flow down the back & right hand side of the core, into the top of the bench, and having cooled further, return along the floor of the bench into the exhaust box.

Sounds logical to me. Keeping a design simple is most of the time more difficult as compared to making it ever more complcated.

Alan Leahy wrote:Some questions about the Port - I thought the recommended height is 2.1 times Base, but I notice on the SKP files its just 2xB
2xB would suit me fine if that is possible, as its easy to build without cutting

The SKP files are showing a very different brick pattern, involving at least 3 seams. Those three seams together with the bricks are usually very, very close to the 2.1B target. If anyhow possible, please use the pattern of the provided skp file, including a separate riser liner.

Alan Leahy wrote:Port width is 54mm for a 108 base, my base is 115 so my port width will be set to 58mm

Yes, that's correct, use the same Base number for every calculation on the core. Also, the dimensions of the lower riser part should be done with this Base figure in mind. In your case, that dimension should be 159x 159 mm. This is also the case for firebox height and width.

Alan Leahy wrote:Port depth appears to be not so crucial from what I have read, 120mm is recommended for a 150mm system, and mine is currently 115mm - with no cutting bricks - should this be increased to 120mm minimum?

Port depth isn't important, specifically in the case of the Shorty. This depth is much less in a reversed system and it'll work nevertheless as it should.

Leonardo, I've had a close look at the drawing and yes, there are some details I would do differently. Here they are, in no particular order.

The core could be shifted to the right quite a bit, so the design would look quite a bit more symmetric.

The six legs under the core could be left out, the horizontal support frame would be strong enough on it's own. The ends of the frame could be resting on the bricks of the bell's wall. about halfway in the wall's thickness and wrapped with superwool.

The exhaust box is correct, but a cleaning hatch is missing.

The bell bricks should show a better running bond, in my opinion. It's correct at the sides left and right but front and back aren't that strong. A single skin bell like this is asking for a generous layer of plaster, with a stucco net in it. Two layers, the net in the first layer and the second being the finishing one.

The heat shield in the left of the bell doesn't need to be that large. Have a look at the build in the Sculptor's Shop, that's a 200 mm system where such a three-sided shield is fitted.

In my view, there's only one real possibility.
Taking out the front of the hearth in the dining room, rerouting the chimney and building the bell and core into that space. But then, I won't face the amount of work and the mess that's coming with such an endeavour.

So it's all up to you what to do.

Leonardo Bevilacqua wrote:As I was "building" the stove in Sketchup I noticed something. My firebrick size is 220x110x60 mm, and it  looks to me that it doesn't fit very well for the 180mm system measurements. I fear it would mean many cuts during the build. Am I right?

Most firebricks aren't a good fit for a 180 mm system. Sorry for that, it can't be helped.

Leonardo Bevilacqua wrote:In the back of my mind I remember reading something about tweaking the measurements to fit better with the brick size one has at hand.  I guess it would involve tweaking the base number B in Peter's formulas. Could you help me doing that?  Or, is it better if I just stick to the standard measurements for a 180mm system?

Tweaking the base number can be done, that's true. But I don't think it would help much, a lot of cutting is what one is stuck with.

Leonardo Bevilacqua wrote:Another question. I understand the strong recommendation to stick faithfully to Peter's proportions in order to have a good result. At the same time, though, I'm sure there's a tolerance within which no big difference can be noticed it the end result (performance wise) but may make the build easier. What tolerance would you say I have with the measurements? I guess it would be in the range of a couple millimeters.

There's some tolerance on the measurements, that's correct, specifically regarding the firebox. This could be a bit higher or lower, wider or narrower and deeper or not. About 10 mm tolerance won't hurt the workings, provided the first and second gate are correct dimensioned.
I delved into older drawings, and found the core of one heater done in Februari 2024. The ink of the design was still wet at the time, and it shows.

But at the time I stumbled over the same problems as you did and here's the solution. On top of the firebox' floor there are two bricks on edge and on top of each other, all around, nothing special. Then in order to get the first port right, all around the structure there's a strip of brick that's 53 mm high. And on top of that layer, there's another full brick on edge also over the port. Now, the height of the firebox is a bit too shallow, about 7 mm but in practise that might be less due to the thickness of the seams. The way I did the riser liner I wouldn't regard that as correct anymore. Better to follow the method of building the walls and doing the liner as a separate item.

Here's a picture:

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".

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.

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.