Peter van den Berg

Jim St Laurent wrote:A 7½ inch core delivers about 42 kWh per load, which calls for a bell ISA in the range of 120–150 sq ft.  
Using the ISA table below, I’m trying to understand how to get the most heat high up into the living space Bell surface area.  

ISA for a 12‑ft Tall Square Bell (5 ft basement + 7 ft living space)
| **Side (in)** | **Side (ft)** | **ISA @ 12 ft  tall |
| ------------------ | ------------------- |------------------------ |
| 24″sq             | 2.0                    | 84.0                      |
| 32″sq             | 2.6667             | 113.78                  |
| 40″sq             | 3.3333             | 147.78                  |
| 48″sq             | 4.0                   | 176.0                    |

Something isn't right here, according to the table on the batchrocket website ISA for a 7.5" system should be between 77.5 sq in. and 101 sq in. So, 84 sq in. would be about right but... a Shorty core is less powerful, tuned down a bit as it were, as compared to the first generation batchrockets. As such, the ISA should be reduced by 15%. Failing that, there's a good chance the core won't get up to working temperature easily, if at all. Mind you, all internal wall and ceiling of the bell is part of the calculation. So, if you are able to do that, use the rule of thumb for calculating the bell and calculate the system size chimney in m² first, multiply that by 255 and you've got the correct ISA in metrics for a Shorty core.

Jim St Laurent wrote:Questions:  
1. Would an oversized bell store more heat higher up into the living space? either more ISA or more thermal mass in the top section

The top venting Shorty core will spew the hot gases up in the bell, thereby driving the cooler gases out through the exhaust low in the bell. This was always the behaviour of the 1st gen batchrockets and is still the case by this particular variant of the Shorty core.

Jim St Laurent wrote:2. Will insulating the core where appropriate help?

Yes, it will, absolutely, especially when starting cold.

Jim St Laurent wrote:3. Would a clever bell cover design with more internal surface area, that won't hold fly ash be an improvement?

No, any obstacles aren't helping, as experience showed, rather the contrary. This effect isn't well understood as yet, I am unable to tell you why.

Jim St Laurent wrote:Based on our discussions, I’m leaning toward an up‑venting Shorty core paired with a 40‑inch square bell and a insulated block wall foundation.

Provided the calculation is correct, of course. By the way, all Shorty core bells are now built with a bypass to warm up the chimney and getting the core up to working temperature in short order. There might be a possibility that the 7.5" system can't be coupled to a large enough bell. Just scale the core up a bit further, these cores are able to run happily on half a load as well. I seem to remember that an 8" system is better to avoid cutting (a lot of) bricks.

Jim St Laurent wrote:I can accept some heat loss into the basement, knowing that part of it will return as gentle warmth through the flooring above. My main goal is to retain as much of the core’s output as possible up into the living space.

Hot air will rise and might go through the  floor above, that's true. But... a brick bell isn't emitting just a bit of its power as convection, most of it's heat is radiation. Nevertheless, with a warm space underneath the effect can be felt upstairs. The bell in the basement will emit quite a lot of heat, since the core is in there, despite the insulation around the core, as experience tells us.

Jim St Laurent wrote:The heater will be located in the basement, and the main bell will extend upward into the floor space above, ending at about 15 inches above finished floor height. I’ve already completed the heat‑load calculations, and a 7.5" system should let me run one full burn during the shoulder seasons and two full burns per day in mid‑winter, which fits my heating plan well.

Hi Jim, a heater in the basement will heat the basement well. And sticking through the floor above it, it will heat that space somewhat. All depending on the size of the part that is in the space above, obviously. Please, build the heater in the place where you live, that is the best option. In case that is on the first floor, build a large footing and place the heater in your living space.

Jim St Laurent wrote:Right now I’m considering either a Sidewinder or a DSRv2 core. I’m leaning toward the Sidewinder because of the compact footprint, but I’d appreciate input from anyone who has built both.

The whole of the batchrocket family consists of four main designs, being: the first generation with a high riser, the DSR2, DSR3 and the Shorty. All those designs can theoretically be built as sidewinders, where the riser or afterburner area is placed beside the firebox instead of behind it. That said, the first gen and Shorty are built multiple times as sidewinders now, I don't know of any DSR2 like that and just of one DSR3 built as sidewinder. The smallest footprint comes with the DSR3, no doubt, because there isn't a separate afterburner space behind or beside it, but on top instead. At the same time, the DSR3 is a real pig to build as a first one and get it right at the same time, I won't recommend that approach.
Looking for a well-behaving heater that isn't a hell of a job to build? The Shorty core is what you want, in straight or sidewinder version.

Jim St Laurent wrote:I’m pretty much locked in on using a double‑masonry bell, with the flue vent exiting high in the basement. I’m also kicking around the idea of a raised core to move the heater away from the concrete floor and make fire starts easier for me.
I’d really appreciate any advice, examples, or lessons learned that could help me refine this design before I start building.

Raising the core is always a good decision, it'll make the day to day life with such a heater appliance much more relaxed.

Jim St Laurent wrote:Questions:
• Which core type gives the smallest footprint at 7.5"?

Usable ones and ease of build and use? Shorty core as sidewinder version, no doubt about it.

Jim St Laurent wrote:• Can a Sidewinder exit into the bell from any of the four directions — left, right, front, or back?

A sidewinder Shorty core can exit to the left or the right, or... up! The straight version can be exiting front or back, or up. A first gen is always exiting up only.

Jim St Laurent wrote:• Is there a repository of SketchUp (.skp) heater designs? I found some of Peter van den Berg's designs already.  Ideally something using imperial firebrick sizes (9×4.5×2.5") or splits.

Most of the batchrocket designs are to be found on the website batchrocket. eu.

The site is in multiple languages, although the Shorty description is in Dutch only, so far.  Google Translate is able to solve the problem of a passable translation, though.

If you know what core and size you want to build, I am able to draw you a design using the correct brick size and layer by layer, for a reasonable fee.

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".