Peter van den Berg

Hi Aeron, a question: is the workshop in use each and every day of the week? In case it's 4 days or less, you have to start all over again in the morning as soon as you come in. A quick response heater is usually the way forward in such a situation. All barrels or a single skin brick bell.

I'll try to go through all the questions and try to provide answers, starting down here.

Aeron Ironbark wrote:- Core material: insulated fire brick vs ceramic fibre board (vs something else)? What's the cheapest and easiest? And is longevity/durability the exchange for that?

Hard firebrick, no question about that. You can insulate the core or not, although I've seen larger mass heaters without core insulation.

Aeron Ironbark wrote:- Location of the flue: I've seen brick RMH's with flues inside the bell, and I've seen them outside. It seems to me that if it was inside, as soon as the burn starts it would begin heating the flue and starting draw, thus minimising (or eliminating) smoke coming into the room. However, it seems to me that this setup would also create quite a strong draw during the main burns, and would thus be drawing out hot gasses before they had given their heat to the bricks, thus making the system much less efficient. Conversely, the flue on the outside means that only gasses cool enough to reach the bottom would ever make it to the outlet, which would mean much more efficiency in terms of heat given to the thermal battery, but how does the draw start in that set up? I've been thinking about a Tee piece at the outlet, and putting in some wood shavings at the start of a burn to heat up the chimney directly? And how does the draw continue? As I understand, any flue situation relies on the temperature differential between the pipe and the surroundings, and in the scenario where the pipe is outside of the heater, and the outlet of the heater is at the bottom because that's where the coolest gasses end up after giving their heat to the brick and thus making the thermal battery a thermal battery, how does the draw maintain?

The batchrocket system has been built for decades now, on every continent, Antarctica being the exception. There are calculations for every system size you might choose, designed in such a way there 's always enough heat transported to the chimney to keep it going. In short: not all the heat is extracted from the gases. Have the chimney external to the bell, with a bypass to help it going in case the heater is stone cold.
The first generation batchrocket, with its long vertical riser blows the hot gases to top, thereby pushing the colder gases at the bottom out. Built exactly to specs and with a good chimney, this one doesn't need a bypass. That said, as a rule of thumb, within 20 minutes of lighting a cold heater the exit temperature of the bell need to be at least 60 ºC.  If this isn't the case, water vapor is big time condensing inside the chimney and all smoke comes back into the house.

Aeron Ironbark wrote:- Secondary air inlet: Does this piece of square tubing have to just be a replacement part, since it will sit directly underneath the primary burn chamber and be much hotter than steel is designed to get? I've heard of fancy expensive metals that can be used to make tubes out of, but I think in Australia it's hard to get. Love to know what folks use for this.

Thomas answered this very comprehensively. Suffice to say: use the latest Shorty core, that one doesn't need a secondary air provision.

Aeron Ironbark wrote:- Sizing the Bell: If i've read the tables correctly, if i go with a 200mm riser, I need a Bell with a 9.4m2 internal total surface area. I've read about people adding columns inside the bell to add surface area, which sounds like a good idea to me to keep overall size down. I've also been hoping to do it as a single skin for the same reason, but is that a bad idea?

A 200 mm system seems overkill to me, it's a 50 m² workshop, moderately insulated. You might get away with a 150 mm one, but scaling up can be done witout steps so a 175 mm system and ditto chimney might be plenty large enough. This system scales up very fast!

Aeron Ironbark wrote:- Bell lid material: What do I make the lid of the bell out of?? I've got the business end of a rocket stove pointed directly at this thing, and I can't find info about what it should be made of... I don't know much about concrete, but it seems like it would just crack? And what does it sit on for support across the span? Another reason I like the flue on the outside is a simpler construction of the lid, so there's that too...

Given enough distance between ceiling and riser, steel t-bars and firebricks would do the trick. Also used multiple times worldwide, including my own red bell heater since two years. OR you could use a Shorty core, much less firebricks in the walls and ceiling possible, because it's tuned down a bit and blows horizontally back to the front instead of vertically up. Bell would also a bit smaller, not to mention cheaper.

Aeron Ironbark wrote:- Bricks and mortar: What bricks for the actual Bell? Plain old building bricks? Solid/vs hollow? And what about the mortar, both for the core and for the bell?

This isn't one question, but a handful instead.
Normal, solid building bricks are perfectly adequate, the denser the better. But anything could be used, including concrete paving bricks, commonly called clinkers. Have a look at a large heater like that:  The Sculpter's Shop heater!
Refractory mortar for the core, clay/sand OR sand/lime/portland for the bell.

Matthew Galloway wrote:Here's the part that I'm not clearly understanding from the instructions:
"Equally, if the firebox is built into the bell then the surface area of the firebox within the bell won't play a role in calculating this area as no heat is absorbed there."
Is he referring the SA of the firebox itself or the walls of the bell that surround it?

Just because people tend to be confused about what should be included into the ISA, I tried to simplify the method. Pretend there isn't a core at all in the bell, just calculate the wall and top and that's it. Reason behind this: when painstakingly calculated, the door opening should be left out, and the core should be included for about 20 to 25 %. When started from cold, the core will take up some of the produced heat, but shortly after that it will emit heat instead.

Calculated the simple way, the piece of wall where the door opening is, will be on a par with what the core will extract. So please, don't overthink this, just take the four walls and the deck, all internal. The result will be exactly fitting for the intended core.

sara ventura wrote:I still struggle to imagine the behavior of the hot air coming from the riser to "open air" inside the bell.
Has it the speed enough to project like a beam to the front wall, and from this first contact with mass ( and heat transfer) bounce and spread upwards?

It might help to view the core exit as a venturi (which it is), gases leaving at a remarkably high velocity. A bare core is different, exit is slow. But coupled to a fitting chimney the whole of the core will step up to a higher combustion rate and higher gas speed. What you describe is correct, the heat wave is travelling almost straight forward and more or less bounce against the opposite wall. In general, with a higher velocity of the gas stream, the less it will be spreading by itself. Until something solid is in its trajectory, as it were.

sara ventura wrote:Is it l like the air coming from an A/C unit, that you can freeze yourself if you're straight in front of the beam, but move away from there and the air becomes slightly fresh but quiet? I hope I pictured the example correctly : )

Yes, that's about the correct picture.

sara ventura wrote:Another question, when drawing the airframe and door adapted to my firebox (scaled to fit the size of my bricks), I just have to scale the air inlets accordingly, right?

The air inlets are large enough as is, although it won't hurt to make those a bit larger, just for sure. Remember, the exact dimensions of the air frame aren't really important as long it isn't cramped. The core will behave itself without a human being fiddling on the air controls. Even better, there aren't any air controls!

Scott Weinberg wrote:Would this oversizing of the flue, effect the performance or workings of a stove
I am thinking about those that may have aquired  a place that hard a large flue, but don't need that much stove size for the needed heat value that the flue could provide for.

Common knowledge seems to go in the direction of one step larger won't hurt, for example from 6" to 7". Two steps up might be too large, all depending on length of the chimney, also circular or not, smooth inside and straight up.

One step smaller will almost certainly culminate into a recalcitrant heater, difficult to start and slow to come up to speed.

sara ventura wrote:My fault, I've drawn a 20cm exit flue because I'll use a double walled pipe, 15cm exit inside. Also, will have a reducer from 20 to 15 at the base so I simplified the drawing, a lot😅

OK then, I am back on track again.

sara ventura wrote:Noted on the bypass, even better for me, construction wise.
That means the bypass flap will endure maximum temps coming from the core, anyone has experience on how does the flap behave after a long time period?

Your concern would be justified in the case of a 1st generation batchrocket build. The Shorty core however, is radically different in this respect. Maximum temperatures won't be at the top of the bell anymore, but opposite the core exit instead. This is better in more than one way, the most obvious one: people tend to live at a lower level than the top of a high bell.

sara ventura wrote:I'll add a bit more ISA, would you say the best place to add space is between the exit core and facing wall?
To have more space for the gases to move easily to the bench?
Or increase the clear space around exit pipe, wall and core? as it is now I have 4 cm on each side.

The exhaust is situated under the core's level, space enough there. I would vote for more space between core exit and facing wall.

Sara, in the drawing the flue is drawn as a 200 mm diameter one, while the core and ISA is for a 150 mm system. You won't need a 200 mm chimney, any particular reason to have one?

Also, the bypass have to be higher in the bell, preferably higher than the core's exhaust.

For to calculating the ISA of the bell, just imagine the bell whithout a core at all. Just four walls and a ceiling, this will be accurate for the chosen size.
Reasoning: the walls of the core will extract some heat from the smoke gases during a cold start, that's true. But after about 15 minutes the core will emit heat to the bell.

Olga Booker wrote:a) I am in doubt as to whether or not it can easily and sufficiently heat a large, high ceiling stone barn. This barn has a floor surface of 100 square metres (about 1070 square ft) and the height of the ceiling is 7 metres (23 ft) at the apex.  I am under the maybe wrong impression that a rocket stove is more suited for smaller spaces.  Also worried about how quickly it could heat that space after a couple of days away when the stone walls have seriously cooled down.

A high ceiling isn't a problem for a pure mass heater, most heat is coming off the walls as radiation. So, there's a relatively low amount of hot air in the shape of convection produced. A large batchrocket is a very potent space heater, have a look at this massive one, which is heating a space of 320 m² (3440 sq ft) with a 4+ m (13.1 ft) high ceiling, very little to no insulation at all in the building. The radiation of this biggie can be felt at a distance of 15 m (49.2 ft) across the entire shop space. Just an example, this heater is loaded three times in succesion 6 days a week. Won't cool down during the nights, so the shop space will be still quite warm in de mornings.
I won't say this the one for you, but it illustrates what is possible with a carefully made design.

Olga Booker wrote:b) We are both too old to build it ourselves and here in France, I have not found a reliable company that I would trust to build it for us.

It's all planning and gathering materials. The work itself can be spread out for many weeks, months even. There are examples of people, like Glenn Littman, who did one layer of bricks each day between coffee time and lunch, while probably having a power nap in the afternoon. It did give other people lots of time to be in time with comments and tips.

Olga Booker wrote:c) I haven't quite come to the idea of an oil barrel sticking out from the corner of my living room, but I'm sure some designs would take care of this.

As has been said by others, an oil barrel is not a necessaty. The whole of the heater could be masonry, and would even look beautiful when been plastered by a professional.

Mason, you mentioning in the first sentence of your post that there is a coal fireplace in virtually every room. Just a thought: could there be a fireplace in virtually every room minus one?

What you want is the idea of an open fireplace burning wood as fuel, looking like an old-fashioned coal burning device but with a lot of modern technology behind it. Unfortunally, that technology do start at the very front of a well-built Batchrocket, certainly not hidden behind a coal-heritage fireplace. Otherwise, it wouldn't be built to specifications and won't do what the technology suggests.

Said all that, here are the questions.
Are you talking about an open fireplace, or what?
In case there is a door, what size is it?
In case of no door, what's the size of the front opening?
What's the cross section size of the chimney and running to what height?

Personally, I did an open batchrocket in France, some years ago. It roared like the very devil and did warm a generously sized bell but its efficiency felt something... to desire for.
That brings me to a possible solution: a Double Shoebox Rocket #3. Most of the time called the Désirée, meaning the long awaited for. This is the shortest core in depth, burns upwards but... it's the hardest and most complicated combustion core to build of the four main batchrocket variants.
When done well, it would burn beautifully, but it requires a complicated door assembly. Never tested as an open fireplace, by the way.

Have a look at https://batchrocket.eu/ontwerpen#DSR. It's in Dutch, but Google Translate is very good in converting the text into readable English.
The whole of development of this core is published on an English language forum, namely Donkey's rocket mass heater forum. Have a read, and please consider all possibilities. If a properly built batchrocket can't be inplemented and you still want one, demolishing the coal hearth is the only option that's open.

Matt Todd wrote:Conclusion
After one burn, I’m sold! 3500 lbs of mass heated to an average of 120 degrees in a short burn with so little wood is a no-brainer. And the infrared heat it puts off is oddly satisfying to feel, even from across a room.

Congrats, Matt! You've seen and felt what a mass heater could do in a room. Also the second burn went swimmingly well as you reported. The wall behind the internal chimney assembly might experience too high thermal stress, keep an eye on it. The Shorty core seems to perform like she should, in my opinion she will stay this way for many years to come.

Welcome in the wonderful world of this oddball system!

sara ventura wrote: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.

Ahhh... I get it now. The intake end of the T to the side, away from the core. Height placement in your case, about the level of the firebox' deck. In other words, somewhat lower than the core's exit opening.

sara ventura wrote: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.

OK, although the bell is oversized for a Shorty core, you can get away with it since you've got an adequate chimney and a bypass. Chances are that you have to run the heater with the bypass open a crack permanently, especially during fall and spring. As long as the build is neatly done, it'll work as it should.