Peter van den Berg

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.

sara ventura wrote: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?

What you wrote there is confusing me. Somehow, I get the impression that you think a bypass is a valve in the core itself. Just to be sure: it isn't, it's an extra opening between the bell and the chimney. Opening the valve in that opening is bypassing part of the bell, so that the chimney heats up much quicker. So, the position of the bypass could be halfway the height of the bell or somewhat higher.
The opening should face the core or the wall.

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%.

sara ventura wrote:As it is now, I'm at 6m2 ISA and I need to reduce it a little.

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.
Don't worry, please read on.

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.

sara ventura wrote:-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?

Better to have the core not lower than the bench, also construction would be much more logical.

sara ventura wrote:-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.

The slabs for the bench could be 60 cm, sticking out on the outer side. Internally, the bench could then be 50 cm or even less. Making the wall of the bench beefier won't help much, the seat will be the warmest anyway.

sara ventura wrote: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.

Correct.

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. Those columns are there for to support the slabs of the bench' seat, while at the same time the wall of the bell above the bench is resting on the slabs. For how to construct this, see my article about the French Bench.

Gerry Parent wrote:I was under the impression that a person could cheat and use a fan and/or a heater to decrease the amount of time before a smokeless fire is achieved.
If so, wouldn't using such devices be able to prove that these numbers are not fixed but are interchangable as you mention?

I am inclined to say you are right, good thinking. But the combustion system would change from an atmospheric aspirated device (is that the right term?) to a blown combustion system, like a wood gasifier. Food for thought, no doubt about it.

OK, the drawing is ready. I couldn't find a good example of the door assembly, I will look for it. The whole of the core is a little bit larger, although it will run without any problem on a 150 mm chimney diameter.

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

Using hard firebricks seems to be the logical way to go, then.

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 interchangable. Maybe, with an awful lot of turbulence induced both the other two T's, time and temperature, could be smaller.

Actually, I am convinced this could be the case, although I am unable to prove it. A young master heater builder from Germany visited me last year, and he was very surprised my Testo gas analizer showed such low CO numbers after 5 minutes into the burn of a cold Pepper Shaker heater. At the same time, the temperature in the afterburner area couldn't have reached at least 850 degrees C, far from it.
Interesting line of thought, don't you think?