A tentative design for a 3 bell + batch box

[fiedia]

I would like to heat as much as possible my house with a single RMH. I came out with a design including a batch rocket stove + 3 bells (one in each room). I tried to scale the bells with the help of a simplified thermal model (see thread). It is my first rocket and I am sure that many things in this design should be inmproved. I came with a lot of questions and I hope you can help me.

The house :





East side with kitchen on the first floor and a bedroom upstairs :


East side :




West side with living room downstairs. The first floor will not be heated by the rocket stove.


West side :

  

The heat source is a batch rocket 200 mm diameter (8”) with some modifications. Heat riser port at one corner of the box; loading from the opposite side through a glass door. It heats the kitchen downstairs.



The heat riser is inside a first bell : the kitchen bell (inner dimensions : 74cm x 24cm x 90cm; bell section / heat riser section = 5.5; ISA ~ 1.25 m², heat riser top to bell ceiling = 30cm, wall ~18cm thick 2 skins)..

An oven sits on top of the batch box. It’s ceiling is heated by smokes flowing inside the kitchen bell. It’s bottom is heated by the batch box.


kitchen bell



Then smokes flow into the living room bell through the wall. This second bell fits under the stairway (inner diameter : 28cm; average height : 200 cm; max ISA ~ 3.75 m²; wall thickness ~12cm two skins).

living room bell



After the living room bell, smokes go back to the kitchen and up to the first floor to the bedroom bell (ISA ~ 2.15 m²).

bedroom bell



Total ISA ~ 7.15 m².

Results from the bell thermal model applied to these three bells :




The first bell has been designed as small as possible to leave more heat for the others. The second bell has been designed to heat as much as possible the living room (round shape, height and thinner wall). We do not want to heat the bedroom to much.

Although the ISA looks good for a 200mm batch (9 m² max), the model exhaust temperatures stay below 100°C. Therefore bypasses will be usefull in case of insufficient draft to heat the whole mass (~3 tons).

Bypass through the oven : two removable plates on top and left hand side of the oven can shunt the living room bell. As smokes will go through the oven instead of the living room bell, it will improve the draft and increase the oven temperature (white oven becomes black oven).

    

According to the thermal model, shunting the living room bell will increase exhaust gas temperature up to 270°C. This should be sufficient to get a good draft.



Living room bell regulator : The living room bell has been designed as big as possible to optimize heating of this room. A sliding plate has been added to shorten the smokes travel inside this bell. By opening gradually this bypass, the heating power of the bell should decrease also gradually.

    

According to the thermal model, opening fully the regulator will almost double heating power inside the bedroom, increase exhaust gas temperature and halve the living room bell heating power.

    

Access doors :

There are 4 accesses for inspection and cleaning :
- in the kitchen through the oven, removing the upper plate will give access to the inner of the kitchen bell and the upper flue to the living room bell.
- In the kitchen through the access door on the left hand side of the oven, it will give access to the left part of the bedroom bell and the lower flue to the living room bell.
- In the living room through the access door at the bottom of the bell
- In the bedroom through the access door on the right hand side of the bell, it will give access to the exhaust flu up to the roof and to the right part of the bell thrue the swivel plate hanging inside (rotating this plate may also regulate the heating power of this bell).




All of this looks very nice but I have absolutely no experience in building such stoves. There are many questions open. It would be nice if you could give me some advices.

Open questions :
- Bell stacking : did anyone already tested two bells on top of each other (as here with the kitchen and bedroom bells) ? Any surprise ?
- Backdraft : I am afraid of backdraft at the access door on top of the batch box. But the bedroom bell is heated by the heat riser through the kitchen bell ceiling. It should increase the draft and avoid back draft. Am I right ?
- Wood load burning time : I assumed a 2 hours burning time at 700 °C (1330 °F) at heat riser’s top. Is it realistic for a 200 mm batch box fully loaded ?
- Heat riser and other materials : do you have suppliers (ideally in Europe) for ceramic pipes for the heat riser, super wool and ceramic boards ?
- Backwall insulation : I planned to use 5 cm aerated concrete to insulate the back of the kitchen and bedroom bells from the house wall. There would be at least 12cm brick between hot smokes and the aerated concrete. Do you have any experience with aerated concrete or other insulation materials for this purpose ?
- Lintel : there are to 72 cm lintels above the oven. One will be in direct contact with heat riser’s smokes (inner skin). I am afraid in using reinforced concrete (I guess that steel expansion will be a problem). What kind of lintel do you use ?
- Doors : any advice regarding how to make a window door (it is a sideloader) ? Can I use standard steel and glass and how thick ?
- What is the best heat riser insulation : perlite or vermiculite ?
- The heat riser port will be on the rear right corner of the box (looking from the front side window). I understood that the primary air inlet should be on the front left of the box. Do you have any drawing or sketchup file for this set-up ?
- Do you see any other issue that I should take into consideration ?

sketchup model : www.ecologie-pratique.org/mediagallery/media.php?f=0&sort=0&s=20180628125039788

[gadget]

None of your pics are loading for me. Interesting concept. If I could redesign my house it would be with a central located mass heater. As it is, my heater is going to the corner of the house, up stairs. Not ideal but it is where it needs to go. This is why I will be heating the air as well as the mass. Something to think about if you find your design is to complicated. Another option is heating water and moving it around where needed. This has its own issues of safety though.

-Gadget

[peterberg]

As I see it, it is an overly complicated design. For example, the batchrocket as it is, represents a proven design. Personally, I know of some people who've built sideloaders but it looks like those are riddled with problems. The fact that the air inlet is in one of the sides and the port in the other while the window is in one of the closed sides isn't very good for burn quality. In order to have the right burn characteristics the door need to be tightly closed and the window will black up partially or as a whole.

Why do you want to reform the design in such a way is one of my questions. In more or less the same space a sidewinder could be built. The round bell in the living space would look nice but it is complicated at least to build like that, the 45º ceiling very hard to implement if not completely impossible to realize. The port in one of the corners won't burn as clean as one in the middle or slightly off-center. To the riser the port should always be in the middle in order to form the double vortex.

Furthermore, it looks like you are relying on baffles to guide the gases while in a bell system that should be done by gravity. Just a thought: is it possible to build the heater in two rooms by making a large hole in the wall? This way the kitchen and the living room are heated, it's possible to implement it as a straight batchrocket and a small bell on the second floor could be done relatively easy.

Oh yes, I've built several experiments 8 or 9 years ago, done as double bell and triple bell on top of each other and it works. Although the double bell is more efficient by 15% as compared to its single brother. In other words, you need to deduct 15% of the original ISA for such a construction. Triple bells aren't any better than double bells so in my view it isn't worth the trouble to build like this.

[fiedia]

Thank you peter for this long and clear response. As you say, it is quite a complicated design and I will do some additional trials with a new prototype bell before tempting to build the final heater. I would like to validate the thermal model with more temp sensors inside the bell and study bell efficiency vs heigth. I will update my posts when achieved (A brick bell prototype to improves standard stove efficiency; An experimental bell thermal model).

Regarding the sideloader, I will forget about it and go for the standard design with the port in th middle. Thank you for your warning.

The round bell was designed to optimize heat radiation in the living room. I do not see it as very hard to build although more chalenging than a square one. Anyway the next prototype bell will be in the living room and I will get new information to scale the final living room bell.

You are right. It would be better if I could just build a big bell sitting across the wall between the living room and the kitchen. A real mass heater in fact. But I would rather avoid digging a large hole in this shear wall. The house is almost 200 years old and sitting on clay. May be the solution is to build the heat riser on the living room side with smokes going back into one single big bell in the kitchen up to the bed room. I will validate this by measuring temperature vs bell height in my next prototype. For such a design, I would have to shorten the heat riser by 0.2 m. Do you see any issue using a 1.2 m riser for a 20 cm diameter batch box ?

You say triple bells are not better than double bells. Did you just split in two or three the same big bell (keeping roughly the same ISA) or did you add bells on top of each other (multiplying by 2 or 3 the ISA) ? Does the 15% correspond to the top surface to total ISA ratio (neglecting the top surface of the lower bell since it is internal) ? I would like to understand your set up. Did you create a post or some drawings or pictures on multiple bells ?

[peterberg]

Aug 13, 2018 23:36:34 GMT -8 fiedia said:

Do you see any issue using a 1.2 m riser for a 20 cm diameter batch box ?

No issues, 120 cm is just a tad above minimum recommended height.

Aug 13, 2018 23:36:34 GMT -8 fiedia said:

You say triple bells are not better than double bells. Did you just split in two or three the same big bell (keeping roughly the same ISA) or did you add bells on top of each other (multiplying by 2 or 3 the ISA) ? Does the 15% correspond to the top surface to total ISA ratio (neglecting the top surface of the lower bell since it is internal) ? I would like to understand your set up. Did you create a post or some drawings or pictures on multiple bells ?

What I did (during an entirely different experiment) is just splitting the bell in three with a couple of different proportions. And also a double bell in two different proportions. The double one came out slightly better than the triple one. I didn't build any double bells in recent years because a single one is the simplest to build with very good characteristics and a bench can be built as part of the main bell anyway.

Here's the post about this experiment: donkey32.proboards.com/post/1170/thread
It might be a good idea to read through this thread, maybe there's something in it you could use.

[fiedia]

Going one step further, in this project.

An old wood stove heats presently my kitchen. Hot fumes are reused to warm a bell in the adjacent room. I wish to replace the old stove with a batch box to get cleaner exhausts and avoid soot and creosote in bells.

House overview :


For the adjacent bell, the detailed information is posted here.

Batch box in refractory bricks :


inner skin, top built out of refractory bricks :


outer skin in reused bricks:
        

Sketchup model : here

peterberg, thank you for all this knowledge and experience you are sharing. Would you be so kind to give me some details on some points you posted ?

Question 1 : How to build with removable refractory bricks ?
I wish to reuse the bricks after one or two seasons for my ‘final’ project, an 8” batch box. I planned to use clay or superwool between bricks but I read from you , in your "DSR mark II" thread :

The whole of the top plate is mortared with a strip of plastic between plate and mortar so it'll fit perfectly but is still removable.

What plastic is it ? Plastic both sides of the mortar ? Is it better than just superwool or clay ?

Question 2 : Can we use vermiculite to fill gap between brick skins ?
There is a black oven on top of the batch box. I would like to avoid superwool as mush as possible. Even if it is biosoluble, I would rather not eat it on my breads and pizzas. I planned to use Vermiculite between box and inner skin as shown below. But I read from you :

Leave the gap open but close it at the top so convection is not allowed to the room. The outer skin will heat up by radiation like an Austrian air gap free-form heater. Heat transfer by conduction isn't possible at all because of the need for the inner skin to expand.

Any loose but hard material like gravel or sand will settle down over time and will crack the outer skin eventually.

Do you think vermiculite is compressible enough to be put between batch box and inner skin ?

Vermiculite around the batch box and heat riser :



Question 3 : Air frame balance
I wished to use your air frame with side inlets for primary air. You wrote in the "6” brick stove from Russia” thread :

Don't forget the tube directly above the firebox' door acts as a balancing pipe.

How critical is this “balancing pipe” ? Have you done some trials without holes for air balance ?

Thank you in advance for your time.

[peterberg]

Jan 19, 2020 1:33:31 GMT -8 fiedia said:

Question 1 : How to build with removable refractory bricks ?
I wish to reuse the bricks after one or two seasons for my ‘final’ project, an 8” batch box. I planned to use clay or superwool between bricks but I read from you , in your "DSR mark II" thread :

The whole of the top plate is mortared with a strip of plastic between plate and mortar so it'll fit perfectly but is still removable.

What plastic is it ? Plastic both sides of the mortar ? Is it better than just superwool or clay?

Ooww... Let me get this straight. The plastic is only used during curing of the mortar, so it adhered to one side only. This plastic can be anything, it should be removed before firing up the heater. One would be able to lift the piece off and place it again without any problem whatsoever. But you could use a clay-sand mortar just as well, this can be scraped off and used again as I did with a lot of experiments.

Jan 19, 2020 1:33:31 GMT -8 fiedia said:

Question 2 : Can we use vermiculite to fill gap between brick skins ?
There is a black oven on top of the batch box. I would like to avoid superwool as mush as possible. Even if it is biosoluble, I would rather not eat it on my breads and pizzas. I planned to use Vermiculite between box and inner skin as shown below. But I read from you :

Leave the gap open but close it at the top so convection is not allowed to the room. The outer skin will heat up by radiation like an Austrian air gap free-form heater. Heat transfer by conduction isn't possible at all because of the need for the inner skin to expand.
Any loose but hard material like gravel or sand will settle down over time and will crack the outer skin eventually.

Do you think vermiculite is compressible enough to be put between batch box and inner skin ?

This is a question of having the gap between inner and outer skin completely open, difficult to achieve in my experience. In case you want to use vermiculite, you need to build the thing first and fill the gap later. A lump of mortar that falls into the gap will connect the inner and outer skin, creating a high risk of cracks in the outer skin. Whether or not the vermiculite is endlessly compressable: I have serious doubts about that. You could replace the superwool by something that's cheaper, like rockwool or even cardboard. That last one will burn away in the absence of oxygen and leave the desired gap open again.

Jan 19, 2020 1:33:31 GMT -8 fiedia said:

Question 3 : Air frame balance
I wished to use your air frame with side inlets for primary air. You wrote in the "6” brick stove from Russia” thread :

Don't forget the tube directly above the firebox' door acts as a balancing pipe.

How critical is this “balancing pipe” ? Have you done some trials without holes for air balance ?

Yes, I did. Without this open pipe the air tend to choose one side, not necessarily every time the same and occasionally but not always changing during the course of the burn. The balancing pipe ended this behaviour completely, air distribution is much more even left and right than without. That's why I said "Don't forget", when it isn't doing anything I wouldn't mention it, don't you think?
Mark also that it aren't just some random holes, which I incidentally tried as well. I drilled some fairly large holes through the side of the duct without removing the horizontal pipe. This didn't work, the balancing pipe need to be completely open in order to achieve the desired effect. Give the air room to go around corners, that's the general message.

[fiedia]

Thank you for your answer.

I will use clay, I guess it is much easier and since it has to last just one or two seasons, it should be sufficient.

Let’s go for the wide open balancing pipe. I was wondering if I could just drill holes but you answered before I even asked !

Regarding filling the gap between fire box and first bell skin, I understand vermiculite is not the best.
Rockwool is not biosoluble, I would rather use superwool.
Cardbox would leave the gap free and I counted on vermiculite or superwool to hold the firebox sides (3cm bricks lying on edge). Well I see no perfect solution here.

Thank you for your precious advises.

Nest steps of this project to be found here