BBRMH with hob & oven - Bell, 6" Batchbox Rocket Mas Heater

[esbjornaneer]

I have now had a weeks use out of the stove I build at the beginning of November.

As the subject says what it is I'll let you look at the pics I took while building it BBRMH pics (hope they work even though they are in Dropbox)

My inspiration was initially from Klemen's thread, thank you all that contributed to his thread! I have read lots more during the month I was away while the stove was drying off and would now do some things differently but this is good for now.

It is working OK, I can't say great as the oven has not reached more than 170C and mostly only 120-150C with the amount of wood that is needed for 24hrs heating. For heating purposes it is working great, uses less wood and giving warmer maximum & average temps than the last stove (a radiant heat stove with flue system).

Cooking on the hob can of coarse only happen while the fire is burning so only once a day at the moment, twice if it gets really cold. And the hob is hot all over. Even though the oven is only really useful for drying off kindling at the moment we have managed to bake in the fire box. 2 good loaves of bread and one banana bread/cake (with 2 loaves of bread being ruined <one burned to coal, the other undercooked> in the process of figuring out how to check the temps in the fire box).
3 sites that have been useful are:
www.finecooking.com/articles/how-to/judge-temperature-charcoal-grill.aspx?pg=0
www.fornobravo.com/tech-resources/controlling-oven-temperature/
www.goodcooking.com/conversions/oventemp.htm
And I have come to conclusion that the FB is ready for bread if the embers are died down and I can hold my hand in the box for 9 seconds before I have to remove it.

I am considering shifting the location of the oven so it splits the gas stream in order to heat the box from 4 sides instead of 2 but I'm not sure how much extra temp I will get from doing it and it would mean breaking down & rebuild part of the front to do it... So not sure if it is worth it?

I have had a fair amount of cracking happening already in the seams of mortar, the fire bricks in the heat riser expanding more than the ones outside the insulation. I guess I did not mix the right proportions of water in the fire cement for it to hold right? but am sealing it up with clay slip as they develop and it seems to work.

The only other trick I have had to learn is to light a small fire within 10cm of the heat riser entrance and let it burn a bit before filling the fire box with the round wood from our property.

I'll come back with dimentions and modified drawings of what was build at a later date.

Your comments and suggestions are most welcome! Especially related to suggestions about how to get the oven temp higher.
Thank you all again.

[morticcio]

Nice stove and very neat build, but I can't help but think there isn't enough mass for a heater let alone an oven. How thick are the firebricks - 30mm? The steel oven and large hot plates combined with thin firebricks it is no wonder the oven doesn't hold the heat for long.

A second skin of standard brick would help add more mass and keep the heat flywheel effect going. Also adding some insulated covers on the hotplates will keep the heat in too. If you're modifying the oven then maybe making this from firebrick will help too? Also do you have a shut off on the chimney to close when the fire has gone out?

[esbjornaneer]

Hi Morticcio & all,
Thank you for your comment, it is much apreciated.
I was just about to ask you about the rendering you did in Dec 2013 but will do it in the rendering thread.

The firebricks are 40x110x220mm in dimention and weigh 1.7-1.8kg each. I think I used somewhere around 350 bricks so for southern Spain I come in at quite a reasonable mass of about 500-600kg. I only fire it once a day and still keep the minimum temps of the main room, downstairs, of the house (where it is) above 19C (high temps of 26-27C have been reached but moslty 25C). I know the winter has been very warm but I am happy with the inside temps and counld fire it twice a day with smaller loads if I needed to keep the minimum up.

The problem with the bricks that are localy available is that they are not heavy so adding another skin it would have to be firebricks if I was to get some dense brick. How thick an earth plaster/cob layer would you like to add to it if you were to use that method for adding mass?

I have a thin plate enamelled chimney pipe comming out of the bell and it is possible to keep my hand on it for most of the burn. Towards the end it gets hot enough to only hold it for a few seconds but most of the heat seems to be extracted. The strange thing to me is that the top half of the bell does not reach the same temperatures as the bottom half (up to the height of the lower part). Does anyone have any thoughts on why that is? I will loose stack a layer of bricks into the bell between the inlet and outlet to see if it makes a difference in forcing the heat up. (I know it shouldn't be necessary but still with a try.)

I did consider, while building, adding a concrete block bell upstairs but am not sure that there is enough heat left in the gasses to work. The upstairs could happily take some more heat. Any thoughts most welcome?

There is no shut off valve on the flue side but I can shut the primary air inlet off with a metal plate that gives a metal to metal tight seal, and I have taken to stuffing a piece of cotton cloth into the p-channel/secondary air inlet to stop that air flow too. And as soon as these are closed in this way the chimney pipe goes cold. It also means that CO2/CO from any embers that are left in the ashes vent out with the little air that might enter cracks.

The issue with the oven is that it does not get hot enough for what we want. I had a brick walled oven in the last stove and was recommended (not from this forum) that a metal box oven would be better at getting the heat into the oven than a brick walled oven. If it were brick it would be more even in temp but I think it would be lower than what I have reached if the box stays in the same location.
Does anyone have any experience/idea of how much more the oven would heat if the gasses pass on 4 sides (both sides, top & bottom) of the metal oven box compared to only on the top and right side with the bottom and left side insulated with 2cm rock wool? There would be a 4-8cm flue gap over the oven and 11-7cm flue gap below if I move the oven.

[esbjornaneer]

I just completed reading Adventures with a horizontal feed (thread donkey32.proboards.com/thread/511/adventures-horizontal-feed) and it seems I can not make a reply there.
Peterberg sayd fairly early on in the thread (page 10 posted Sep 17, 2012 at 10:57am)that he did not like the embers left when testing a core brought to him. I am getting lits of embers and way more ashes left in the fire box after the complete burn than the video shows in his last post on page 24. Can anyone comment on why this is? Wood too damp (I am getting some bubbling out of the ends of some pieces) / Primary air inlet not being at the bottom of floor, the V (due to reusing door from previous stove) / any other reasons? I don't think it can be due to damp in the stove any longer as it is close to a month since starting to use it.

It was great to see the burn in the last post on page 24 in the thread!

[esbjornaneer]

I have now added the vital data as a spreadsheet and a jpg (for those that do not wish to open a xls) for those that are interested in the Dropbox folder mentioned in the first post.
I have deviated from the formulas on the FB dimentions and the primary air inlet. And am not sure I have understood how to measure the HR height (so don't use that formula unless you know I have got it right).

I'd love feedback if you feel it could be done better (for future (re-)builds) and if you think I will be able to add a secondary bell with maximum dimentions as visible in red (it can be made smaller).

[satamax]

Esbjornaneer, best way to cut on the ashes, make the two slants out of insulating firebricks, or even the whole firebox.

[esbjornaneer]

Jan 5, 2016 13:21:55 GMT -8 satamax said:

Esbjornaneer, best way to cut on the ashes, make the two slants out of insulating firebricks, or even the whole firebox.

Thanks for the advice Max. (I've just completed reading about your recent build.)

I doubt I will be able to get hold of insulating firebricks here in the mountains of southern Spain. I know you used AAC's (air entrained concret) blocks for your insulation around your FB. Do you think they would work as the sides of the V shape (slanting sides) of the FB? Would they need firebrick on top, how thick?

As it is I have 4cm insulation below the flat part of the floor (4cm thick refractory but non-insulating firebrick, the slanting sides are corners cut off same type brick so they are made up of brick triangles stacked next to each other) and 6 cm insulation on both sides. (Insulation is rock wool throughout the build.)

And will it make a difference with the insulation I already have?

Many thanks for suggestions and advice, Esbjorn

[Vortex]

Best place for an oven on a rocket stove seems to be above the riser. Maybe you could make another one there and keep the original for drying your wood.

[satamax]

Well, air entrained concrete in a rocket stove isn't worth much after my latest experiment. Prototyping at best. iIt doesn't hold. Tho, if you want to make temporary slants with it, just as a proof of concept, before buying real insulating firebricks. I say, why not!

[esbjornaneer]

Is it because of trying to enter an image link starting 'https' instead of 'http' that I can not get images to show?

I have now uploaded the drawings of the actual build as plan and section, with descriptions of what each part of the stove is and how the gasses flow. (In the Dropbox folder from first post.)
As mentioned in a previous post the insulation is rockwool. What the plan picture doesn't show is that the HR is insulated with 10cm rockwool on the side of it that is above the FB.

It would be great to know what people consider 'well insulated' to be? It is mentioned often but I have not actually seen reference to more than 2" of vermiculite/perlite. Have I done an overkill as rockwool has double the insulation value to vermiculite/perlite?

[esbjornaneer]

Jan 6, 2016 10:07:15 GMT -8 Vortex said:

Best place for an oven on a rocket stove seems to be above the riser. Maybe you could make another one there and keep the original for drying your wood.

Thank you Vortex. Yes that would be the best place to have it as it is the hottest, but frying needs hotter temps than oven baking so wanting to have them both I had to place the oven downstream from the hob.

I have added some ideas of alternative placements of the oven in the DB folder too. All the options still allow system CSA flue passages but in two of them it is split in two streams. How much of an issue would that be? Which of the options do you think would provide an oven with more heat (given that the flue gasses are the same temp)?

At tne moment I have a P-channel that is only metal where it goes through the FB and its roof, like the first permutation of them that Peterberg intrduced in thread Adventures Horizontal Feed. Would it be possible to have a P-channel of the second permutation, i.e. one that is a rectangular section welded into an L shape where the metal is exposed to the flue gasses that pass back over the FB for the horizontal part of the channel before it goes down throught the FB? Or would it over heat the air and cause a problem, like metal deteriorating too fast?

Many thanks again for any feedback, Esbjorn

[Vortex]

Yes, I was thinking of some kind of high temperature bypass flap to direct the hot gases to either the oven or hot-plate, depending on what you wanted. Sorry I dont understand the question about the P-channel.

[esbjornaneer]

Hello Vortex,
Peterberg shows it on page 4 (Aug 12, 2012 at 11:13pm) (of the thread in my last post) in a pic, this is the version that I have at the moment. And talks about the relationship between the temp of the air in this secondary air inlet and the suck generated by the HR.
Page 7 (Sep 8, 2012 at 4:19pm) shows the L shaped version.

I wasn't expecting you pesonally to answer the question but rather hoping for Peter himself to make a comment. I know the L shapes P-channel has been sandwitched between the FB & a 'black oven' in a Belgian/Dutch build that he talked about in Klemen's thread but the diffenence I am suggesting is that the metal would not be protected by the firebrick floor when the gasses pass over it. This is the video of the Belgian/Dutch build

Hope this explains what I mean Vortex

[peterberg]

@esbjorn,
The P-channel in that particular build doesn't need to be protected, it's air cooled. It's just a nuisance having a duct in the middle of the oven floor so I designed something around it.

[esbjornaneer]

Thank you Peter!

So I am back to the question of how much difference in heat it would make to move the oven... anyone? Even your thoughts on the subjet are welcome. Is it worth the hassle to move it? Into which position?