Rocket cob oven

[mark203]

Hey guy's,

I just wanted to share with you an oven that I'd built over the summer. It is a cob oven heated from the exhaust of a rocket stove that is built into the foundation. The pipe of the rocket stove feeds into the wall of the oven and on the opposing side is the chimney which is about an inch above the floor. The foundation is made of salvaged concrete from a demolished parking garage of which I cut into blocks(I do NOT recommend this).

Unfortunately heating the oven exclusively with the rocket did not meet my expectations. I then discovered an alternative way to fire the oven which I'm very pleased with.

I first load the oven with wood and shut the door. I then begin a fire in the rocket stove which heats the wood inside the oven. The heat from the rocket alone will ignite the wood inside the oven.The key to making this work is by keeping both fires going simultaneously The reason for this is because the fire inside the oven is receiving air from the exhaust of the rocket. The rocket is preheating the air before it reaches the oven fire, so there is no cool air what so ever. It's a double whammy!

After a minute has passed from lighting the rocket there is no visible smoke until the internal fire starts. This does not last long however. The smoke cleans up pretty quick but I haven't timed that part yet.

I have exceeded 900 degrees with this technique in under an hour however the oven was still warm from a day or two before. I think the floor was 150 degrees from the start and the upper dome was over 200. I'll report back with my findings when starting with a cold oven.

In conclusion, I do recommend incorporating rocket stove technology into your future WFO designs. It has proven to be very efficient with the method described above. Experiment!

[LYMENH]

Re: Rocket cob oven

MARK203,
I've been noodling over a design for a rocket fired bake oven, and am on my way to starting one. My thought was to make an inlet AND an outlet on the back of the oven floor. Then fire the rocket stove into the oven, with the front opening closed, so that the exhaust exits the floor vent. Any ideas why the rocket didn't heat the oven up well?
Doug

[shuboyje]

Re: Rocket cob oven

Rocket stoves are about efficient combustion. Pizza ovens are about brute force. Burning only the tip of the wood like a rocket stove does to get it's efficiency you would need a giant stove to put out the same heat as a standard sized fire in a wood fired oven. That is part of the reason the oven didn't get hot.

[LYMENH]

Re: Rocket cob oven

That makes sense, shuboyje. But I think I'm still going to try it. At the worst, I'll patch the rocket stove holes in the floor, and have a regular bake oven on my hands. I'll let you all know how it turns out.

[jislizard]

Re: Rocket cob oven

I am interested in building a rocket stove BBQ, the Pizza oven doesn't arrive until they restock so I have time to plan.

Most of the You Tube vids had flames shooting out of the top so I am not sure if it will work, maybe I will just make a longer chimney as I don't want everything flame grilled/burned to a crisp.

I have read that the combustion is so complete that you basically get CO2 and steam (I will ensure that I pay a bit extra Carbon Tax to offset the extra damage I am doing to the environment).

Does the rocket stove still impart the flavour of a traditional BBQ? It looks like most of the cooking is done in pots and I guess it is a stove and not a BBQ.

Cheers

[mark203]

Re: Rocket cob oven

shuboyje is correct. There is not enough fuel burning simultaneously using the rocket. I believe that you can still get the temperature pretty hot but it would take a lot longer. If I was to build an oven using a rocket again I would use less mass and cook over a live fire.

I too have been thinking about a rocket bbq. I'm just going to cut a hole in the bottom of a weber and insert the exhaust. Quick and easy.

-Mark

[jislizard]

Re: Rocket cob oven

I too have been thinking about a rocket bbq. I'm just going to cut a hole in the bottom of a weber and insert the exhaust. Quick and easy.

-Mark

Would you be using any fire bricks in your weber to make the heat more even and longer lasting?

I usually cook over embers which it doesn't look like you could do with a rocket stove, looks to be flames all the way.

With firebricks I am hoping to even out any hot or cold spots, spread the cooking area over a larger surface and also keep the BBQ hot for longer, but being a newbie I am not sure if that is how fire bricks work.

I am also confused as to why you pack the chimney with vermiculite or perlite for insulation rather than just having an enclosed air space around the fire box and chimney. It seems that perlite and vermicultie are insulators because of their air content so I would have thought that air would be better, as long as it was protected from the wind (to prevent cooling on the surface)

MArk

[LYMENH]

Re: Rocket cob oven

Mark,
Air is a very good insulator, however big areas of air set up convection currents, and still transfer heat. So you want little pockets of air. An enclosed air space around the chimney would be better than just a single wall, but perlite and vermiculite are just that much better.
Doug

[jislizard]

Re: Rocket cob oven

Thanks for that, I use perlite and vermiculite in my garden soils so I have a bit around, makes sense about the convection currents, I seem to have forgotten basic physics along with everything else since I left school.

[Gulf]

Re: Rocket cob oven

Thanks for that, I use perlite and vermiculite in my garden soils so I have a bit around, makes sense about the convection currents, I seem to have forgotten basic physics along with everything else since I left school.

There are different grades of vermiculite, which I found out recently. The courser, (larger the granules), the better. I bought some horticulture grade vermiculite here in the states which is very fine and not as good an insulator.

[david s]

Re: Rocket cob oven

What makes you say that finer grade vermiculite is a poorer insulator? They all seem to be the same density to me. If you pick up a bag of fine stuff it weighs the same as a bag of the course stuff.Although the spaces between the grains are smaller for the finer stuff their total volume will be the same. A bucket full of large marbles will weigh the same as a bucket full of small marbles. This would indicate that there would be no difference in the insulation value between small and large grain size. Correct me if I'm wrong. Is there any data on this?

[jislizard]

Re: Rocket cob oven

Interesting. Small marbles would have smaller gaps between them, glass is denser than air. A volume of small marbles would be denser than the same volume of large marbles.

I am thinking of a bucket of sand and a bucket of bowling balls rather than marbles to illustrate the size difference. You can get more grains of sand in a bucket and less air, whereas a bucket of bowling balls will have maybe two balls in it and loads of air.

However, marbles don't contain air in them, whereas vermiculite does.

Anyway, I am most likely to use perlite. Rocks that float on water are hard to beat, in any situation!

Unless I am mistaken, I forgot about the existance of convection currents a moment ago.

Edit: of course the proportions would be the same, with the same number of contact points around the spheres regardless of the size of the sphere. If the spheres are all the same size. If they are varying sizes then there will be some differences and also where the marbles come into contact with the sides of the containers, smaller marbles will have smaller air gaps between their surface and that of the bucket, whereas larger spheres will have large gaps around the surface of the bucket.

Also smaller particles can be compacted driving out more air space. Not marbles though. I take it that the insulation material is not going to be compacted but loose filled.

I was going to mix up some concrete and perlite but it might be better just to use loose perlite as it need not have any structural purpose.

[david s]

Re: Rocket cob oven

Perlite is a better insulator than vermiculite, but as soon as you add cement to it there is not much difference. Use it loose if you can because it will insulate better and you won't have the problem of removing the water.
A 10;1 vermicrete mix has about the same mass of cement in it as mass of vermiculite. and for every 10 L of vermiculite you add about 3 L water.
I've also found (and I can't explain why) that a 50/50 mix of perlite and vermiculite mixes up with cement and water way better than either of them alone. It might be like mixing alcoholic drinks where 1+1=3

[mcdowellramage]

Re: Rocket cob oven

Mark203
Could you describe your rocket stove, perhaps a diagram. Once your oven comes to temperature do you keep the rocket burning to help sustain the temp?
rob

[Gulf]

Re: Rocket cob oven

What makes you say that finer grade vermiculite is a poorer insulator? They all seem to be the same density to me. If you pick up a bag of fine stuff it weighs the same as a bag of the course stuff.Although the spaces between the grains are smaller for the finer stuff their total volume will be the same. A bucket full of large marbles will weigh the same as a bucket full of small marbles. This would indicate that there would be no difference in the insulation value between small and large grain size. Correct me if I'm wrong. Is there any data on this?

I am a newbie to the whole concept of pizza ovens and vermiculite as an insulator. Very sorry if my post has mislead anyone about insulation efficiency. But, my searching this sight and trying to find an ecomomical way to build my oven has came across many post's that recomend course vermiculite over fine vermiculite.