Earth oven and rocket stove mashup?

I am very pleased with the simple wood-fired earth oven that I built some 20 years ago. However, the labor required to keep the floor hot has reduced my use of the oven: I must rake the banked fire back onto the oven floor to keep the heat. Shoveling coals back into the oven to reheat the adobe mass is a common practice when using hornos or earth ovens. Once the hot coals have reheated the oven floor, the oven must be swept to prevent ash from contaminating the food.

Today I read Benjamin Dinkel’s process of converting a BBQ to a rocket bread oven in this thread:
https://permies.com/t/257726/rocket-ovens/Rocket-bread-oven-existing-BBQ
What I like about his BBQ conversion is the potential for conductive heat at the base of the oven so that he can make pizza or flatbread for a crowd: the floor of the oven stays hot by feeding the rocket stove below the chamber: no need to bank a fire, rake or shovel back the coals then mop the floor.

Instead of Benjamin’s repurposed steel oven housing, the oven chamber here is an adobe dome atop a floor of fire brick. Below the firebrick is an insulating layer of perlite. Below that is a 4’ x 4’ square block of solid adobe mass. The perlite thermal break was designed to keep the cold adobe pedestal from cooling off the firebrick floor. While the perlite insulation works as intended, the firebrick does not retain enough heat for extended cooking.

I have 3 questions for the experts:
Would an adobe rocket stove carved into the adobe pedestal accomplish the goal of sending heat up through a 3” layer of adobe, a 3” layer of perlite, then 3 inch thick firebrick to keep the oven floor hot?
The adobe oven vents through the oven door. Could a rocket stove similarly vent through a front, rather than a rear or vertical flue? (My oven is built against an adobe wall)
Is there an existing example of a wood-fired oven that is heated from both an in-chamber fire and a rocket stove below that I could simulate?

Thanks for considering this possible labor-saving modification to the earth oven!

Hi Amy,

I'm glad you liked my post.

I think heating through 2 layers of brick and perlite sounds pretty hard. Is there a way you could get the hot gases to go underneath the firebrick? Take out the perlite and let the hot gases go through under the firebrick and then exit somewhere inside the adobe dome? If it vents through the top front, probably best in the rear somewhere? You could save yourself all the woodfire inside the dome this way probably.

I'm working on a design with an adobe dome for my next pizza oven. So I'm definitely interested in what you are doing.

Do you have a photo of your oven?

Really happy to hear that you are making an adobe oven Benjamin. Using the materials that are available from the land is very satisfying. I followed the 1st edition of Build Your Own Earth Oven by Kiko Denzer and found the book amazing and easy to follow. I subsequently bought the 2nd and 3rd editions. The 3rd is excellent with new information that I wish I would have used in my original oven. Here is another builder recommendation for the 3rd edition: https://permies.com/t/223765/Build-Earth-Oven-Kiko-Denzer
You asked,

Is there a way you could get the hot gases to go underneath the firebrick? Take out the perlite and let the hot gases go through under the firebrick and then exit somewhere inside the adobe dome?

Yes, that is exactly what I need to do. The only problem is that the center of the floor might collapse. The bricks might be set tightly enough so they will enable a modification. I could try temporarily "gluing" the brick grid together with wood and flour paste, which would burn off when I complete the modification directly under the bricks. I'm willing to take a chance on the floor becoming unstable.
By following the ratio for oven door height relative to dome height, the gasses exit the front door perfectly. This led me to think that, by removing a brick from the back of the oven floor, the exhaust from the rocket stove below could enter the adobe dome, rise and heat the dome, create the important convection to heat the dome then (like the smoke from the fire inside the dome) exit the door of the dome.
I will take some pictures of the oven and the door exhaust in a few days so the possibilities become clear.
Thank you for your interest!

I built a rocket fired cob oven about eight or nine years ago as my first actual rocket project. I followed standard cob oven design, with an L-tube rocket below and the riser exit spreading under the floor to five vents around the edges of the floor. I had a heavy mass of bricks spanning channels, topped by a thin layer of sand and the firebrick oven floor. It bakes well, but the floor takes hours to come up to temperature; we generally bake on metal racks set an inch off the floor. If building it again, I would arrange it so that the fire channels were directly below the floor firebricks so that only one layer of brick needed to be heated from below. Rows or pillars of one brick wide set on edge to support the floor bricks would be the way to go. I would suggest just taking out the floor bricks that do not extend under the dome walls and building your rocket firebox with horizontal flue channels as described, leading to maybe three vents around the edges of the floor.

Some pictures of my oven...

What a wonderful project Glenn! Thank you for explaining the build, sharing photos of each step and how you would make adjustments. Your experience gives me courage to take the leap and actually make some modifications to my oven.
Your suggestion:

If building it again, I would arrange it so that the fire channels were directly below the floor firebricks so that only one layer of brick needed to be heated from below. Rows or pillars of one brick wide set on edge to support the floor bricks would be the way to go. I would suggest just taking out the floor bricks that do not extend under the dome walls and building your rocket firebox with horizontal flue channels as described, leading to maybe three vents around the edges of the floor

I can visualize everything except the vents. The oven floor is 24" diameter. Would this mean cutting an opening the width of the flue channels of 3 rear bricks (opposite the door) running above the flue channels? Would you be willing to share the diameter of your floor and the corresponding size of the 5 vents in your oven? Maybe the total vent area would help me come up with the other variables for the (7"?) L-tube.
Thank you for your thoughts here Glenn!

Glenn Herbert wrote:If building it again, I would arrange it so that the fire channels were directly below the floor firebricks so that only one layer of brick needed to be heated from below. Rows or pillars of one brick wide set on edge to support the floor bricks would be the way to go.

I've seen people building with glass bottles under the floor bricks supposedly as "insulation". If one were to cut off the tops and bottoms of the bottles, could they support the floor bricks while allowing the heat to move under the floor and not melt over time? Or have worse happen to them?

That's a really interesting idea Jay! You really got me thinking in a new way. I appreciate your input.

After reading The Rocket Mass Heater Builder’s Guide, it seems that this particular rocket stove / earth oven mash-up is not possible due to the limited riser height. The “deal breaker” for a rocket stove under my existing earth oven arises from the following limitation:
- The adobe pedestal sits on a concrete slab.
- The distance from the slab to the bottom of the firebrick floor is 28.5”
- Subtract 6” for a reasonable burn tunnel, and 2.5” clearance at the top of the riser for the heat channels, (28.5” minus 8.5”) the maximum possible riser height measured from top of burn tunnel is 20” (recommended height is 48”).

I could try an approach that is flatter than a rocket stove like the Patsari. Unfortunately, the heat will probably not get hot enough to accomplish the objective of generating significant oven heat from below the firebrick floor.

Thanks to Benjamin, Glenn and Jay for your suggestions. For now, it's back to the drawing board!

Hi Amy.
I think it could still be possible. First thing is that you don’t need to subtract the height of the burn tunnel from the height of the riser. So you would still have 26”.
The important thing is that the feed tube is not longer than 1/3 of that, which would limit the height to about 8”.
With a short riser the burn might not be 100% efficient and maybe flames would come out too sometimes. But in your case I don’t think that would be a problem.
I’ll dry stack some firebrick when I get home
from my vacation and see how your measurements work.

I would advise an L-tube rather than a J-tube for this application. That is what I did, granted with more riser height available. Make the firebox 30" deep and you should have enough flame length for good combustion.

My core has a 6" x 9" x 30" firebox, a 6" diameter riser of around 30" from the floor, and a web of horizontal channels above that. Since there will be around 12" of horizontal channels before opening to the oven space, I think you will have enough flame path. By the way, my oven floor is six firebricks wide, so a bit less than 27" inside diameter. A 24" oven should be easily heated by a 6" L-tube with dimensions like mine.

Well let's keep going!
Benjamin, my understanding from watching "Dimensions of stove by James Fox on https://permies.com/t/226771/messing-vermiculite, the 4th video, is that the riser measurement is taken from the top of the burn tunnel. Take a look and let me know if that is how you hear the riser length definition. Very nice series of videos for my learning!
Glenn, can the burn tunnel be any shorter than 30"? The length from center of the 24" circle plus the oven doorway and a 3" threshold shelf is only 22". Ideally, I'd like the length of the burn tunnel to be 21" and at most 24". Will a shorter length be effective?
I also want to add another detail about the purpose of the L tube: it is an adjunctive heating approach. I will continue to light the initial fire in the oven which heats up to 900 degrees F in one hour. The rocket will enable continuous cooking by keeping the floor hot for extended flatbread / pizza baking without redistributing embers across the floor then banking them and mopping the floor.
This point is important because it allows lower performance for the backup floor heat. All the rocket has to do is maintain the oven temp.
My next experiment was going to be placing a firebrick 2" above a fire to see if the flame actually heats the upper surface of the brick. This should tell me if the underfloor rocket will actually maintain the oven temperature.
Thank you both for your encouragement!

The upper half of my bell is firebrick, and one section that will be against the chimney is just a single layer now exposed. Within an hour of starting a fire, this surface gets too hot to touch, while exposed to room air and 2' or so away from the top of the riser. Your floor will get plenty hot, not for fast pizza cooking, but for supplementing and steadying internal temperature.

There is no reason the riser needs to be centered under the oven floor. It can be toward the back a bit as long as the spreading channels give good coverage toward the front.