I'm planning on building a smallish rocket-fired oven (for pizza and bread). I've seen a fair number of posts and images on the subject, but most of them look like they'd do a good job of heating up the dome of the oven, but maybe not such a good job of heating up the hearth/floor.
There's a thread over on the donkey forums that shows an oven with channels under the floor that channel the exhaust from the heat riser into the oven (though it's not really clear where the exhaust goes after that—out the open door?). I'd guess that these channels would heat the floor up nicely.
Extending this concept a little further, I thought what would happen if the heat riser was offset relative to the oven so that these channels passed all the way across under the floor? Then the hot gases would enter the oven, flow across the inside of the dome, then exit through some sort of slot or holes at the opposite side.
Something like the attached image (which is looking from the front of the oven, with the feed tube and exhaust both at the side).
What do you think? Could this work? Or would something simpler work just as well (or better!)?
Mine might be one of the ones you saw, so I can tell you that underfloor ducting like you show can heat the floor well.
http://donkey32.proboards.com/thread/1301/all-adobe-mud-cookstoves-ovens?page=3 I used a double layer of brick as well as an inch of cob/sand to even out the heat as well as possible. The heat shoots up the side & back walls, across the roof, and out the door to the chimney. I haven't been able to bake in it yet - this winter came on hard and fast, and has not let up at all, so the mostly-finished oven is under wraps until thaw.
I would advise fewer, larger ducts with a good 5-6" of cob/brick to let the heat spread and soak into the floor mass. That is if you want to use it like a traditional oven, heating up and then coasting. I have read about rocket ovens continuously fired and relatively low mass.
For my flue gas exit, I built a chimneyed double-doorway arrangement like Ernie Wisner shows in a video. It will not likely reburn in the chimney like his; the combustion should be relatively complete in the riser and underfloor ducts.
Thanks for that! Actually, yours wasn't the one I saw, but it looks like they both work the same way. That's a really beautiful job you've done there with the stonework. We have lots of local basalt around here, so I'm inspired to do something similar.
Is the floor insulated? How did you incorporate insulation between and below the ducts? I'm guessing that the most effective strategy would be to insulate below the ducts (and maybe in between them too), so that you won't lose heat from the floor into the material below. But also not to insulate above the ducts to maximize the amount of heat transferred into the floor. Does this sound about right?
I'll be interested to hear how yours works when spring comes. I hope you'll post some photos here
My "insulation" around the combustion zone and ducts is a layer of glass bottles with strawcob packed around them. The ducts are not insulated from the floor, but separated by 2 to 4" of cob, and a layer of sand and firebrick. This should make a significant mass to hold even heat. I plan to brick up the firebox opening after heating, for standard cob oven use.
I will certainly post pics when I have test results to share.
I lost track of this thread, and have used the oven a number of times now over the past two summers. It works well, though not getting as hot as I had hoped - 400-500F, fine for baking but low for pizza. The thick floor mass between the underfloor ducts and baking surface may have been a factor. For baking while firing, a thinner floor mass would probably let it get hotter faster.
On reading various discussions, I went with firing continuously while baking, which works well and uses very little wood. There is zero smoke after the first five minutes of starting the fire.
I have posted a couple of photos elsewhere on Permies.
He was expelled for perverse baking experiments. This tiny ad is a model student:
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