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Ken Carman

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since Oct 10, 2013
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Recent posts by Ken Carman

Thanks for your quick response, but I am a bit confused about the answer. I read the post and your anser, and it sounded like they were having a very similar problem, and you addressed the vertical exhaust chimney.
A. I'm confused how that eliminates #5 (10" heat exchange run) as the problem.

B. Does it matter how big any cross sections are past the burn tunnel & chimney, as long as they aren't smaller than the burn tunnel cross section? We were thinking that maybe we have too much space above and/or around the barrel . . . or in the larger "cavern" where the cleanout is, just before entering the 10" run. Can those cross sections be too big, or do they make no difference?

C. Is our only problem our vertical final chimney?

5 years ago
Well . . . been having problems. We get fairly decent draw at first, but as it heats up, the draw diminishes, and fuel starts burning upward as much as down the burn tunnel.
Here's what we have; fuel chamber 7" X 9", burn tunnel 7 X 7 1/2 (52.5sq")and 15" long, chimney 7 1/2 X 7 1/2 and about 38" high above the roof of the burn tunnel. {All fire brick, as suggested}
We have about 4" gap above chimney to barrel, and at least 3" (average) gap around and between barrel and outside of chimney. Perlite/slip mix makes the square chimney round, but very thin at corners (about 2" thick in centers). The barrel is in fact off center, pushed toward the exhaust side, with the opposite side resting against the perlite so that all air is flowing down one side which has a gap entering our cleanout of about 70 sq ". This cleanout area is probably over 100sq" at it's widest spot. Our primary heat exchanger is a 3/8" thich 10" pipe that runs 40", then a 90 elbow, then about 20' more before transitioning into a temporary cinderblock chimney that is 7 blocks high.

So, why has our draft been so weak?

1. Is our feed/burn chamber too big?
2. Is our tunnel too long at 15"?
3. Do we have too much space between the top of chimney and the barrel? (at least 4")
4. Is our cleanout chamber too big? (at least double the cross section of burn tunnel)
5. Is our primary heat exchange pipe too big ? ( at 10" diameter)
6. Is our temporary chimney inadequate?
7. Is our fuel inadequate? {we have been burning split up pine pallet pieces, wanting too save our oak for needed use}

BTW, yes, we did purchase the pdf book.
5 years ago
We are working through our list of available materials (without having to purchase). We would now like to use 10" ID X 1/4" steel pipe for our exhaust run. Would this cause a significant problem if we build our "J" burn chamber entirely out of firebrick (including internal chimney), which would give us about 6 1/2" X 6 1/2" square inside the "J" In other words, what happens if our exhaust run is bigger than the run inside our outer barrel? Would it help if we condensed back to 6 1/2' square before heading up our exterior final chimney? Would we be better off using the 10" metal pipe for our internal chimney too?

How important is it to utilize stone in our bench for heat storage? We have unlimited sand (and clay), but would have to locate stone (as we live near the ocean with no native stone).
6 years ago
Great info! I appreciate this very much. I am, however, not sure that I clearly understand all that you wrote about the cinder block portion. It seems that you are saying that we need to use fire brick for the first six feet of exhaust leaving the bottom of the outer barrel, and the rest could be cinder block, right?
Are you saying then that we should only utilize a straight run if using the block? The only reason I was going to do the 180 was because of all I had read about the heat going through too quickly, and not enough being absorbed. What if we had it only run a few feet, and then turn back? We can do that, or run the full 17' or so with no turn. What do you recommend?

Oh, and I'm confused about how making the bench high(mass greater) will save my back. I understand what you said about the 33 not radiating as much heat as a 55, but didn't get the connect to bench with greater mass. Wouldn't a portion of the success of the draw have to do with the chimney outside?

Again, many thanks!
6 years ago
Thanks for your reply, but yes, we are wanting to do more than keep things from freezing which is what we did last year. I did store a lot of water under the pallet tables I set up, and in fact added 130 degree water from a 55 gal buried in a hot compost pile, but this year we want our tropical plants to do more than just survive the winter.

So, back to my questions about the cinder block, straw, and drying the bench.
6 years ago
We are making a rocket mass heater for our small (10X20 ft) greenhouse. Our plan is to have the exhaust flow through one side of a row of cinder blocks before turning 180 (using bricks) and flowing back the other side before exiting to an outside chimney.

My first question is how well would the cinder blocks hold up as our exhaust chamber when covered in our sand/clay mixture? We are building our burn chamber and riser chimney out of used fire bricks which will then be covered by a 33 gal steel drum. We plan on burning as small of fires as possible, as we are in the south and only using this as supplemental heat to keep temps a little warmer than they would stay overnight otherwise.

Second question: We will be using the mass heater as a bench to hold plants. Do we want straw in our mix? Do we need it? Our bench (covering the cinder block exhaust channel) will be around 3' wide, and about 18" to 24" high. We will be adding 4 part sand to 1 part sticky clay.

Can we dry the bench out by building very small fires?
6 years ago