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David Hughes
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I'm a newbie on the forum, and to rocket stoves as well. Glenn H. addressed a question re: a formula, etc. for a stove used in an aquaponics application. It triggered a couple questions in my mind since I'm trying to get my mind around the principles of rocket stoves. Does the area around the j-tube structure (pipe with insulation making up the feed tube and riser) have to be in the neighborhood of 2", or what space does that need to be in relation to the j-tube structure? Ex., if I have a 6" OD j-tube, then 2" or 3" of vermiculite, mortar, etc. around the j-tube (insulation), could I then have more than 2" between it and the outside of the barrel or whatever I come up with to use for the unit? I'm thinking more like a conventional looking wood stove, possibly larger pipe and maybe even plate welded up in the shape of a square, with the exhaust exiting at the bottom like the rocket stove typically has. The stove would be used as a heating unit in our house. I just don't want the 'mass' look of barrel, masonry couch, etc., just a stove to heat about 1200 sq. ft.

Thanks in advance for your responses, David
 
Erik Weaver
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Location: S.W. Missouri, Zone 6B
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If I understand your question, I'd say the answer is a qualified yes.

Whether there is a limit to how large the space between the fire riser (that is what the insulated vertical element of the j-tube is normally called) and the outer barrel (or welded up plate in your example), I do not know. I suspect not within reason; but what is "reasonable" is then the question. Think in terms of bell masonry heater design. So long as the containment area of the bell is less than that which will cause all the heat to be sucked out and vapor to condensate before rising out of the heater (through the chimney, into the atmosphere), it will work (assuming a properly designed and installed chimney is in place, and other factors that constitute a good draft, pulling the exhaust out of the house).

The next question, is why?

Why would you *not* wish to benefit by having thermal storage of the heat? It is this which moderates the heat released into the room. Without substantial thermal mass, the rocket heater works a lot like a wood burning stove: hot when burning and cold when not burning. The difference is the rocket heater burns through the wood much more quickly than a wood burning stove. For many reasons this is a very good thing; however, one reason this is *not* a good thing, is unless you have thermal mass to capture and slowly release all that fire energy (heat) it is less comfortable as a heat source, and requires being burned more often, and when burning requires more attention than a common wood burning stove (Peterburg's batch box eliminates that headache, but cannot alter the physics of high heat production without thermal mass to capture and then slowly release that heat: a high rate of energy production is only half the system; capturing and slowly releasing that heat is the other half of the system).

I burned a test rocket stove/heater this past winter. I was primarily concerned with testing the basic build and operation (how I feel living with my dragon, as it were). Since I also built this on my carpeted living room floor, above a basement which I did not add support joist under, I did *not* add thermal mass (too heavy). The result is the barrel got the place pretty darn warm, and even hot at times (roughly a 1350 sq.ft. upstairs and 1200 or so in the basement; it did not heat the basement at all, so far as I could tell: heat rises, after all). It was nice when burning, in terms of heat output. And the things in the living room warmed to some degree, I'm sure, but nothing like a proper thermal mass would. As a result, temperatures began to drop as soon as the fire stopped burning. So I would run the temps up to 80 degrees F or more, and then cover up under my electric blanket whilst temps fell back to 50 F or so, and then either go to bed or start the heat/cooling cycle over again.

I still saved a lot of propane. So it was worth the trouble, in terms of money saved. I may have even paid for the supplies I bought to build the prototype (I'd have to check my records to be certain of that; if I didn't pay for them outright, I paid for a lot of the materials in saved propane costs). But I also learned a number of other things. Namely, for *me* the batch box is a better design, because I'd rather read a book than get up every 5 or 10 minutes to tend the j-style feed. And I really, really do need to add thermal mass; for me that has always been the plan, and this years build will accommodate that; but had I been doubtful of the benefit of thermal mass to capture that heat and slowly release it into the room over a period of hours, instead of minutes, I'm convinced I'd consider the thermal mass to be a good investment, both in efficiency and comfort.

There are all kinds of ways of making thermal mass look nice. I would not give up on that easily (in fact, I would not give up on that at all). I certainly recommend planning on adding thermal mass, be that bells, benches, or a design that has the appearance of a more traditional masonry heater. There are many beautiful designs, so it is hard for me to imagine not finding something you would enjoy living with.

That's my 2-cents. Add a couple buck and you can buy a cup of coffee
 
Judi Anne
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Do a simple image search for masonry stoves. Not all, but some designs can have all or most of the benefits of a rocket with a much more "generally accepted" appearance. The exteriors can be finished in a variety of ways. "Finnish masonry stove" images in particular have a very sleek modern appearance and a friend has hers tiled over that fits right into a "Mediterranean decor".
You certainly aren't the first to object to the "barrel and bench" appearance of many rocket builds.

This thread might be useful to you. http://www.permies.com/t/32821/rocket-stoves/Rocket-Mass-Heaters-Scandinavian-Slavic
 
David Hughes
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Thanks so much Eric and Judi Anne!! I understand the principle of needing mass with higher temps generated with the rocket stove vs. the conventional wood stove now. I'd mistakened the burn time, thinking that smaller amounts of fuel (smaller portions of wood) could possibly last as long as 3 or 4 logs in the conventional wood stove...Wrong!!! Wishful thinking I suppose. So, I get it better now and with Judi Anne's help with other design models I think I can proceed with incorporating mass with a design that better fits my decor. Thanks again!!!
 
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