If you are burning wood then it’s simple to heat water. Only reason not to may be aesthetics. Otherwise it’s a lot of available BTU’s dying to do some useful work and just going up the flue instead. Right now I’m researching these cheap diesel space heaters and wondering how much water you could heat bleeding the heat off the exhaust pipe.
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90 degrees F? That surprises me. I thought it would be much higher. Where did you get that figure?
Don M
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The key is in the bell and laminair flow/ layers. Rindert is right.
Edit
95 is very low but not impossible. Draft is formed in the riser, so cooling of the gases can reach a very low temp without effecting the burn.
Rindert, if you want to go that low, count at least one m2/kW in the bell. Numbers for the riser are available.
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I think that’s for the final flue temperature for a Rocket Mass Heater, with most of the heat absorbed by the stove’s mass, usually cob (clay and hay, or similar). Unless you’re happy with tepid water, 90F isn’t going to happen with a water heater 
Even in a Rocket Mass Heater, a cool flue temperature like that is iffy. You wind up relying on the heat riser for all of your draft. Folks sometimes need an auxiliary fire at the base of the stack, or forced draft of some sort to get things lighted and going.
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m2/kW? I’m not sure what that is. I think you want a well sealed bell. If the rest of the system has a little pressure loss, you might have positive pressure in the bell, compared to the room.
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10,8 sqft/ kW for heat exchange area. Hot gasses need surface and time to cool of. It can be water, cob, air whatever.
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Ahh, heat exchange area; obvious once you know the answer
Thanks!
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Sorry. 1 m2/10 kW or 1 sqft/ kW. To late. I should be sleeping
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A very useful number, and one I can probably remember
It’s mid afternoon here, I have no excuse
The reason I’m cautious about low flue temperature:
I came across this neat idea for heating a tent:
https://www.notechmagazine.com/2014/03/crimean-ovens.html
I built a scaled down (bad idea) version in our back yard. For the fire box, I used an insulated J-tube rocket stove, with the burn tunnel feeding the brick-lined trench. No riser (also bad idea). I had about a 1 meter stove pipe at the end of the 1.5 meter trench. I never did get it to draw very much at all. Cool, heat absorbing, horizontal ducts are much harder to get to draft than I would have guessed.
Edit: If I were to try it again, instead of the trench, which was about 6 inches (no charge for mixed units ), I would use a shorter, wider, and much higher “trench” which would work like a heat bell. I think this would be easier to start up, and could warm up slowly without killing the draft. Also lower pressure drop.
I’m partial to bells.
Probably because I’m a little dingy.
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This is all correct in my experience. The math is outside my wheelhouse but I do run a RMH. A regular one that burns sticks, not batch loads. Built with a ceramic fiber lining in the heat riser pipe and a 55 gallon drum for the bell. 30 foot run of six inch duct buried in a 24 inch high 32 inch wide cob filled bed. The duct turns and comes back to the bell area to exit. My exit flue stays between 76 and 86 degrees but mostly in the 70’s. With that long of a duct run I should have used 8 inch duct in the bed because the flow gets real lazy at time so I run a booster fan in the flue. No way to change one of these systems once they are built. This is in my greenhouse and I originally considered coiling copper tubing inside the barrel to heat water for a large storage tank I have in there but built a separate heater for that task. I guess since there is some interest in this I will do a video tour and explanation of how it all works. Decided not to do anything in the GH right away so none of the systems are running right now. Too little energy for the job of keeping the place warm through the next couple of months. I’ll start heating it in March to move my next years seedlings into.
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Right. But I think if I put a spiral baffle inside the bell it should set up an efficient counter current exchange situation. I didn’t draw the baffle in before, not sure why.
Rindert
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Flue temperature slightly above 90F
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The conventional wisdom, if there is such a thing for Rocket Anythings, is that the cross sectional area of the system should be about the same all the way through. If you design for that with the heat riser and the area between it and the bell, you would need a very large bell to keep the cross section the same in the spiral. I think.
I don’t know how critical this is, but it may be important if this is your only, or main, source of draft.
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You bring up a very good concern; flow restriction.
I have dissected two of these and I have a third one that heats the water for my house.
The one for my house has an 18in diameter tank and a three inch center flu. It’s probably a 1995 model or there abouts. It works well, so I am basing a lot of my decisions on it.
So then I would make the heat riser 3in, with 1in of insulation all around it. Then I would need a 3in space all around the insulation where the baffle will be. If you add all this up you get an 11in barrel. If the square space, that the combustion gasses see as they travel down past the baffel, is 3x3in and the barrel is 28 inches tall you get approximately 9 turns in the spiral. So then if we approximate the barrel diameter at 1ft and pi at 3 we get a path length of 1x3x9 or ~27ft. That 27ft is not so much different than the 20ft of ductwork in a cob bench. A 3in riser in an actual rocket stove would be too restrictive to allow turbulent flow. But I plan to burn natural gas, propane, or hopefully stored woodgas,.not wood, so think turbulence is not necessary. Laminar flow works for my existing water heater, so I think it will work for what I propose.
That 27 foot dimension may need to be reduced by redoing the spiral. No big deal. At this point I feel like all these numbers are not too far outside what is tried and true. At this point I feel like this will work. I’m going to make a cardboard model.
Rindert
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Here’s a wild idea. The heat riser and bell provide a good, hot combustion environment and draft. They also form a counter-flow heat exchanger. Flame goes up, hot gases go down. It’s sort of a coaxial, upside down U-tube. Suppose you build a heat riser, really well insulated, with an insulated pair of elbows to connect to what was the flue outlet of one of your water heaters. You have the same counter-flow (kind-of), a long burn tube, and you can keep the twisty plate inside the water heater. The cooler exhaust now is at the bottom of the water heater. You would probably want an exhaust stack to help with the draft. It may not work quite as well, and it might be hard starting the draft, but it would be a lot easier to fabricate.
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If you are finding this discussion hard to follow then you can get a lot of additional information about the subject from this site.
I built mine after reading this book
rocket mass heater book - Google Search.
The authors are part of the permies community and all the formulas used in the book are somewhere in the permies site.
If I were to build another heater, and this is somewhat different from what Rindert and Kent are talking about, I would go with this design.
Batchrocket.eu - Introduction
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Yup, I don’t know if anyone ever built one, but it has at least been thought of.
Rindert
BTW. The ‘twisty plate’, as you call it, is usually called a spinner. Where my thinking about spiral baffles, and spiral path heat exchangers started years ago.
LINK (Rocket-Waterheater very simple to build (rocket stoves forum at permies))![[Thumbnail for rocket-water-heater2.jpg]](https://www.permies.com/t/39135/a/20598/rocket-water-heater2.jpg)
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Yes, that’s what I had in mind. Those stovepipes at the top could sure use some insulation.
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Looks like we’re not the only ones concerned about water heaters.
Rindert
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