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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My water heater’s exhaust regularly measures 420F. I installed a grill thermometer in the flu pipe. It just keeps me motivated to keep working on this.
Rindert
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Rindert, 420 F is more then 200 C! Way to much for a gas powered heater. Biden is right! If those numbers are common in the US, government should take action. The further under 100 C the better. I tried to keep it at 60 C, a lot of condens anf efficiency above 100 %.
My woodgas boiler needs 420 F to function ok and that hurts to leave it to the birds.
And if you are investing in a new boiler , go for instantanious condensing type.
Edit, you need low temp process water of course to get the boiler condensing. Sanitair water under 140 F and floor and/or wall heating 85 F.
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Part of that is they need heat to make it up the exhaust pipe so CO gets pulled out, but that seems too hot.
They also make the signature 900 series now which is supposed to be 30% more efficient. It is called the condensing type. I don’t think it eliminates the exhaust flu like the furnaces do though.
You can switch to the tankless system as well.
If you want to switch to electric you can get the heat exchanger type which might be cheaper to operate then propane if that is what you have. The real bonus to that is you don’t have the exhaust vent that sucks heat out of your house.
There are tax rebates for the super high efficiency ones, but I don’t know if they make up the cost difference with your usage pattern at this point.
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I recently replaced an old gas waterheater and gas dryer. I purchased a electric heat pump to replace both. As a single person i am saving about $100 a month. I am pretty sure the old water heater was horrible on gas consumption. That unit was probably 40 years old had the burner replaced 2 times iirc.
The dryer take something on the order of 1.6kw to dry a load but it is slow a little over 2 hours.
I don’t remember the actual power consumption of the water heater. They where definitely more expensive to buy but i am happy with my lower monthy bills.
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I looked at water heaters at Home Depot not long ago. You can still buy one, like what I have, for $600.
The problem with the high efficiency ones is they are expensive and require a lot of expensive maintenance. Contractors will tell you to go with the old ‘B-Vent’ type.
I don’t think it needs to be that way. I think anyone with reasonable welding skills can make something better.
Rindert
This is a full scale layout. I have not yet built one. Ask me any questions.
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