Thanks so much for your time Allen,
I know that my post may look as if it came form someone self obsessed and not prepared to listen to reason or experience but that couldn't be further from the truth.
In fact it exists because I know I cant possibly have got this right with my first conceptualisation, what I don't know is how wrong I am yet.
This is one of those times when feeling a little deflated is actually refreshing ... All be it humbling at the same time.
It isn't about space, it is about hot water. We will be off grid with very limited power and being in the UK very limited sunshine for the most part.
I absolutely could have a stone / cob mass vertically or horizontally in the immediate vicinity of the core in fact that was my first thought.
However once I started thinking about how to add in a heat exchanger it occurred to me that, provided I could get it to work safely, placing thermal storage tanks in the same space that the stone mass and ducts would need to occupy would actually store more heat. E & E say in one of their vids or pod casts that water is the best heat store, just before going on to say you shouldn't mess with it unless you are aware of the risks and can design them out.
"Understand just applying high temperature ....... "
Alas no, I fear that enthusiasm got the better of me there.
I appreciate the need for the insulated riser but hadn't given much thought to how that was working as a 'pump' - my assumption not a fact.
I looked at the stove concept and simply saw a thermal pump but when I looked at the RMH concept I failed to appreciate that the very same pump was being augmented with a second stage, the cooling gasses moving downwards.
My conceptual sketch shows clearly that I failed to consider the system as a whole when thinking about the mechanism for driving the air flow. I am suggesting cooling gasses that are moving upwards which is not going to work unless the riser is capable of creating a great deal of pressure differential.
Is that something that has been measured or calculated ? How much differential is created in the riser and how much in the barrel in a conventional system?
I am not too discouraged as placing the heat exchangers in a section of downdraught is actually a far better plan and will produce the counter flow I need without reversing the water flow through multiple exchangers.
A point well made but also one I have considered extensively. Since I want, in fact need, to ventilate when the heater is not running an HRV is almost a given.
The house is an old stone build and moisture would quickly make the space uninhabitable without significant ventilation.
The HRV will work with forced extract as opposed to forced supply so that the space is negative as this will give the best results as far as I understand it and it appears that you agree.
The problem that I envisaged is that the very negative pressure that keeps the space healthy would negatively impact the gas flow in the heater which is why I envisage using what will effectively be a room sealed unit.
Of course I may have overlooked something fundamental but provided that the ducts are big enough I don't at this point see why it wouldn't work.
However I am now aware that E & E abandoned an air supply below the feed tube as it was causing smoke back and I can see how that might be an issue which leads me to think that ducting the air would probably work OK but that I will need to have at least some primary air going in at the top of the feed tube.
I am also aware that the co-axial flue may be a poor plan as it directly works against the flow by cooling the exhaust and heating the incoming air which will/would reduce the flow of both!
"6-8 hrs of close attention ....."
Isn't that a function of the size of the heater. I get that a few sticks, however well they are burned, isn't going to heat much mass but then I wasn't thinking of sticks ....
There go's that box again, I just don't seem to be able to stay in it! Perhaps actually knowing where the sides are would be a good start
Anyway ... Part of the reason for thinking water mass is the ability to move it away from the heat source when it is hot and replace it with new cold mass. I reasoned that this approach would allow me to extract heat more quickly and therefore utilise a bigger fire. The second thought was to use more massive fuel in much the same way as a bale burner consumes the end of a bale that is fed in as it is consumed.
I am not suggesting that I will be burning bales but I hope to be burning the end off logs that are sitting on a 'grate' for want of a better analogy. (Concept sketch attached)
Primary air is drawn under the fuel, and probably a little from the feed tube to combat smoking, whilst secondary air flows under the grate support plate directly to the burn chamber.
The grates will be 3mm stainless, welded and will need toi be sacrificial I suspect.
I expect I will have to play with this in a mock-up to have any chance of getting it to work.
Water, thermo-syphon and Un-vented System ...
IT will not be sealed, that is just asking for an explosion, my reference to sealed was on the DHW side and even that is more risky than a vented system which is my preference.
You may be correct about the coil arrangement shown, although the flow in all the elements exposed to a temperature gradient is the right way.
Given that I now think the coil would be best placed in the down draft chamber, barrel in conventional terms, I dont think it will be an issue but there is a little trick that can be employed if I need more flow.
Ironically it works the same way a RMH does. You create a long vertical loop with its feed leg, upward insulated and the return leg uninsulated. The water cooling in the down leg gets more dense and drives the system.
Obviously you loose some heat in the process, you have to or no work is done but since the loop could be a nice polished pipe in the living space that isn't much of an issue.
This type of thermal pump was employed in large bore single pipe heating systems, mostly commercial, for years to drive water round horizontal pipes supplying heat to floors.
Calculations ... Well I suspect it will be more trial and error, OK probably mostly error, rather than calculation. I suspect that the thermodynamics and flow calculations required for such a complex space will be way beyond me even if I could find the information I doubt I would recognise it.
It is interesting that you say use a tall riser, that seems to me to be intuitively correct, but I suspect it will need to be a larger CSA to counteract the pressure drop.
My original thought process was to use a square riser, as I expect to be using kiln brick, but the cast solution is very appealing, either way I will offset the burn tube so that a turbulent, hopefully a vortex, is created.
I have already looked at Web4deb Channel and you are correct It was very interesting although I would not be keen on using aluminium fins in a potentially corrosive hot environment.
There is another design I like the look of but I will need to do some calculations on exchanger area as it may noy have enough without being stupidly big.
Heat exchanger on rocket heater - Apostol Engineering
I would be most interested to hear what everyone else thinks.
Pleas keep the questions and challenges coming, the more I have to consider and explain my thoughts in context's I haven't thought about the closer I will get to actually being able to build a test rig.
Or perhaps just starting again because I was way of base in the first place
There are some deal breakers here that may turn out to be insurmountable problems.
Fuel feed and long burn time being the one I am currently most worried about. I cant believe that feeding logs hasn't been tried and yet it I haven't seen a single example of this being done, that is worrying.
Looking forward to your thoughts, collectively