Masonry rocket heater with built in water heater question.

Good afternoon everyone.
I'm new to the site, but there seems to be quite a bit of knowledgeable people here. I was hoping to get some pointers on a project I'm working on.
I have a small, two room cabin that I am trying to heat. I built this out of 8' landscaping timbers to give you an idea of the scale. There are two 8' square cabins connected by an 8' dog trot. I have a couple of heater cores like you'd get out of a central unit on your house. I thought about running hot water through these using s thermosiphon system ran off a j-style, poured "concrete" rocket heater that I saw online.
The design was simple enough, but since its a solid pour, I need to hammer out all the details before hand. I have a few sections of 10" stove pipe I thought to use as a stack. I figure I can wrap 1/2" or so copper tubing around the stack before I pour so it will be set in place. The burn chamber I figured I could construct a square framework. I think the mass of the "concrete" will hold heat to give me a nice warm fireplace, and also heat the tubes enough to get a strong supply of hot water to my radiators.
Here's where my questions come in. I've never done ANY of these things. I understand the concepts of all of it, but are there any certain EXACT ratios that need to be followed?
For instance, I'd like the capability of a long burn so you could start it before bed and it would stay going, or at least warm until morning. So can the "j" portion, for wood feed, be taller to accept longer kindling and it not effect efficiency? With a 10" exhaust stack, is there a particilar size that the burn chamber needs to be? I fired 10" all the way through but I honestly don't know.
On the water heating part. I know that hot water will rise and create a flow, but if the entire system is basically a loop of 1/2" copper tube, appox. 30' long...will that circulate? I worry that the radiators won't draw enough heat and the water temp will equalize in the tube. This would be fine as far as a heating stand point, but if it gets too hot and turns to steam...I just worry lol.
Anyway, I could go on all day. I hope my ramblings aren't TOO confusing. Thanks in advance for any help you all can provide.

I'd like the capability of a long burn ...

Efficient burns are fast burns with plenty of air. If you smolder the fire to make it last all night, you will consume a lot more wood for the same heat.

The better option (IMHO) is to burn fast and store the heat.

Yes, I understand the fast burn. I see quite a few designs with a relatively short "wood storage" chute, about a foot. This J- tube has a vertical wood feed. So if it were taller. Could I use longer slats of wood to create a self feeding burn that would last longer? I didn't mean to smolder it, just didn't want to choke it out with too long of an intake full of wood.

Wood doesn't always cooperate with falling into the feed tube. I go by and shake mine in the feed tube periodically. It usually gets the fire going better. The height of the feed tube shouldn't be changed. You could put some sort of rack over it to hold the wood. However, I recommend that you concentrate on storing the heat.

Ok, great.

So I'd thought to have just a beefy poured refractory, what I see you guys calling a core. Having never used one of these I'm not sure how long the stone with hold its heat. I'm picturing a 30" square, with about 5' or 6' tall poured riser. That would have a 10" inside pipe with no secondary. I'm not sure how long that would take to heat up, but I'd guess it would hold a fair amount if heat for several hours.

The core should be insulative to keep the fire warm. After combustion is complete (after the heat riser), then start storing the heat.

There is nothing beyond the riser.

uh-oh

Jeremy Stewart wrote:
I have a couple of heater cores like you'd get out of a central unit on your house. I thought about running hot water through these using s thermosiphon system ran off a j-style, poured "concrete" rocket heater that I saw online.
...
I think the mass of the "concrete" will hold heat to give me a nice warm fireplace, and also heat the tubes enough to get a strong supply of hot water to my radiators.

Hi Jeremy,
About a year back, I posted a note about producing hot water by extracting heat from the thermal mass (below boiling temp) instead of the stove itself (boom-squish style). The idea is to make the system more simple, robust, safe and stable, all because of the much lower and stable temperatures in the thermal mass. Is this what you are having in mind?
Link to my post: https://permies.com/t/22031/rocket-stoves/Quick-Easy-Rocket-Stove-Hot

I think we are on the same page, yes. I could not view the video on the post you linked below, but judging by the descriptions I think so. There would be minor differences as far as usage. Meaning I simply need the water to transfer the heat from one location to another where it can then be delivered via radiator. I work with large gas fired boiler systems at my job, but we use pumps to move the water. My main concern with this setup is...too efficient? It's hard to turn a fire OFF. The greater the temperature difference between the hot and cold sides, the stronger the siphon. Too great you'll get thermal shock, maybe stall the system. Too little, the water temp would equalize, also stalling the system, but leaving a very hot radiator in my little rooms. That might also create some overheating of the system and cause boil off.

I thought that running the water coil through a significant amount of mass surrounding the riser would heat it enough, and long enough. I'm not so sure it's the most efficient way of getting heat where I need it but the other options won't work so well. Can't have an open flame, no room for a thermal mass or an indoor stove of any kind. I don't want to resort to propane heat when I know there should be an alternative. Just have to work out the kinks.

Jeremy Stewart wrote:I simply need the water to transfer the heat from one location to another where it can then be delivered via radiator.

Hi Jeremy,
This company sells double combustion stoves that can be fitted with a water-heating module:
http://www.walltherm.ca/
They also have a solar panel module that integrates with their hot water tank and controller.
Their system can use both very hot water from a stove radiator, and mildly hot water from a solar panel. Therefore, it should be perfectly feasible to set up a similar system with a radiator embedded in the thermal mass of a rocket stove. They use a small circulation pump, which is cheap, needs little power, and allows great control of the radiator temp. This is standard hardware, so I feel quite confident about giving it a try.
As you say, there still is the question of how to locate the heat exchanger in the thermal mass. I am thinking of embedding a recycled radiator straight into the thermal mass with a slanting angle, without getting too close to the fume pipe. The idea is to get a constant, comfortable temperature, instead of designing for a fast heat-up with boiling risks. I would definitely fit a safety valve on the system. Anyway, the whole set-up should be very much the same as for a solar panel, swapping the solar panel for a radiator put into the thermal mass.
Besides, if you choose floor heating, the design temperature is barely above you target room temperature, so you actually would rather avoid high temps. Instead, the huge thermal mass at mild temperature of the mass stove provides an ideal large buffer at constant temperature.

Jeremy did you want to extract heat from the stove and store the heat in water then pump the water to radiator in different part of your house? If that is what you want to do this video might help, it is the last parts of the build. you might want to see the other parts. https://www.youtube.com/watch?v=EVpseNZI9vY. Here is another video on a working thermo siphon system, it is at 9:20-9:40 minutes of the video

Beautiful video Larry, thanks for the link.

As for heating a house, I would collect the heat from the thermal mass rather than from the fume pipe. The temperature you need for radiator heating or a hot water supply should not exceed 60°C/140°F, so I would rather place my exchanger in that temperature range in the thermal mass. You also get a much more stable heat source.

Frere

You only looked at the second video and were the heat is taken from the system. The TWO video links show the concept of thermo siphon, no mechanical moving parts, so no electricity needed for system to work. You will not freeze if you do not have POWER, like in a winter storm, as long as your water did not freeze because water is your heat storage median. The heat is siphon off the top the barrel in this rocket fireplace and you only need to load it once an hour, unlike a 6" system 15 minutes, that this rocket fireplace replace. The author of this video did a concept build and and state that he would build a 16 in feed system next time with 25 feets 1/2 cooper coil to get better heat transfer from the to of the barrel. Here is the video link for the coil at the top of the barrel being build.

https://www.youtube.com/watch?v=fYqQd27Zbwk Part 4 of the build. The working thermo siphon is in part 7

Thank you for your comment Larry, and yes, thermo-siphoning would be ideal. As I live in Spain, the mass heater should be plenty for heating. I would place a radiator in the thermal mass for hot water production in winter, when the solar panels can't keep up. The Walltherm hot water tanks are designed for a double input (Walltherm stove + solar panel). Ports and fittings are also provided in the tanks for mounting an optional electric resistor as back-up. I would use that full system and just swap the Walltherm stove for a rocket stove with a radiator embeded in the thermal mass.
As for power, the plan is to cover the whole southern roof slope with solar panels in order to have a confortable supply.
This project is at a very early stage though and I am still looking for a property to purchase, but I'll let you know if it goes ahead...