combo passive solar thermosiphon + rocket stove water heater design

looking to get some feedback on a combo solar / wood fired water heater design

this will go into a straw bale house at 8000' in southern colorado, design priority places an emphasis on anti-fragility

primary heating element : a passive solar heat collector using a thermosiphon glycol loop copper tubing heat exchanger inside the water tank, preferably WITHOUT a pump involved. would also like to have the ability to further heat the water using an integrated rocket stove, for cloudy days or occasions when more hot water or hotter water are desired

- will commission a welder build / modify a custom water tank that allows the upper end of the rocket stove barrel to to nest under it for conduction heating
- solar loop in / out connects to a large passive solar water heater on the outside of the south-facing wall

have seen quite a lot of rocket stove water heater vids on youtube but most of those are using copper tubing to make a heat exchange loop around the rocket stove interior J-flue. have not seen anyone attempt to nest the barrel under a water tank for direct heating via conduction, which seems like a simpler approach. can't imagine why it wouldn't work ?

( see attached illustrations )

some considerations :

- water tank will obviously need an aperture at the top to prevent pressure build-up
- this system is envisioned as very hands-on / monitored while heating using the stove - if temp gets too high / water boils solution is to let the rocket stove fire die and manually add more water from the well inlet
- rocket stove exhaust outlet run under a short ( likely cob ) bench and up through the roof
- weight of water tank is supported on ring stand with legs / rocket stove bears no weight
- water tank will be well insulated

- servicing the interior of the rocket stove barrel will prove challenging in this setup. if the j-shaped chimney inside it built of high quality refractory bricks how likely / often would one expect this to pose an issue ?

purpose of this water heater : provide hot water to the kitchen sink, bathroom sink, and a small soaking tub

- soaking tub will be insulated and exposed to incoming sunlight on the south side, so it should be possible to get the water in there lukewarm by end of day before adding any solar / wood heated water to bring it up to soaking temperature

- daily use will require filling the water tank in the morning so the solar collector can heat the water and using the soaking tub in the evening

anything i'm missing / misconstruing / missed opportunities before i attempt this ?

thanks for your time and attention ! 🙏

I think your tank and stand design might be unnecessarily complicated/expensive.
Steel barrels are already designed to stacked 4 high.
Even with a full barrel on top of an empty one, you will probably be fine, as long as the bottom barrel is properly set.
If you bolt angle iron vertically around the circumference of the bottom barrel, you should have a very secure arrangement.

With your passive solar  preheat ,keep mind the collector could become an emitter as soon as the liquid in the coil is hotter than the liquid in the collector, so you may wants include a shut off valve to prevent this.

this is a good point, though i don't think 55 gallons would be quite enough for the envisioned use. do you think the rocket stove steel drum would potentially support the weight of 2 drums welded together for the water tank ?

good tip on the shutoff valve to the solar collector loop as well 🫡

These type of ideas are often presented on the forum but rarely do we see the results.
I really dont know how well your idea would work for you, J tubes are really great stoves, the internal riser makes the concept very user friendly.
However a 6” J tube built to the common spec will struggle to maintain 300c on the top of the barrel even a 8” wont run much hotter apart from peak burn when they can reach 500+ but maintaining that sort of heat is not so easy.
I have made several J tubes that differ slightly from the standard spec and do get a bit hotter but you still need a good strong draw and smallish dry wood to maintain 500c .
The first gen batch box stoves get much hotter but do have a tall riser.
My point is … a gas burner flame runs at 2500c so 300-500c on top of the barrel might not meet your exportations?  

i don't mind at all if it takes 2 hours or whatever to heat the water : ) the point is to do it in an anti-fragile way that doesn't rely complicated devices or fuel externalities beyond my control

this is def gonna happen in whatever final form the design takes

i will absolutely document the process and post the results here as documentation / discussion fodder for anyone else thinking similarly to reference 🤝

here are some updated renders of the evolving design ; )

I like the idea. I would suggest to consider installing the tank in such a way that it would be directly exposed to fire. I cook quite frequently on my outside dry stacked masonry stove and have noticed that pots placed on the top cast iron plate heat much slower than when exppsed to direct fire. That's why some cooking plates for masonry stoves have removable rings.

Also I would suggest to not use BBR firebox for this task. It will be heating only when burning. Massive amount of energy in the form of coals radiation will not work much towards heating the tank. Some simple L-tube would probably work the best. When the fire dies out, the coals will keep heating the water, radiating a lot of heat towards the tank's bottom.

Cristobal has a good point with the direct exposure to fire there. Maybe cut out the top of the red barrel in the render.

The render so far looks like a J-tube design to me? The bench could also be a bell, correct?

I agree with the other posters, the setup seems a little complicated not to mention the cost of getting a custom tank and stand built. I guess if the stand is supported by the floor and not the RMH that should be fine. Heat transfer is a function of area and it doesnt seem like you would get much area from having the fire riser partially (I dont see dimensions) envloped by the tank. I wonder if anyone has thought about putting the exchanger (copper coil and jacket) around the outside of the fire riser instead of the J-tube? Good luck!

cutting out the top of the rocket stove barrel ( and maybe reinforcing the bottom of the water barrel with some thick steel ) a good idea

as is using an L-shaped flue instead of J-shaped, so heat from the coals will contribute to warming the water

very glad i sought some feedback here : )

Question.  Do you actually need the tank over the stove barrel?  The bottom can't be insulated so you will have radiative heat out the bottom.  Yes it is into a heat bell so maybe not a risk?  Its size will limit storage capacity.  Guessing you will want more storage.  As you stated it will make getting to the heat riser for maintenance difficult.  Material to build the barrel will be somewhat difficult and expensive if done so it doesn't corrode.  ideally would like a much smaller amount of stainless or other material exposed to the fire.

Alternate answer.  There are various youtube videos showing the water "tank" aka wood box, being made with 2X material, plywood, lined with insulation and then that lined with an EDPM which is rates to 200 degrees max working temperature.  Personally since I am a bit over 4000 feet in elevation the boiling point of water is worst case 203 degrees.  So I couldn't ever get much over the 200.  And if my water tank is big enough even getting it close would give time for the fire to burn out.

So my thinking is to build the barrel instead.  Use say 1 1/4" black schedule 40and wind it into a spiral of barrel diameter or slightly larger.  like a giant spring.  Say 3 or 4 full wraps.  Spiral into that say 1/8 inch flat material run thru a slip roll and weld the 2 together into a solid barrel.  The squish boom discussions sound like that size pipe is big enough to keep it from all flashing to steam at once and into a non pressurized system it should be safe anyway.  Now the question it corrosion protection.  Treat this as sacrificial?  Or can the inside of the pipe be cleaned and electoplated in copper or nickel.  Plating inside a pipe would be difficult but what if a piece of monofiliment was blown thru to pull a centering brush with the power wire.  It would be a slow operation.  One other alternate would be an active corrosion prevention system with a sacrificial anode elsewhere in the tank.  Final option here is maybe glass line the pipe.  Still researching this part so no solid guesses as to desired answer here.  Final high price alternative would be the 30 to 40 feet of stainless pipe.

So lets start at the "tank" and work thru the system.  Inside the tank an insulated snorkel up to say 2/3 of the height of the tank and out thru a bulkhead fitting.  From there into Tee with the main part aimed down and an air break aimed up so the whole tank will NOT drain if a leak happens somewhere other than the tank itself.  Take the line down to the bottom of the spiral.  Question in the thinking here is do I want to do a Tesla valve where it goes into the spiral so there is even less chance of it back feeding?  Head pressure backward vs forwards and cold vs hot water should prevent it but a Tesla valve would give added protection.  

Then on the outlet of the spiral a heat riser pipe going as straight up as possible.  (short runner pipe on an angle allowed if needed)  The outlet should go a couple of inches above the water level for the system.  Now want to drop this out and into what amounts to a giant funnel open to the air but with a steam condenser built into the lid.  Condenser borrow from alcohol distillation tricks.  Small amounts of steam generated should return it back to the water.  Now at the bottom of the vertical pipe want something to direct any bubbles into the middle of the pipe so it works like an air lift pump running on steam/a bit of air when needed.  Most of the time the expansion of the water combined with convection should lift the water above the water line so it is flowing into open air and thus a non pressurized system.  Now some large glass bottles above this water line neck down should provide a reset-able refill for the system with easily visible site glass.

From this insulated funnel into piping looking like an ES(program won't let me do just the character) laid on its side.  It is all inside insulation.  This creates a trap to seal the tank.  Only time the water level gets high for the water to return to the tank.  A air vent on the high of the up curve of the pipe, of the tank will let its volume change up and down while preventing it from siphoning the trap completely empty. (air vent back into the top of the funnel

Now for the solar end my thinking has been looking at drain back systems which means I would need a pump.  But done correctly I can get rid of the complexities of heat exchangers and antifreeze.  And if I can do it with PEX tubing, greatly reduced freeze risk if something fails plus reduced cost.  And low wattage DC magnetic drive pumps are readily available.  No seals to fail on the pump and with some in the 30 watt range not really a big solar panel to drive and only needed when the sun shines anyway.

Third component of the thinking is for summer months to include 2 more pumped loops.  One to ground for final cooling inside going to hydronic ceiling system inside  And one to ceiling hydronic system so the hot air inside the house is doing the preheat on the tank.

PS you are showing your well in and hot out backwards.  Cold to the bottom and hot out of the top.  Add a stratifier if needed so you don't mix them.

any suggestions what sort of insulation / material i should consider for insulating the water tank ?

i think his blue rendered barrel is like a giant pot on top of a stove ---the red rendering part of the drawing ---so hot water is going to generated from the bottom ---which should in some part heat his copper coil --the well water in would just be topping evaporating losses ---with a ballcock fitted---perhaps there should be another hot outlet to the system

andrew stevens wrote:any suggestions what sort of insulation / material i should consider for insulating the water tank ?

Rock wool would be ok but like most high temp insulation it needs to hold lots of air so you cant really tightly strap it in place.
In an ideal world a second skin 2”+ away and filled with loose vermiculite would be very effective.