combo passive solar thermosiphon + rocket stove water heater design

Nov 08, 2025 20:12:58

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 ! 🙏

Nov 09, 2025 02:54:57

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.

Nov 09, 2025 09:50:40

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 🫡

Nov 09, 2025 13:53:05

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?

Nov 09, 2025 18:18:04

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 🤝

Nov 09, 2025 20:07:55

here are some updated renders of the evolving design ; )

Nov 09, 2025 20:30:59

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.

Nov 10, 2025 03:04:59

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?

Nov 10, 2025 03:42:16

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!

Nov 10, 2025 18:14:27

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 : )

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