Rocket stove for hot tub heating

Hello everyone!

I have an insulated hot tub, it holds about 700L of water. I've been designing a rocket stove to heat the water and I'd like to ask if you can spot any issues in my design.

I have 5m of 28mm copper pipe that will be coiled inside the chamber, with natural water flow.
I've used 150x100mm burn tube and magazine because that is what I already have. Magazine is extra long so I can load it with "normal" firewood and not need to keep watching it.
I've left the other end of the burn tube open for more airflow and to clear ashes.
Heat riser is 140x4mm pipe, this I have to buy.
Burn tube is offset to the side of the heat riser, my thought is that this would create a better vortex = cleaner combustion
The "barrel" is an old air compresser tank. Outside diameter is 300mm.
Flue pipe is 160mm HVAC pipe since it is cheap. It will be longer than in the drawing, now it would blow gases in my face 😁
I have some mineral wool I could use for insulation (not sure if it can handle the heat?), though on a future update I would surround it with rocks and try using it as a sauna kiuas.

Edit: the copper coil will be positioned in a way that it will always go uphill, so no steam or air can get trapped.

CSA
140mm          153cm2
160mm          201cm2
100x150mm  150cm2
Barrel             467cm2 (this is the approximate barrel area, with heat riser and copper pipe included)

Welcome to Permies, Catrik! And good luck on your first rocket stove experiment.
You will need all the insulation you can put around the core, as the steel will be stealing heat from every part of it and keeping it from getting as hot as it should be for full combustion. The burn tunnel and riser especially need insulating. You don't want to heat the copper coil directly from the riser, but from the descending gases out of the top of the riser - let the fire stay as hot as possible as long as possible, and you will have less smoke to worry about. Since you are not looking for space heating but water heating, I would insulate around the barrel as well. If you have a good hot fire, the steel will not last a long time... depending on conditions, maybe a few seasons if you use it often. At least you are not trying to keep a house warm all winter with this metal core; it would probably burn out the first winter.

28mm is pretty good sized copper tubing, and might be big enough to keep from flashing to steam near the top, though I suspect you will get a good bit of that when the fire is going well. Gravity flow is not very fast, and you would be safer if you had a pump to keep it flowing faster. The shorter the run of copper to the hot tub, the easier it will be for steam to escape without bursting the tubing (not to mention getting better circulation.)

I expect that your tall feed tube and low air intake will allow the fire to climb up the feed in preference to going through the rest of the system. A cap on the feed might defeat that, but would probably let the whole feed get hot and burst into flame when you open it to add wood.

The burn tunnel and riser especially need insulating.

Do you mean I should insulate the riser inside the barrel? In that case I would need to change the dimensions because there is not that much space. I could buy 100 or 114mm riser pipe, then cut and weld the 100x150mm burn tunnel smaller to keep the CSA about the same. I don't really have a gut feeling for what size is enough for decent heating time and if my copper coil would be a limiting factor or not, so if I can use smaller pipes that's always a little bit cheaper and less work.

If you have a good hot fire, the steel will not last a long time... depending on conditions, maybe a few seasons if you use it often.

I'm guessing I would be using it on average less than once a week, so not really worried about that.

I expect that your tall feed tube and low air intake will allow the fire to climb up the feed in preference to going through the rest of the system. A cap on the feed might defeat that, but would probably let the whole feed get hot and burst into flame when you open it to add wood.

Yes my plan is to have a cap on the feed, however I did not think of the heat build up. I will have to keep that in mind.

Yes, the riser needs insulating. It doesn't absolutely have to be very thick, even an inch/25mm of mineral wool would probably be adequate. Is there a pipe size between 150mm and 100mm? I would be loathe to reduce the size of your system below 100 x 150. Smaller J-tube systems are notoriously finicky and difficult to make work well, and in any case you would be severely reducing your potential heat output. I would want as an absolute minimum 25mm clear between outside of insulated riser and inside of copper coil, and some space between coil and barrel would help air circulation and heat transfer a lot too. More space would be better.

I think rather than reducing the size of your riser, I would abandon the compressor tank and look for a steel drum. Around here they can be gotten for $10-$20 US in excellent condition, often free for an older one. The extra space will make everything much easier and probably more effective.

I have no advice on the design front, but I did see this new video, fresh out of Wheaton Labs, where Uncle Mud discusses what he did. Maybe this will be helpful.