Where should I place themosiphon coil in a rocket stove?

Any thoughts on where to best place a water coil for a thermosiphon loop?

I'm hesitant to place it in the burn-tube, as I don't want to crash the temps, and putting a loop in the burn-tube is
difficult to design while avoiding any downturns in the plumbing to avoid heat traps or live steam. (NO ELECTRIC PUMPS!)

If a coil is placed on the outside of the drum, that seems very inefficient with the low temperature delta.

This leaves a couple options; coil hanging on the inside of drum/bell, or a coil wrapped around the Vertical Burn Chamber just
prior to insulation.

I like the later, as this will keep the Vertical Burn Chamber (made from stainless pipe) cooler, and any soot build-up will
be easier to brush free from the inside of the VBC, where one would expect it to condense, rather than having to fiddle with
scrubbing a long & unwieldy loop of copper tubing. (Monotube boilers are a bitch to clean without the aid of a steam lance.)
One-inch copper thermosiphon tube should move enough fluid to prevent steam cavitation. Thoughts?

My design will not include a cob mass bench. After the bell, it will exhaust straight up thru the roof. My cabin is two stories,
380 square feet. Stove will be located downstairs. Water loop will thermosiphon to a tank. Warm air will cycle by convective
loop thru a large grate and stairwell. TP valve near stove, mixing valve on storage tank for safety. YMMV. I'm located in
the NW USA.

Failing this, I've got a 15 gallon stainless beer keg, and I'll make a batch heater using something like Erica's canning rocket stove:
https://permies.com/t/18988/stoves/Erica-Wisners-Rocket-Canning-Frying#161360

I am not an expert, however instead of a long bench I would build a tiny 3-4ft bench filled with water to extract the heat from the exhaust,
thus helping with airflow, thermal mass and a nice place to siphon off heat without messing with the burn chamber.
I have also seen a few systems that had a hot water heater on top of the 55gall burn drum, you could use that and then just pump/pipe it up to the 2 floor.

The exhaust at the start of the exhaust bench is normally over 600F, and the bench takes it down to 90F.
I am not sure how you plan on configuring your system but a regular chimney is going to have a exhaust temp at the roof/end of 500F, and that is such a waste.

It might even be possible to have a vertical rise (chimney) to the 2nd floor and then a 90 degree turn to a regular RMH "bench" on the 2nd floor.
Then the usual 90 turn outside the house up to the 2nd floor roof/gutter. However I would be concerned about the weight of the thermal mass on the 2nd floor.

Water(4) can hold 5 times as much heat as soil(8/10) . http://en.wikipedia.org/wiki/Thermal_mass.
Water transfer the heat from the exhaust to itself(0.6) at a faster rate than soil(0.2) too. http://en.wikipedia.org/wiki/List_of_thermal_conductivities
So it could be 3 times shorter and hold the heat 5 times longer.

I personally use a copper coil right above the heat riser, much like a monotube boiler. This keeps it out of direct flame and in the cleanest possible exhaust. There is a mild bit of soot on the pipe after a full season of use and a lot of experimental use. Here's a diagram of how my system is laid out. I am using a pump for mine, since my stove is about 25' from the heat exchanger.

If the distance was tightened up, I see no reason why a thermosiphon wouldn't work. Many really old cars used this method of heat transfer for their cooling system.

I've got a cheaper, simpler design that I've been working on. It involves taking a portable air tank, removing the safety valve, and welding a threaded bushing on the opposite end of the tank. 1/2" iron pipe would then be attached to the two ports and welded for a seal. The ends could then stick through your burn drum and again be welded to form a seal between the drum and pipe ends. You then have 1/2" hot water connections ready for use. Add your T&P, mixing valve, and expansion tank and you are all set. The slower rise in water temperature due to heating 5 gallons of water is no big deal, since you are getting heat off the burn drum pretty quickly. Since no steam is being generated, the tank should last for years, at least as long as the burn drum.

The hot water can then thermosiphon to a storage tank, and in turn, be thermosiphoned to a radiator upstairs, giving a decent amount of thermal storage and good heat for the upper level of your cabin. No pump necessary. The system will have to be engineered to burn just enough wood to radiate heat from the burn drum and keep the water around 180 degrees for best efficiency. In my rocket stove, the exhaust is so cool that paper will not discolor when placed over the exhaust.

SO what you guys are building in not s rocket mas heater and most likely not a rocket mass stove. With that being the case be very mindful of the "regular fireplace" chimney hazards.
The barrel over the heat riser creates a second after burn of the gases. You coil above the heat riser is subject to temp of up to 3000F.
The air in the chamber is in excess of 600F it is very possible that explosive steam could be produced in a closed loop coil. esp as the pipe get more wear and tear.
I failing RMH might give you a smoked out house but a failing close loop coil next to your family/friend face is slot worse.

This is definitely not a rocket mass heater in the traditional sense. The mass has been replaced by a circulating loop of water. I keep mine outdoors for additional safety. I ran a closed system last year and never experienced a boil without trying to in order to test the fail safe measures. Safety should always be the #1 concern. That is why it is important to work the details out mathematically, then experiment on a smaller scale to understand what you are up against. If you do your homework correctly, you will succeed when you scale up. Because all combustion has occurred before the exhaust hits the coils, there is no danger of creosote and chimney fires. Fire erosion is another non concern as direct flame doesn't come in contact with the copper coils. Its kinda like the old boyscout trick of boiling water in a paper cup. As long as there is water in the cup, it will not burn.

Keep in mind what a BTU is - the amount of heat to raise 1 lb of water by 1 degree F. By knowing the btu content of your fuel, burn rate of your stove, the water capacity and flow of your system, and the BTU capacity of your heating system, etc., it is quite easy to engineer a water based system safely. If you are a strictly seat of the pants engineer, I would recommend looking into other options. A little math and science can go a long ways, so I recommend regardless of what method you choose to utilize rocket stove heat, that you look into the theory behind it all for best efficiency and most importantly, safety.

The fact that the water pipe coil never gets to 200F, most likely means that second ignition of the gases never occur.
And I fear that you are still wasting alot of heat by missing that 2nd burn and also from the exhaust, the temp of teh exhaust in a regular RMH goes from 600F-90F.
Its also possible that the water gets to a supercritical state and just waiting to exploded without you knowing, just waiting for a crack to exploded.

Heating your house with the hot stove-radiant water heater is still pretty awesome.
And lots of fun figuring out the burn rate and carefully adjusting the feed rate even when you are not actively watching it.

The secondary burn does occur, the coils come after the heat riser. Heat is only pulled out of the exhaust after all possible combustion has happened. The exhaust on mine runs about 250, so it isn't as efficient as a RMH. The convenience of being able to transfer the heat from one stove wherever I choose makes up for that. By adding more coils, I think I can approach the efficiency of a RMH, however. The cost of copper is what has prevented me from doing this yet, though.

Thank you George, Bengi, for the excellent and thoughtful replies.

Yes, water and fire are dangerous. I have experience with live steam, boilers, and horizontal steam engines (heat engines!).
It's the kind of activity that should be kept out of the house, and enclosed in nothing more substantial than a 3-sided shed.

Applying rocket-stove tech to hot water production (or steam!) can revolutionize the practice. Thanks for the report on
the creosote non-issue. I may apply this to a mono-tube boiler for live-steam (outdoors only!) Cleaning soot and chopping
copious amounts of wood is what kills much of the fun of the live-steam hobby.

With regards my cabin project, placing the water-loop on the stove output side should keep the heat-pump aspect of the rocket design
functioning well. (The temperature delta is a little weak, but I DO want to AVOID steam.) I found another thread where Eric suggested this location as well.
Which brings me to another question:

If I replace the cob bench with a water-loop, might I turn the exhaust vertical, so my coils might be wound in progressive horizontal loops, thus avoiding any downturns/heat traps?
Then I can route straight up thru the roof (while keeping the heated side of my loop at the lowest relative position to my storage tank, improving the
thermosiphon.)

Final thought: rather than taking the exhaust off at the lower OUTSIDE portion of the bell/barrel, might I plumb the exhaust thru
the top of the bell, with the hidden portion of stove-pipe passing almost to the bottom interior of the bell/barrel? (With a thermal blanket
wrapped around this hidden portion to prevent the equalization of temps.) In this way, the rocket could be made a little
more compact.

Hi
I want to put out a concept for Hot Water heating adding to an RMH. Therefore not compromising having a whole other unit just for Hot Water heating, if that makes sense.
It is thermosiphoning, mains cold water goes into header tank which also functions for relief of pressure and expansion.
The copper coil should be encased with very good insulation, like 150mm cellulose, where the box can be made out of sheet metal or 2 different sized barrels upside down to achieve a cavity to fill insulation.
So the copper coil has lots of surface area within a confined well insulated space to exchange the heat to water. Looks alright? Shall i have a go?
Let me know if you can point me out something...
Cheers
Oli

The above is a near duplicate of this post: https://permies.com/t/28282/rocket-stoves/rocket-stove-water-heater-designs#454299

Let's keep the discussion in that thread to avoid more duplication and confusion