Requesting hot tub guidance`

Hello all,

I am trying to design a rocket mass hot tub and come in search of advice:  Thank you!

I have attempted to post a drawing of my basic concept, labelled "hot tub plan 1," but if it fails, here is a synopsis:
It would be an outdoor system.
I hope to create a heat cyphon with a copper coil positioned around the barrel of a 4" j-tube system, and with its ends communicating to tank aperatures of high, hot water inlet and low, cool water outlet, respectively.
I would position an IBC tank so that it was conductively heated by the rmh's exhaust system inferiorly while being ciculated with hot water from the ciphon.  The tub's sides would be insulated but its top would remain open during stove
operation.

Here are my questions
A. First of all, is this a sufficiently open system to be safe with a copper coil?
B. Secondly, Are there resistance variables defined by water weight that affect the safety of such a system used with a nearly full 275-gallon IBC tank of water?
C. I would also like to install filters on the interior of the tank over each aperture to prevent debris from accumulating in the coil.  Would the added resistance of a thin, non-insulative membrane contribute dangerous resistance
to water flow through the coil?
D. How do I calculate coil tubing diameter?  Should it be consistent between outlet and inlet?
E. As drawn in the image labelled "hot tub plan 2," Would it be desirable to elevate the tub enough to allow cool outlet water to issue from the tank bottom at the same level as, and feeding into the coil's top?
Would the resulting water weight be sufficient to push heated water down the coil and back up and out of a spigot overhanging the rim of the tank by a few inches so that it could act as pressure-relief and temperature gauge as it passes
through the intervening air between siphon and tank?

I am interested in using one of the 4" precast refractory j-tube cores from dragonheaters.com
http://www.dragonheaters.com/4-dragon-burner-rocket-heater-core/

I have searched and found nothing specifically devoted to this topic, but please forgive my oversight if this has been previously covered.
I welcome any advice, criticism or precaution.

Thank you kindly,
Brian

Hello all,

Just checking back to see if anyone might have any feedback on my last post.
Hopefully the drawings are not completely indecipherable.

Thank you all!
Brian

HI. That's what I want to build basically.  I've used the term "thermosyphon" or "gravity" system. Your second drawing with the tub, the "load" or "heat emitter", elevated above the "heat source" a few feet will flow better. Not sure if the first one would flow much at all.
A rule of thumb, and it's against code, is never place a valve between the heat sorce and the load (tub). If someone absent mindedly shuts it off you could have a Big Bang. Since it's a "open" system you don't need a T&P relief valve.
I built a coil without a drain and it froze and cracked.
Make it so you can access the coil to inspect it and clean creosote.
I think 3/4" pipe is good. Not too big, not too small. Easy to bend. Fill with sand or salt then bend.
I'm using a stock tank because I move around a lot right now.
Happy soaking

Many thanks, Jeremy!

Your good advice is duly noted.

I will experiment with the second plan.

I will likely arrive at questions along the way and will report successes and failures as they occur.

Thank you,
Brian

Any time Brian. You cam PM me also if you have questions. I looked at the second drawing again and noticed the coil is backwards. The cool water from the bottom of the tub should go to the bottom of the coil.
I'm building a small version in my camper van this week. The tiny coil is heated by a small propane burner. Adjustable from 1K btu to 10K btu.

Another thing that really helps heat the water faster and reduce fuel usage is to cut some rigid foam insulation to fit nicely in the tub. On top of the water. Really helps keep it hot between uses also. Buy the good pink or Blue foam. The white styrofoam falls apart.

Thanks, Jeremy,

I was, in fact, worried about coil flow...it does make more sense to keep cooler water at the bottom!

If I were to use Geoff Lawton's design per his youtube video (link below), and if I kept the coil horizontal as his is, would the thermal siphon still function?
https://www.youtube.com/watch?v=1oDpmmsqHwQ&t=628s
There's a diagram at 10:31 position.

That is to say, if I still elevated my tank above a horizontal coil and used an open hot out line flowing back up and over the rim of the tank, would the hot water still siphon as effectively along a horizontal coil?

Here's a drawing:

Also, because Geoff's system is essentially a "double boiler," is it safe to assume the water therein will not flash to steam?  In a system where the coil makes direct contact with the barrel, is there any chance of it flashing to steam beyond the speed at which hot water can be propelled out of an open-ended coil outlet?
I should probably make a miniature, too, to see what works best

Thanks!
Brian

HI Brian.  Like that video and heater. That guy, Jeff,  is great. How can I become so seemingly laid back and confidant. Lay off the coffee would be a start lol.
I've considered similar systems. Water jacket boilers, wood fired, were given a poor rating as far as efficiency so I looked at other designs; Batch heaters, coils, solar preheat, gasification stoves, etc. Just remember the "three T's" I learned from Ianto Evans; time, temperature, and turbulence in the combustion, firebox, and heat transfer department. All 3 of these are improved, over a wood fired boiler in the water jacket design in the video. 1000 degrees is hot compared to 500-600 in a big firebox wood fired boiler. There's a lot of other factors of course such as mass, surface area, delta T, conductivity, creosote, etc. My thinking has switched to low mass systems as I'm doing mobile systems the past few years. Youre higher mass. There's a couple assumptions or oversights in the video. Like a good salesman he didn't mention that a high mass stove and water jacket water mass and heat transfer takes time to warm up. I'm dubious that this design can perform like a on-demand water heater unless the water in the jacket is hot already??? Did I miss something? I did get a bit distracted.
If it's inside a warm space and used everyday the time lag is reduced. I've gone back and forth over rocket stoves a lot. Building a "upside down" fire, feeding, mass, etc. I like that one can adjust the "personality" of the stove. Leave more exposed metal to create a "flash" stove and you have a quick temper. Embed more mass a slow and even temperament. I've thought of a movable fire insulation cover for the barrel so one can heat up a cold space rapidly then cover it once the space is warm. The heat would go into the mass then for slow release. Or it could be designed to go into your tub mass after the house is flash heated? This is the fun of designing ones own system.
As far as safety I think they are both open and similar but don't quote me. If the water mostly leaked out the residue could flash.
If you find a good stainless water tank in the USof A please let me know where. However, I did just discover the beer brewers are modifying stainless kegs to heat water. Finding someone to weld stainless is tricky. Several places have turned me down for liability reasons. But if you say it's a open system they might change their tune.
Oh, I would do a vertical coil in the water jacket if you want reliable thermosyphon. As far as I know high points are possible air locks. The design in the video is a pressurized water system going through the coil.
There's a lot of variations isnt there? Have you started gathering materials. That can take some doing and time.  Best wishes

Regarding safety in the previous message there would be very little water to make steam in either a coil or water jacket that have leaked. A bit more in the jacket perhaps? But in either case as long as it's open it would "run out of steam" before becoming hazardous. Keep children away from the out vent in case it spurts over a little.

Hi Jeremy,

I don't know how quickly Geoff's tank would heat up.  It seems that all the hot air from the heat riser goes directly into circulation around the hot water tank.  Geoff seems to imply that positioning the tank horizontalally

forces the hot air in a spiral around the tank.  I wonder how re-positioning it vertically would influence this airflow.

I know little about welding.  I'm trying to resolve the design, first, then I'll approach some friends or family that weld (I'm legally blind or I'd learn :)
As for materials, I've got a tank and will probably purchase the 4" dragon heater precast core.

I'll do a modified design plan drawing and post it soon.
I'm thinking of Geoff's system, only vertical so it will siphon.  This will, however, require my elevating the tank significantly, or sinking the stove in order to establish the vertical siphon gradient.

HI Brian. How's it going on your design. Yes, the 'relative placement' of components in a system is a challenge. Also, I'd be curious to ask Jeff how long it takes for hot water production from stone cold fire up.

Can't help you on the design front, but this video might leave you some clues.