Rocket Hot Tub! - PTJ 2022

I'd love to do a project like this!  One thing I'm considering is incorporating a rocket stove into a wooden ofuro-style tub.  We don't have the appropriate cedar or cypress, but I'm wondering if I could fell and mill up some black locust for the project.  Any wood-meisters have any input for me?  

Attaching a picture of the basic idea.

I've watched all the resulting videos in this search:
https://www.google.com/search?q=ofuro+outdoor+diy&client=ubuntu&bih=602&biw=1280&hl=en&tbm=vid&sxsrf=APq-WBtjtuDzwudW8ey5xPAE-8cW0ewm1g%3A1649192957098&ei=_a9MYuu_BbKGqtsPhu2-kAw&ved=0ahUKEwirsv7C6v32AhUyg2oFHYa2D8IQ4dUDCA0&uact=5&oq=ofuro+outdoor+diy&gs_lcp=Cg1nd3Mtd2l6LXZpZGVvEAMyCAghEBYQHRAeOgQIIxAnOgYIABAWEB46BQgAEIYDUPkBWNcFYPkGaABwAHgAgAGUAYgBsQSSAQMwLjSYAQCgAQHAAQE&sclient=gws-wiz-video

I'm wondering how a rocket design could help with some of the issues I had with my wood heated hot tub.

I bought a house that had one, it was made by the previous owners. It took all day to heat the hot tub, feeding it wood for hours. Once it we brought the water up to temperature, it was easy to 'over heat' and we would then end up adding cold water.

It was hard to hit the "sweet spot" for temperature and it took a lot of wood and a lot of time to heat up!

Ashley Cottonwood wrote:I'm wondering how a rocket design could help with some of the issues I had with my wood heated hot tub.

I bought a house that had one, it was made by the previous owners. It took all day to heat the hot tub, feeding it wood for hours. Once it we brought the water up to temperature, it was easy to 'over heat' and we would then end up adding cold water.

It was hard to hit the "sweet spot" for temperature and it took a lot of wood and a lot of time to heat up!

What you need to have, Ashley, is a cut-off PLUMBING valve from the wood heating source, be it a barrel stove, rocket mass heater or any other wood style heating. It could even be that it is not a total on/off valve but one which allows you to reduce the hottest water to varying volumes while allowing the introduction of some cooling water.

As long as this hot tub/spa has motorized jets, appropriate mixing to ideal temperatures should not take too long.

OF COURSE, the above means that there must be some type of pressure relief valve so that huge pressures NEVER develop in any portion of the water heating system. The old Snorkel stoves were great [never had one] so I don't know how folks handled whatever needed mixing was needed to get to ideal tub water temps.

Just had an odd thought after reading this thread:  was considering my dream greenhouse with a rocket hot tub added to it... previously I had thought about having a rocket mass heater for the colder winter nights in my someday greenhouse (with the hot barrel safely away from the plantings of course.)  So, my odd thought was to have one central rocket burner "section," then perhaps a damper to switch between a hot tub and a slab-bench-mass that runs most of the length of the greenhouse to flow the hot air through?  Cuz sometimes you feel like a soak, sometimes you don't!  That would allow control of humidity as well as heat output into the greenhouse.

Or maybe that is just overkill and I should plan to snake a single flue around a hot tub AND through a slab bench mass?   Build the hot tub INTO the slab bench mass?  Could a damper-type switch be used to create heating zones in a home using one central rocket mass heater and alternate paths through the house and out?  Or perhaps pull-up metal plates that open a flue path or push down to close it, allowing both (or multiple) paths to be open (to varying degrees) at the same time?   Or would that mess up the air flow and adversely affect the burn?

Or maybe I just shouldn't try to think so late at night....

Here's some first thoughts...

a) The barrel could protrude into the tub, rather than sit outside of it, in order to transfer more of its heat directly to the water
b) The stratification chamber can warm both butts and water
c) The whole structure could be surrounded by insulation, such as perlite-cob for the floor and hempcrete or other lightweight fireproof (and hopefully fire-friendly) insulation

I have a snorkel stove hot tub. It takes 3-4 hours to heat it. When the water is 90 degrees, don't add any more wood no matter how anxious your bathers are. It will be 102 degrees about a half hour later. If it's 106, people will go in and  out quicker. I find adding water is a waste of water. I put a plywood cover over it and go again the next morning. I got 2 free aluminum fin solar collectors and hooked them up last fall. Just now getting ready to try that. I get the most use out of it as a cool pool in summer.

I can't wait to see what comes of this project, should everything work out. I *need* somewhere to soak my bones come winter, and plan to build a "hot tub" tub / shower into my roundwood timberframed cordword/cobwood cabin utilizing a RMH.

Hot water soaks are MANDATORY up here in the winter, even if only once every 3 or 4 weeks.

The BTUs needed for a good hot soak in 50+ gallons of water are somewhat enormous, plus the conundrum of transferring heat from a rocket to the water both safely and efficiently can be tricky - very interested in what sorts of ingenious solutions are worked out for this.

We live in a similar climate as The Lab and my thoughts go-to keeping that thing from freezing solid and cracking the sides out during the winter if there isn't a whole lot of usage.  Could something like this rocket soaker maybe incorporated into a Truly Passive Greenhouse/WOFATI design or would the moisture/humidity be too much for a structure like that?

I’m curious what permies with hot tubs do for maintenance!

Is there an alternative to all those chemicals?

I bought a house that had a hot tub once…. Once I found out what the prescribed processes were, I didn’t want to do it, or get in anyone else’s!  It was a long time ago, and I think I gave it all away!

My motorhome furnace pushes hot air directly outside. So I want to capture that poor design/lack of insight loss for a hot tub. The real need is for a cool water dunking or sitting pool in 120° summer, so in addition to figuring out how to get the hot air into the water (some way to attach a pipe wrapped in copper tubes?) I need to figure out how to clean the water (plants?) so the water (rain harvest is the only source other than delivery) can be used all year long and thus conserved, as I live in a desert.

I was at a festival and had an amazing hot tub which was a very large blue plastic tub with pipes coming in near the top and out near the bottom. It was powered by two central heating radiators which we're leaning together with a small fire underneath. The water heater up, came out at the top of the radiator and up into the hot tub and cooler water ran out near the bottom and back to the lower inlet valve on the radiator. It was very portable and could be moved around on a trailer. Great temperature too.

Jadie May wrote:My motorhome furnace pushes hot air directly outside. So I want to capture that poor design/lack of insight loss for a hot tub.

Hi, Jadie May. The hot exhaust air is not a very voluminous heat source and collecting said heat would also be a wee bit difficult PLUS one never wants to impair that exhaust flow because that is moving the highly poisonous, as in deadly to humans carbon monoxide out of your motor home.

Jadie May wrote:The real need is for a cool water dunking or sitting pool in 120° summer, so in addition to figuring out how to get the hot air into the water (some way to attach a pipe wrapped in copper tubes?)

This cool pool is going to warm up right quick in 120 F summer heat. If it is cooling pool why add ANY heat. We go from a hot sauna into a mountain river which is about 40F in the Fall. One can easily get used to being in such a temperature to cool a body. Especially when it is 120F outside air temps.

Jadie May wrote:I need to figure out how to clean the water (plants?) so the water (rain harvest is the only source other than delivery) can be used all year long and thus conserved, as I live in a desert.

Hot, sweaty bodies can, in such a climate, make water very stinky very quickly which can lead to all sorts of health concerns. I personally would look at using some type of disinfectant that local swimming pools use.

Ashley Cottonwood wrote:I'm wondering how a rocket design could help with some of the issues I had with my wood heated hot tub.

I bought a house that had one, it was made by the previous owners. It took all day to heat the hot tub, feeding it wood for hours. Once it we brought the water up to temperature, it was easy to 'over heat' and we would then end up adding cold water.

It was hard to hit the "sweet spot" for temperature and it took a lot of wood and a lot of time to heat up!

I made a rocket stove hot tub and it takes about 2-3 hours to get ALL that water up to suitable temperature.  On the plus side, it’s sort of like compound interest, in that the first hour or so the temperature doesn’t rise much.  But as it warms and gets cycles back through the copper tubing buried within the cob, it’s fetter warmer and then hotter much faster.  I use a laser thermometer and estimate when to stop feeding the stove.  

Mark Persky wrote: I made a rocket stove hot tub and it takes about 2-3 hours to get ALL that water up to suitable temperature.  On the plus side, it’s sort of like compound interest, in that the first hour or so the temperature doesn’t rise much.  But as it warms and gets cycles back through the copper tubing buried within the cob, it’s fetter warmer and then hotter much faster.  I use a laser thermometer and estimate when to stop feeding the stove.  

Nice photo! How is the rocket heater set up/connected to the water in the tub? Is there another thread with information on your tub?

would it not be possible to run the burn chamber under the tub and the barrel inside it?  

Water is an incredible heatsink, I would be really interested to see what temperature the barrel top reaches with several inches- perhaps half a foot- of water above it and several feet around it.

Would make for an amazing thermal battery for a greenhouse in the winter.

Jerry McIntire wrote:

Mark Persky wrote: I made a rocket stove hot tub and it takes about 2-3 hours to get ALL that water up to suitable temperature.  On the plus side, it’s sort of like compound interest, in that the first hour or so the temperature doesn’t rise much.  But as it warms and gets cycles back through the copper tubing buried within the cob, it’s fetter warmer and then hotter much faster.  I use a laser thermometer and estimate when to stop feeding the stove.  

Nice photo! How is the rocket heater set up/connected to the water in the tub? Is there another thread with information on your tub?

Thank you.   I am a total layperson so forgive any terminology I use that isn’t correct.

Basically I built the rocket stove around a coiled up length of copper tubing and connected the ends of the copper tubing to the side of the tub.
When I fill the tub, the water will make its way through the tubing and then I start the heating process.  As the water warms, it rises up through the copper tubing within the rocket stove and ultimately makes its into the tub.  This creates a siphon effect and draws colder water at the bottom of the tub into the lower portion of the copper tubing and INTO the rocket stove where THAT gets heated.  This is a continuous loop and the after gets warmer and warmer, hotter and hotter.

For a much better explanation and pictures of the entire build, here is a link:

https://www.instructables.com/Wood-fired-Rocket-Stove-Hot-Tub/

Hey all,
I am excited about this project too. I am pretty short on time right now so i will have to focus most on the design ideas thatmost excite the residents of the Lab who will be using it and the PTJ attendees who will be building it. I will also try to give your lovely posts a little love. There is so much greaf stuff to comment on already.

As I understand it the Lab residents want to use the hot tub socially so they are pushing for a bigger tub even though it will likely not be treated with chemicals so it will need to be drained and filled frequently and it will take a lot longer to heat up than a small two person tub would take.

Who here is planning to join me for the build in July?

Would love to have something like this for the cold Montana winters! Also curious about maintaining cleanliness without chemicals.

Regarding maintenance and cleaning.  I am having a really weird and probably impossible idea.  Would it be possible to have a small floating island of water-cleaning plants that can live in the  tub when not heated, then be transferred to a "holding pool" during heating?  Like a chinampa, with iris, american water lotus, I don't know what else.  Park it in the hot tub when not in use to help keep it from getting stanky.  

I'm skeptical, because it seems like the ecosystem would be constantly recalibrating across a crazy amount of variables.  But then, roots can be pretty incredible purifiers, and some are quite resilient and robust . . .

For maintenance, there are solar powered pool ionizers or UV sanitizers that float on top of the water.  The ads say they can reduce your chlorine use by 85-90% but i would just reduce it completely and maybe hand clean or change the water once or twice a year if needed.  I don't know everything about them, but a few different kinds are available on Amazon so anyone interested could read their descriptions, specs and reviews to see if they are worth investigating further.  I don't know whether any purchased inputs need to be added regularly as with normal pool chemicals, but I would hope not.  Perhaps worth checking out if the water is going to be left uncovered and exposed to sunlight.

Hmmmmm

The idea of the floating island got me thinking. I don’t think I would want to take the plants out and put them back in. I think that might be too hard on the plants but maybe  you could have a reed bed or purifying type community or bio filter,
And maybe run the water through that living filter. For people that kept the hot tub hot you could have a counter current heat exchanger so that the water cooled off before it went through that bio filter and then regained its heat before it went back into a hot tub.

All of that is just conjecture I have no idea if it would work, and no experience with hot tub or swimming pool maintenance

I now have a couple of hours of talking to mud on the phone about this.  I think we have gone through about 8 general design approaches.  

Looking for something that will seat 6 people.  

Feeling like the smart way to go is something that might go into hot tub mode once every month or two - the rest of the time it would be empty.  

Also, when the urge to do this strikes, it would be wise to heat it up quickly.  

Another idea is that this will be experimental - so something with room for future expansion.  Test now, and add later.

Another is that a leaky tub is good - if you do your hot tubbing in winter and somebody forgets to drain the water before the water turns to ice - everything sorts itself out.  

We talked about a variety of stuff where we heat from underneath.  We talked about cedar tub options (cooper style).  We talked about wood boxes.  

I think where the conversation is now:   what if we make a hole in the ground, fill it with a lot of clay, and then add a bunch of rock with a sand and clay mortar.  Then run a fire in that for about 18 hours to vitrify the clay.  A drain in the bottom that is plugged with a simple rubber flap.  Next to all of this is an unpressurized rocket water heater.  We fill the tub with hot water and can add more hot water as time passes.  It might take two to four hours to fill the first time, but later we can add a second water heater to cut the time in half.  

Kinda wanna get feedback on whether the vitrification would take.

??

How about a cob hot tub?

Those are from a book on cob printed by the cob cottage company of Cottage Grove Oregon Linda Smiley and  Ianto Evans were selling it at the cob class I took from them.  

It has photos from all over the world, beautiful practical and durable examples of what can be done with cob.

The build and cure for the cob, then the plaster, then the waterproofing might take too long for your jamboree, but I thought it would be of interest.

paul wheaton wrote:
I think where the conversation is now:   what if we make a hole in the ground, fill it with a lot of clay, and then add a bunch of rock with a sand and clay mortar.  Then run a fire in that for about 18 hours to vitrify the clay.  A drain in the bottom that is plugged with a simple rubber flap.  Next to all of this is an unpressurized rocket water heater.  We fill the tub with hot water and can add more hot water as time passes.  It might take two to four hours to fill the first time, but later we can add a second water heater to cut the time in half.  

Kinda wanna get feedback on whether the vitrification would take.

??

My understanding of this process invites skepticism.  For clay and aggregate to fire without shifting and cracking, the properties and ratios must be very specific.  Testing for site-harvested clay to reach this level of functionality is significant.  Meaning, vitrification without massive cracking is possible, but not easily achieved.  

It seems to me that putting a roof over it and using waterproof burnishing methods it will yield better, more reliable results.  But I would love to be proven wrong.

Love the idea in general.

paul wheaton wrote:what if we make a hole in the ground

Not sure how intensely the ground freezes over your way, but here on the east coast at similar latitude, filling a hole in the ground with hot water would be a losing battle from mid winter through early summer. Perhaps above-ground makes more sense in that context, and though I'm partial to the idea of a cob and rock tub, something along the lines of a cedar tub would reduce the need to fight thermal inertia of an outdoor mass in chilly weather.

Sounds like a job for tadelakt. An ancient Morrocan plaster technology for waterproofing plaster surfaces. It uses a lime-based plaster topcoat that's then sealed (burnished, really, with a polishing stone) using olive oil-based soap. I've done this to a shower, but it's typically used for sinks and traditional soaking/bathing tubs, some hundreds of years old and still perfectly waterproof.

See Michael Ochs' book Tadelakt for inspiration and details.
Tub photo from this video: https://www.youtube.com/watch?v=PBn5W_-g1Xw

 

I think where the conversation is now:   what if we make a hole in the ground, fill it with a lot of clay, and then add a bunch of rock with a sand and clay mortar.  Then run a fire in that for about 18 hours to vitrify the clay.  A drain in the bottom that is plugged with a simple rubber flap.  Next to all of this is an unpressurized rocket water heater.  We fill the tub with hot water and can add more hot water as time passes.  It might take two to four hours to fill the first time, but later we can add a second water heater to cut the time in half.  

Kinda wanna get feedback on whether the vitrification would take.



Hello rocket hot tub enthusiasts from this permie potter.  I will weigh in with what I know about clay and vitrification.  Earthenware clay never truly vitrifies according to technical terminology.  Vitrify means glassify, i.e., liquify at high heat causing a solidified/solid glasslike clay matrix that has extremely low/no absorption of liquids like one finds on porcelain teacups, sanitary ware like tubs/toilets/sinks.  A fired earthenware creation, while durable due to some chemical bond + chemically combined water burned off, will always absorb water-- similar to a flower pot. Freezing would be a factor-- also like a flowerpot. A glaze can mitigate lots of the porosity issues-- but with earthenware, water usually finds its way through little crackles in the glaze and can sog the clay substrate. Potters do absorption tests on our clay bodies by weighing our dry fired sample first, soaking it in water for 24 hours (or boiling it for 1 hour), then weighing again, to find out exactly what the water absorption factor is.  So!   We have not done these tests yet with the Lab earthenware clay and unless it is unusually non-porous, it will behave like a fired flowerpot and soak in/hold lots of water.  Not sure if this would have any advantage/disadvantage for a hot tub situation.  As far as making a large hole and lining with clay, -- clay cracks+fissures are often a feature of inconsistent foraged clays, attractive for artists using them, but less so in construction applications.  This might be fastest way that water might be drained into the surrounding dirt/soil.  Fired fine clays crack the most--which is why potters often combine several clays together creating what is termed a clay body.  A clay body is sourced from several clays with different particle sizes plus a tempering agent like sand or grog.  A good, forgiving clay body with a wide temperature range would produce fewer cracks for sure.  The most forgiving ones produced in the US are clay bodies for Raku ceramic teabowls-- these coarse clay bodies are very sandpaper-y to the touch but do not crack as much and can withstand very uneven temperatures.  

Overall, if this might be built with with Lab earthenware, then there needs to be a way to seal it.  I do not know what the best way would be.  If I were building an outdoor hot tub, I would be tempted to use (re-use) porcelain tile and create an artwork of the thing -- plus use exterior grade cement mortar between the tiles. I would also support the structure with a ferrocement base to keep it together, because this is a lot of work to potentially lose. Ferrocement is expanded metal + rebar smeared with concrete.  I get it if these might not be the first materials permies would want to use due to high carbon footprint and possible toxic gick.  Anyway these are my thoughts.  Lisa

Lisa, I'm so glad you're here on this thread. Thank you for bringing your insight.

In a bid of quasi-related frivolity -

Every time I hear about the Rocket Hut Tub, I think of the movie Hot Tub Time Machine.  I don't even think I ever saw it.  Just one of those memorable titles.  

BUT!

I recently found out a juicy tidbit - we have a Hot Tub Time Machine celebrity in our midst!!!

That's right, Permies' own Ashley Cottonwood, in a starring (read: "extra") role alongside John Cusack, Rob Corddry, and Craig Robinson!



You can't miss her - look for the yellow coat!



Way to go, Ashley!  You're famous!!!

We have plans to make a hot tub with rocket stove too. Kudos on this post... very inspiring!

One point about Japanese personal (one-person) hot-tubbing that might be of interest to hot-tubbers elsewhere: the water (and its heat) is largely conserved a few days at a time. People there shower and wash well before getting into the tub each night. The whole family uses the same tub water for a few days before it is drained and replaced. A cover is put over the tub when not in use so that a lot of the heat remains. In this manner, not so much energy is used in heating.  Also, when people shower, they are often very conservative with water. They'll wet themselves, then turn the water off when they are soaping and scrubbing and shampooing themselves. They'll turn the water on again to rinse. Not everyone does like that, but a lot of them do.

...on fireproof insulation... there is a brand called Roxul. It is made from stone. I've had a piece 1 inch thick x 8 x8 inches resting on my hand and put a propane torch to it.. fire on the hand!! ZERO heat penetrated the insulation. Also, if it gets damp it retains its insulative properties..

Mark Persky's design is awesome. As for the cleaning, how about rigging an in-line UV filter to the bottom line, so that it sanitizes the water as it recirculates through the system?

That way, you could technically re-use the same water for extended periods.

Why not drain the tub after a few uses, before it becomes a health hazard, when ever THAT is.

Use the water as slightly grey water, run it through a reed bed, fill a swale.  Imagine water as a multiple use resource, it can give us blessed soaking time, then go on to another appropriate use.

Dave Dahlsrud wrote:We live in a similar climate as The Lab and my thoughts go-to keeping that thing from freezing solid and cracking the sides out during the winter if there isn't a whole lot of usage.  Could something like this rocket soaker maybe incorporated into a Truly Passive Greenhouse/WOFATI design or would the moisture/humidity be too much for a structure like that?

So I think putting it in a passive greenhouse is a great way to go... I hot tubbed in the snow in Colorado with people who kept their traditional energy hog hot tub in a hot tub room/greenhouse made from a concrete slab (required for their commercial hot tub) with a track set into it on three sides to hold a bunch of secondhand sliding glass doors.  The forth side housed the electrical breaker box (traditional hot tub) the lights for the room, and also served as a "changing area" or basically a place to leave your snowsuit so your clothes didn't freeze on your body on the way back to the house.  Because the spa was heated the room was always warm, so potted tropical plants that loved the moisture did very well there. Aren't raised beds just big pots, basically? And if you build it yourself or use a different style of hot tub that doesn't require a pad like a portable one, you don't need a poured floor. They also said the tub was much cheaper to heat than their neighbors' because the room offered another buffer from the outside cold, more warmth stayed in the room. Another nice thing was that with the cover on it the windows were clear from outside, but after we'd used the tub the condensation made them opaque for longer than I was willing to be outside in snow with wet hair. they had something set up to drain the tub into a collection thing in the room for plant watering when it needed to be drained about twice a year.

You may know this, or not want to hear this, but in Japan, we shower first to scrub off the dirt before we enter a communal bath to soak in the hot water and rejuvenate our bodies. It's a pretty good system. All folks cleanse their bodies before entering the communal pool, called onsen 温泉 in Japan,  then once in, relax and enjoy a communal soak.

I've been soaking in the communal waters for years now, and it's a great comfort to me in more ways than one.  So I encourage you to build out a hot tub with a shower or cleansing area where folks can wash first, then relax in the hot tubby.  It keeps the communal pool cleaner, there's a kind of respect around those who are also using the communal pool and it allows everyone to relax into their own skin.

If I'm speaking the obvious, my apologies.