Rocket Mass Year-Round Hot Tub?

Short version: I want to build a hot tub that integrates a rocket heater AND a bunch of thermal mass that will stay warm enough year round including through a Minnesota winter.

Long version: I have been interested in rocket mass heaters for a long time but never built/used one. I seem to recall reading about them in the early 2000s? I live in a city with strict code requirements and have little to no hope of building a rocket mass heater integrated with my house. However, my wife really wants to have a hot tub, especially in the winter, in our back yard. We tried a crappy inflatable one and decided it is worth the time/money/effort to do a real one. We are OK with using chemicals to keep it balanced on an ongoing basis (most of the discussion I've seen here focused on brief-use hot tubs with no chemicals, filtration, etc). We may even try to use the heater/filter unit from that inflatable hot tub to handle backup electrical heat and filtration.

We have looked at normal hot tubs but we're not excited about the cost, the apparent fragility, or the power use. I have read a thread here about using a rocket water heater to heat the hot tub, and several discussions here and there about various thoughts around rocket heater and hot tubs. So far I have not seen much discussion of what I have in mind, namely, a tub that integrates dry thermal mass and insulation to stay hot, hopefully for days at a time, from occasional firing of the rocket mass heater. Long term I have a bonus idea to run a heat exchanger and supplement our gas furnace from the hot tub/rocket mass heater as well. This would get us the benefits of the rocket mass heater without the code/insurance issues of having one built into the house.

I also saw the thread here about using ferrocement to build the tub, doesn't seem like it went anywhere.

My questions are: What is good about this? What is dumb about this? What kind of materials should I be using to build this and create thermal mass? Could I likely build enough thermal mass + insulation to keep the thing hot for, say, 3 days at a time from a single firing even in 20 below zero F weather? We are planning to live here until we die (another long story) so I'm happy to put in some money, time, and effort for something that will be nice, last, and do the job.

Hi Brock, welcome to Permies! Years ago there was a hot tub display at my local fair. The salesman said that his hot tubs were heated entirely by the waste heat from the pumps. The shell was insulated really well, so it kept the water nice and warm. Now, Minnesota gets quite a bit colder than Washington does. But I think it could be possible to achieve your goal.

I'm thinking of something like this but instead of a bench stratification chamber, have that be underneath the seating of the tub somehow

https://www.youtube.com/watch?v=SgR7z9sHsXI

Hey Brock.
You’re talking about adding thermal mass and insulation.
I think you should focus on insulation with the build because you will already have a gigantic thermal mass with the water.
The tricky thing will be to transfer the heat from the rocket to the hot tub.
I’ve been thinking about it off and on and haven’t seen or come up with anything that seems long lasting and low tech.
If you use copper piping with a large enough diameter and the tub water level is higher than the rocket you could use a thermo siphon. But that would mean you need to get under the hot tub somehow. Or have a slope/ terrace.
I’m super interested to see what the community here comes up with.

Benjamin Dinkel wrote:Hey Brock.
If you use copper piping with a large enough diameter and the tub water level is higher than the rocket you could use a thermo siphon. But that would mean you need to get under the hot tub somehow. Or have a slope/ terrace.

These folks seemingly did a successful thermal siphon.

https://youtu.be/ZRg7-7rSYL0

I would much rather have the body of the rocket mass heater be the body of the hot tub. I’m thinking of having the gubbins in one corner and the exhaust pipe running around under the benches. I ordered a couple of relevant books. Wife is on board with the project.

Pretty easy concept to place a hot tub over a heated mass with insulated sides and an insulated lid.
Use a batch box of similar and feed the heat into a low flat bell or bench, the bench could be made from cast concrete to support the water.
However, at the end of the day a rocket stove is just a smallish efficient  wood stove and will have its limitations  so dont expect to easily heat large volumes of water in cold weather with one burn, more like multiple burns over several days.
Also this could be a quite expensive build.

Three days at -20F at hot tub temp is a stretch.  But you can should stay above freezing and not need to fire it for that long.

Build it like the big octagon tubs with a bench around the outside and deeper in the middle. You can leave one side full depth with a backrest for the deep soak. The bench is the stratification chamber.

Need to have a really good insulated lid. Need to have a little bit of mass between the Strat chamber and the tub so the water doesn’t suck the heat TOO fast.

If you search YouTube for cowboy hot tub you can find uninsulated J tube rocket sunk into cattle tanks. I had one years ago, but used it as a cattle tank, it was the cheapest deicer I could build. It was a 55 gallon barrel pocket rocket design. I could get a 650 gallon tank from iced over to simmering in an hour at 5f if there wasn’t any wind.

I designed and helped build a rocket fired hot tub some years ago in Massachusetts. It was a crude affair with an all-cob J-tube feeding a cob-surrounded "bell" between boulders with an old cast iron tub perched on/between them. I was not able to stay to see it operate, but was told that it got the tub hot enough, though because the bell cavity extended up to the edges of the tub, the tub rim got too hot for comfort. It should have had the cavity stop halfway up the tub sides.

So a rocket hot tub is perfectly feasible, and if built with more sophisticated materials and layout, should work in your conditions. I agree that insulation is key, both around the bell enclosure and on top of the tub. The mass you need to make the bell and support the tub combined with the water will be sufficient if properly insulated. I would absolutely advise building the combustion core next to or under the tub, with the tub elevated enough that the hot gases can directly heat the tub floor. Steps leading up to the tub are not difficult and raising it will minimize the construction complexity and operational maintenance. If you have any slope or hillside, you are ahead already in layout ease.

Glenn Herbert wrote:
So a rocket hot tub is perfectly feasible, and if built with more sophisticated materials and layout, should work in your conditions. I agree that insulation is key, both around the bell enclosure and on top of the tub. The mass you need to make the bell and support the tub combined with the water will be sufficient if properly insulated. I would absolutely advise building the combustion core next to or under the tub, with the tub elevated enough that the hot gases can directly heat the tub floor. Steps leading up to the tub are not difficult and raising it will minimize the construction complexity and operational maintenance. If you have any slope or hillside, you are ahead already in layout ease.

Thanks, I'm waiting on the Wisner rocket mass heater book to make sure I have my head screwed on straight, but I'm thinking of putting the feed box in a corner, down to a J pipe and then up into a barrel (same as bell?) which would protrude from the top of the whole setup. I would then like the exhaust pipe to run around the perimeter of the tub somehow, under the benches, and come back around to the barrel and use the "kiss the barrel" approach Paul recently talked about on the podcast, to help drive the draft up the vertical portion of the exhaust. I am a little concerned about getting the draft strong enough in this situation, I understand I would need the horizontal bits of the exhaust to slope up as they make their way around.

One big hole in my thinking right now is the water-holding aspect of it. I'm looking at ferrocement or stainless steel for the water-contacting bits of the tub. I worry that cob will not hold up well in this situation if it gets any contact with water, especially on an ongoing/repeat basis. We are also considering tiling the inside. I am concerned about thermal expansion between the hot bits and the water-holding bits, and definitely don't want any cement to crack due to thermal cycling. In that regard, perhaps stainless is better.

I will put together some drawings and calculations once I have the Wisner book and submit it for further critiques. I am also concerned about how this will go over with the Department of Making You Sad. It sounds from my research like I can perhaps do it as a masonry heater but that remains to be seen.

Biggest problem with a wood-fired tub is controlling the temperature. Max temp for most electric hot tubs is 104F or 40C with a high-limit that automatically shuts the tub down around 118F. If your body overheats, you may get dizzy, weak and even pass out.

Even in an electric tub with thermostat control, you can become hyperthermic especially if you have preexisting conditions

https://www.nbcchicago.com/news/local/two-elderly-women-stuck-in-hot-tub-at-kentucky-cabin-rescued-by-first-responders/3814261/

Both were overheated, developed hyperthermia and became unresponsive. One member of the group stayed in the hot tub to keep their heads above water while a fourth person called 911, authorities said. By the time rescue crews arrived, the caretakers of the cabin - who were also called about the incident - had gotten the women out of the water.

One woman was partially alert, while the other was completely unresponsive, authorities said. A search and rescue team member, who is also an EMT, assessed both women.

The unresponsive person was taken to a shower, where cold water and ice were used to cool her body's temperature. The less critical patient had ice and cold compresses applied and was moved outside, where she was cooled off by a hose.

After roughly 20 to 30 minutes, both women began to recover and were taken by EMS to a local hospital. As a result of the incident, the Wolfe County Search and Rescue team issued the following reminders about hot tub safety:

The recommended maximum immersion time is between 15-30 minutes.
Ensure you are capable of exiting the hot tub once you enter, especially if you have any sort of disability.

If you're wood firing then the challenge with a hot tub is more on the insulation side than the fuel side. Rocket stoves excel at using fuel efficiently, but that's an entirely separate thing from keeping the hot thing hot. As others have noted, it is very easy using wood fuel to get your water up to hot tub temps, and even well beyond that into unsafe territory. No rocket stove required, just burn a campfire under a tub for a few hours. But without insulation, when the fire goes out the water will pretty quickly go back to uninspiring bathtub temperatures, assuming you have a reasonable volume of water for the typical hot tub. To hold temperature for a few days like you're hoping for, you'll need very thick insulation, but really that has nothing to do with a rocket or non-rocket stove. And as others alluded to, there is no need to worry about the "mass" part of "rocket mass" here -- the water itself has significantly more thermal mass per unit volume than any building material you're likely to have heard of. If the specific goal is to increase thermal mass, then instead of building thicker walls you'd be better off increasing the volume of water. But the temperature difference here is just too much for thermal mass alone to be enough; you need insulation for this to be practical.

You probably already know about these, but in case you don't, I wonder if there's some inspiration you can gather.  I had one of those "hippie hot-tubs" 30+ years ago.  There is a fair amount of work involved.  If you're not using any jets, you'll need to stir the water frequently while it's heating up.  There's a learning curve in getting the water to your desired temperature without overshoot.  Or else you'd need to add cold water and let it overflow.  Also, consider cleaning when choosing your tub material.  The cedar wood would develop a slippery slime that needed to be cleaned off frequently.  You'd have to drain the tub, scrub it with a brush, rinse and refill.  Remember that water can contain the bacteria that causes Legionnaire's disease.  So water kept at around 105 F can potentially be a breeding ground if you abstain for some form of disinfection.   Lastly, while I loved the hot tub in winter, I rarely used it in summer.  Just didn't find it to be a great experience.  My preference has been to have an indoor bathtub big enough for a reasonable soak.  And even with that I find I tend to just do the utilitarian shower.  Good luck with whatever you try.

https://snorkel.com/product-category/wood-fired-hot-tubs/