Rocket for a 'portable' hot tub

Hello Permies!

I've been reading the forums here for a while, but I thought that with my latest project I would finally make an account to get some input. So, a friend and I are trying to build a semi-portable hot tub. The idea is that we can take it to beaches nearby (we're on the Canadian West Coast), fill it up with ocean or lake water and enjoy! (Plus, its a great little project to play with in our spare time) We have the hot tub part pretty much figured out; really its just a bunch of wood slats that we put together on site and bind with truck webbing/tie downs (think a large wooden barrel, with straps instead of metal bands). So, you can probably tell that in this context, 'portable' really means 'can move it about 500 feet after carting it in with a vehicle'.

What we're working on now is the heat source. The only wood-fired hot tub I've used before was in Norway, and was a standard 'snorkel' tub (wood stove immersed in the tub itself). I'd say it probably took 4 or 5 hours to heat up to something a decent bit above lukewarm. We'd like to be able to get to a higher temperature (ideally, 'regular' hot tub temperature of 100-106F), and *hopefully* in less time as well. We realize that the heating ability/time is related to the volume of water used, and we're going to decide on the final tub volume after we have picked and tested a stove design.

The initial plan for the heat source (and still the backup plan) is to make a simple barrel stove out of a 55-gallon drum. Nothing fancy, just a sideways barrel with a riser/chimney. We would then use copper pipe and create a thermosiphon from the tub, to a copper coil in the chimney, and then back into the tub. I'm not a big fan of this idea because a) bending copper pipe is expensive, finicky extra work (even with ice/filling it with sand), b) it might not produce as much heat as we want, as fast as we want, c) I've never used a thermosiphon design before and I'm not confident it will be able to heat the tub to the desired temperature in the desired time and d) junk in the unfiltered ocean water could clog or bake onto the thermosiphon and really interfere with its operation.

So, this makes me want to try a rocket stove with the hot air flowing directly into the hot tub water (no thermosiphon). I own a copy of "The Book", and I've played with basic designs before, but never with any real purpose. Since I really want this project to work out, I'd really love some input from the permies community! I've got a general idea of what I'd like to make, but I have some questions. But, before I get to those and the stove design, here are all the considerations that I have:

- The stove has to be able to be moved about 500 feet, set up and packed up with reasonably minimal effort and time.
- It has to be able to heat the hot tub!
- It, ideally, would be able to use larger chunks of wood than the tiny pieces usually associated with rocket stoves (think 'batch' stoves). Why? We'll be using it while camping, and trying to chop up knotty driftwood into small pieces is no fun (but, note, it is possible, if we have to).
- We're going to be using it on the West Coast of Canada - so the wood could very well be wet. Wet means anything from 'a bit of water on the outside' to 'we could fill up the hot tub by squeezing out this log'. Obviously if the wood is completely soaked we wouldn't necessarily expect to be able to get the thing up to temperature, but it IS something we have to be aware of.
- Ideally, we shouldn't have to feed the stove every 15 minutes. 30 minutes would be better, even less frequently would be best. If we're in a nice warm hot tub on a cold night, nobody wants to get out!
- The stove will NOT be used frequently, and so the usual issues with metal risers, etc aren't necessarily a problem.
- There will be all sorts of rocks, sand and dirt around. Rocks might be useful for added (imperfect) insulation?
- It would be nice if it could be built cheaply!


So, with that said, here's an exceptionally well-drawn, professional-grade, art-prize-winning drawing of what we were thinking of:



Base/Combustion chamber:
- Would likely be built/moved as a single unit. The feed, burn tunnel/combustion chamber and an optional batch-box would all be included in a larger box surrounded by an insulator. I was inspired by posts like the one here: https://permies.com/t/29470/labs/rocket-mass-heater-shippable-core.
- The batch-box is optional, but would be nice in order to use slightly larger pieces of wood (less chopping, more time between tending)
- I'm aiming for 6" diameter for now
- If there was no batch box, I was thinking about making a magazine/feed tube that was larger and closed. Air would enter from a different set of holes. This would hopefully give me more time before I had to tend the stove.
- Any feed magazine or batch box might be a detachable unit from the main burn tunnel/combustion chamber.

Riser:
- 6" steel tube
- Insulation would be from a ceramic blanket wrapped around the riser.
- Ceramic blanket would be held on by chicken wire or metal ties.
- Height of riser would probably be 3 to 4 feet.

Transition to hot tub:
- Inspired by the standard 'down draft' design, after the riser all tubing will lead to the hot tub.
- Unlike the standard 'down draft' design, I continue using a tube, rather than a barrel - heat can only go one way, rather than all around a barrel space.
- I can either transition directly to the hot tub heating pipe, or create a larger space so that the heat intensity is reduced a bit before entering the next set of pipes.
- Would a larger space/bell at the top of the riser be of any benefit?

Hot tub heat exchanger:
- I could have the heat enter from the bottom of the tub or the top. I think the top is preferable, since that way the air cooled by the water has a downward trend in the same flow/direction as the exchanger.
- I don't know the best way to maximize heat transfer to the water while maintaining a good draft. I was thinking one of two options: a) airflow splits at the top of the water into 2+ smaller ducts, with an attempt made to maintain the same overall cross-sectional area, or; b) there is only one pipe, which snakes back and forth. Option b would be a lot easier, but I'm worried about losing flow with all of those curves.
- Eventually, airflow all combines and exits the tub and heads up the chimney and out.


What do you think? Is this design feasible? What modifications would you make? Any and all advice is appreciated!

Not addressing other factors, I don't see any reason to exit the tub at the bottom. Just turn the duct and head straight back up - that gives more heat exchange surface and makes construction considerably easier.

I doubt there is any benefit to a chamber above the riser - even if the gases expand, they will only cool by wasting heat to the air. Induction (drawing in cool air to dilute the superhot air) would probably not be effective since you need to go down significantly after this point. I say just get to the water as quickly as possible.

For the fuel feed, the drawback to a batch box is that it burns intensely hot and can't easily be stopped if your water overheats. Rather than a tube above the J feed, I would use a cage of vertical strips of sheetmetal, maybe V-crimped for rigidity, with gaps between, held together by hoops on the outside. The idea is to present a smooth vertical interior surface with nothing that a knot could hang up on, while leaving lots of open space so the feed can't form a hot chimney. With the whole thing being in the open air on sand/rock near water, I don't think safety is too big of a problem, as long as the whole thing is properly stabilized.

Thanks so much for taking the time to reply!

Glenn Herbert wrote:Not addressing other factors, I don't see any reason to exit the tub at the bottom. Just turn the duct and head straight back up - that gives more heat exchange surface and makes construction considerably easier.

We were thinking that the current design would improve flow - the air would cool in the water and would fall in the same direction as the piping flows. We thought going up in the water might cause issues with flow, which we're trying to be very careful about since our fuel may be damp. That being said, in retrospect I think you're right... the extra work of making and sealing a hole in the side of the tub probably isn't worth the gain of flow. Also, it would be nice to have the added surface area!

Glenn Herbert wrote:For the fuel feed, the drawback to a batch box is that it burns intensely hot and can't easily be stopped if your water overheats. Rather than a tube above the J feed, I would use a cage of vertical strips of sheetmetal, maybe V-crimped for rigidity, with gaps between, held together by hoops on the outside. The idea is to present a smooth vertical interior surface with nothing that a knot could hang up on, while leaving lots of open space so the feed can't form a hot chimney. With the whole thing being in the open air on sand/rock near water, I don't think safety is too big of a problem, as long as the whole thing is properly stabilized.

At this point, the tub getting too hot would be a great problem to have! That being said, if we find its a problem we thought we'd install a simple flue device in the top run of piping which would direct the hot air from the riser directly into the chimney rather than the heat exchanger. We'd use this to direct the air out if things were getting too hot. Its a finicky bell/whistle that we're not too worried about right now, but maybe we should be?

RE the feed cage: I'm not sure that I'm properly picturing what you're describing... something like this?



I really like this concept, since it would be light and easy to transport and set up. Keeping it open to avoid becoming a chimney is a great idea! This leads me to a question I've had for a little while now. As I understand it, its pretty important to keep a relatively-constant (or very slightly increasing) cross-sectional area throughout the piping to encourage good air flow. In the standard J-tube design, the intake is partially blocked by the wood in the feed, and so you'll get a lot of variability in the actual cross-sectional area of the intake depending on how the wood is positioned. Wouldn't it be better to have a closed feed and have a seperate, constantly-sized air intake elsewhere (like the holes in my initial drawing)? What would be the effect of a closed feed magazine/stack on air flow, if air can't actually escape out of it?

Thanks again!

You have the slats & hoops idea correct, with one point to clarify: it needs to be completely vertical, not funnel-shaped, so the sticks don't slide down and get jammed together. It needs to be such that if a bunch of sticks fits in the top, they will slide all the way down into the feed tube.

"What would be the effect of a closed feed magazine/stack on air flow, if air can't actually escape out of it?"
The effect would be that the fire would burn up into the closed box and generate a big mass of hot gases, which if you opened the box to put more wood in would make a fireball that would take your eyebrows off at the least.

The bypass damper in the duct to let heat go straight up instead of overheating the water sounds very practical, and reachable from the tub without getting out.

Hi

Our rocket heater has an attachment specifically designed for hot tub heating (thoroughly tested over a year). The stove is compact and can easily be carried by two adults. Check out my recent posts or www.perfectburn.co.uk

thanks