Rocket hot tub

Hello,

I am looking for some advise. I have a plan and I don't know if it has any merit, so I would love some feedback from more experienced rocket builders.

I have a 1.2m diameter aluminium tank which I've cut in half. One half is a cold plunge pool. The other half I want to turn into a hot tub.

I tried making a rocket fire already which had the flames running over a copper coil... but the heat transfer was so slow that I would have had to wait for half a day just to heat up the water. I also made it from old benchtop offcuts which began disintegrating under the extreme temperature. So it eventually destroyed itself anyway.

I figured that direct heating would work better. So I thought why not run the burn tunnel in direct contact with the base of the tank. You can't really get more direct heat transfer than that.

My question is whether the dimensions (see attached images) will allow me to run this properly. As I said I'm not very experienced at this stuff so I would love someone else who is to let me know whether what I'm proposing is feasible.

It seems that burn tunnel is longer than recommendations that I've seen before. What happens when the burn tunnel is longer? And what effect does the width of the burn tunnel have too?

The burn tunnel is currently 300mm wide. This means that the burn tunnel has to be a minimum of 1068mm long (see attached image) plus the space required for the chimney and feed. I could make the burn tunnel narrower which would mean the chord length across the edge of the tub is a shorter distance to cover. But I'm also not sure whether this is important and what impact it would have reducing the width of the burn tunnel. It seems there's some competing variables in this equation.

Would love to hear your feedback! Thankyou

Hi Tom;
Since you are not building a mass heater, specifications are much more forgiving.

With your current plan, I think you need to stop trying a J- Tube design and switch to an L- Tube instead.
Your burn tunnel is very long I doubt it would work well as a J but as an L it should be just fine, as you can push your coals toward the riser.
Insulate below the core and around the outside of your tank to keep the hot where you want it.

Wish I still had access to the pictures to share. We heated out water with a rocket stove for 8 or so years and it was quick and stingy with fuel use. We could heat 50 gallons from ambient temp to 150 in under an hour with about 1/2 a 5 gallon bucket of small split wood as long as the bucket. Biggest problem was flashing the water to steam and blowing out the sacrificial link. I had the 1/2 in 50 ft copper coil inside the rocket stove. It was made out of metal and burned out after 8 or so years. Now I heat water with solar powered electric water heater elements. I did post picks on several of the older homesteading / off grid forums as BiggKidd.  This would have been in the early teens 2013 or there abouts.

Thanks for your replies

Thomas, I have a question about the L tube design. Is that a lot more work to keep attentive to the fire to make sure it doesn't burn out? I guess the gravity feed of the J shape would mean that you wouldn't have to be there so much. Because that would be something that seems important, particularly if it's a decent amount of water. I reckon it'll be 400 litres or say 100 gal.

Something that came to me as I slept was to have the burn tunnel turn a corner. This could potentially shorten the tunnel length. How short should I go though? All I really have heard and have been operating on is that the dimensions need to follow a ratio of 1:2:3 . This would suggest that the length of the burn tunnel would even be fine in my original drawing assuming I can get a riser that is tall enough. Would you agree with that statement or is there something else I am missing?

And yes, I agree that insulation is key here.

Larry kidd. I looked up your project. Nice work man...and it sounds like really good performance. Seems similar in concept to what I tried. But I wasn't able to get anywhere near the performance that you're suggesting out of it. I wonder why... hmmmmmmm

Tom Wilson wrote:Thanks for your replies

Thomas, I have a question about the L tube design. Is that a lot more work to keep attentive to the fire to make sure it doesn't burn out? I guess the gravity feed of the J shape would mean that you wouldn't have to be there so much. Because that would be something that seems important, particularly if it's a decent amount of water. I reckon it'll be 400 litres or say 100 gal.

Something that came to me as I slept was to have the burn tunnel turn a corner. This could potentially shorten the tunnel length. How short should I go though? All I really have heard and have been operating on is that the dimensions need to follow a ratio of 1:2:3 . This would suggest that the length of the burn tunnel would even be fine in my original drawing assuming I can get a riser that is tall enough. Would you agree with that statement or is there something else I am missing?

And yes, I agree that insulation is key here.

Larry kidd. I looked up your project. Nice work man...and it sounds like really good performance. Seems similar in concept to what I tried. But I wasn't able to get anywhere near the performance that you're suggesting out of it. I wonder why... hmmmmmmm

I think the layers helped a lot. The way the burner and riser were insulated with wood ash and the multiple propane tanks held the heat in and stirred it around the copper coil.  Also with the bubble of the tank with the large layered coil inside gave the airflow a chance to slow down a little and absorb the heat. The coil was wrapped like layers of old stove eyes giving a LOT of heated surface area and the heat had no choice but to rub across it had no way around.

Another thing you want to fasten the coil(s) to something to give it some rigidity so it doesn't fold up / collapse. I learned that the hard way.

Hi Tom;
The problem with a J-tube and a long burn tunnel is the fire will find that it is easier and shorter to burn up the feed tube rather than the riser.
Your drawing does not show how tall you were thinking to make the feed tube.
On a standard 8" mass heater the feed tube is 8" square and 16" deep, the burn tunnel is 8" square and its roof is no more than 12" long, and the riser is generally around 4' tall.
Trying to enlarge those numbers will create friction and ultimately the path of least resistance is backwards up the feed tube.

With an L-Tube you can use as large a tunnel as you like, with as tall and big a chimney (riser) as you want.
Yes, you will need to push coals towards the riser, big deal, with a larger burn tunnel and no J bend for the feed tube you can fit in larger longer wood.
You were going to be feeding a J-Tube every bit as much if not more and I promise at times it will find the feed tube wood is very much easier to burn.

Large L-tube designs work perfectly for boiling maple sap into syrup, very similar to heating water in a hot tub.