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False Profit Labs built itself a wood-powered on-beach hot tub as a side project. After all the blowing stuff up that we do, we decided that we needed break.

Basically, we took an inflatable hot tub out to the Ocean Beach here in San Francisco, filled it with salty ocean water, built a bonfire, set a heat exchanger up on top of the fire, and pumped the saltwater through the exchanger until we had a nice, hot tub to lounge around in.

Here's how we did it!

Note: Thanks to our photographer friend Loren for taking some pictures at the beach. The well-lit nighttime photos are his. Naomi Dabby took our intro photo, below, at Priceless.
 
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Step 1: Build the Heat Exchanger

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The heat exchanger is basically a coil of copper tubing that spirals up the inside a vertical chimney. We're trying to get as much heat from the fire as possible, including the hot air and smoke that rises up from it.

We made our heat exchanger out of 3/4 inch tubing, various fittings for the ends, and 12" diameter cylindrical sheet metal duct. The whole thing ended up being about six feet tall. Click through the photos to see the exchanger under construction. Whit coiled the tubing, stuffed it up into the duct, and then brazed the connections onto the ends.

Whit says the copper got work-harded as he coiled it, so that first step was more annoying (and resulted in a less-regular coil) than we expected. But it worked just fine.

Step 2: Mount it on a Frame

Next, we welded the heat exchanger onto a frame that would sit neatly on top of a bonfire. Brett donated an old steel end table that gave us the basic shape. Ben cut a hole in the top with an oxyacetylene torch, welded angle iron to the legs for stability in the wind, and then welded the exchanger on top.

The whole contraption made resembled some sort of crazed steampunk sleigh. In the last photo, Ben demonstrates the "crazed" part.

Step 3: Head to the Beach!

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Get your Materials together and go. Photos of the other pieces of the puzzle are below: An inflatable hot tub, a small generator to power our pump, the pump, and a bunch of hose to hook everything up.

We brought a bunch of firewood, too, of course. We stuff everything into a couple cars and headed over to the beach around sunset.

Step 4: Fill Your Tub with Ocean Water

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The photos on this step show what should have happened: We'd pump ocean water into our hot tub.

In actuality, we hit the beach at low tide and the water line was more than 200' from where we were allowed to build bonfires. Our hoses wouldn't reach, and we were worried about our pump choking on sand.

Thus did a saga of bucket-hauling begin. It took six of us a few hours to hump 200-300 gallons up the beach to our hot tub. We used big buckets, coolers, etc.

Next time we'll figure out how to use a pump, because... damn. This step definitely made the eventual soaking in the hot tub that much more awesome, however.

Step 5: Bonfire!

Next, we set up the heat exchanger and built a nice big fire underneath it. We had to make sure to get our water running before the fire got going. Otherwise, the exchanger would get absurdly hot, and we didn't want to risk our fittings melting off or the tubing itself getting slagged.

Note that our exchanger would have worked better if it had some partial windscreens on the sides of the frame. A lot of the heat was getting blown out from under the exchanger, so we would periodically add sheets of plywood to block some of the wind. The plywood would continually burn, of course, so it was a bit of a chore to keep adding more.

On the plus side, the water in hot tub was getting warmer by the minute. The stuff coming out of the exchanger was scalding hot!

Step 6: Explain Yourself to the Authorities

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As you can imagine, our steampunk-sleigh-electric-generator-inflatable-hot-tub contraption attracted some attention. It wasn't long before a ranger cautiously approached to ask us what the hell we were up to.

Brett was ready for him, though, armed with a stack of printouts on beach usage regulations and email correspondence with various bureaucrats discussing said regulations. He unleashed a torrent of minutiae at the ranger, somehow still maintaining total politeness.

The ranger, bewildered, simply said, "Look, I just want to know that you're not going to try to use that inflatable thing as a raft."

"Ah," we said, "No."

He was satisfied with that, so our hottubbing was on!

We didn't take a picture of the alarmed ranger, for obvious reasons. The photo below is by Flickr user psychofish (Some Rights Reserved).

Step 7: Enjoy!

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The pictures tell it all. We spent an hour or two soaking in the hot, hot water (we got it up to 104 degrees!)
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DoğanT22 days ago

Are you crazy? :)

ntense997 months ago

The cover photo looks like FUN! HAHA, well I also made a wood fired coil-based heating system: https://diybarrelstoveoutdoorfurnace.wordpress.com/

Boy I looked at the first pics and it looked like as if the wind was not warm so how did you get out all wet and freezing?
Wyattr551232 years ago
dig a hole down to the water table and use a shirt to filter out the sand
chuckyd3 years ago
It seems that a three-sided box would cut down on the wind action and capture a great deal more heat for the exchanger. It seems that very little heat is using the exchanger, actually. Also, maybe you would need such a hot fire, and use a smaller fire that would last longer.
Aron3133 years ago
I she wearing a bathing suit?
Tizaro3 years ago
perhaps if you had a barrel for the fire (think hobos under a bridge kind of barrel) then just mount the exchanger ontop of that?
80$man3 years ago
nagutron. Nice work, looks like a great idea for a beach party! I like it!

Question: how did you design the heat exchanger? Did you use cross flow heat exchanger theory or did you 'wing' it? I you would like to improve the design, I could send you some calcs+suggestions? I'm glad you guys used copper tubing, high ductility for coiling and great conductivity for heat transfer!
nagutron (author)  80$man3 years ago
Totally winged it! We won't be doing this again in the near future, but if you have sketches for better designs, please do attach them to this comment thread for the benefit of people who may be attempting this now.
80$man nagutron3 years ago
Nagutron, time is working against me now, I will revisit this project in December, I will attempt to design a proper concept, but for now here are a few pointers for people who want to improve the design:

Preface: please don’t build a heat exchanger that can produce a steam supply directly to the pool, run the heated water through a mixing vessel first! Safety, steam burn like bloody murder! Experiance hurts!

Guidelines: Take into account two mechanisms of heat transfer:

Convection: heated air transfers energy to the water whilst flowing over the pipes, thus increasing pipe area (reduce diameter and increase amount of coils and/or increase coil length and/or adding fins to the piping), funnelling more air across the pipes with a larger cone section in the bottom leading to a converging chimney section would also aid convection.

Radiation: the coals of the fire radiates energy through LOS (Line Of Sight) infrared, thermal radiation, thus the increasing the LOS area of the piping closest to the fire would increase thermal radiation. I would advise adding a short fins to the pipe to aid in radiant heat transfer in a coned section closest to the fire.

Further: the addition of large fins to the tube surface would not necessarily lead to improved heat transfer, where exists an optimum shape. It may be best to stick with multiple thin coils of a substantial length, possible with the addition of modest fins. Start small and experiment!

Finally: Counter flow heat exchanger perform better than parallel flow, but establishing natural flow in the water from the top to bottom of the heat exchanger is problematic. It would probably require the tub to be above the top inlet of the heat exchanger for buoyancy to be sufficient to establishing natural flow (or require forced flow). Natural counter flow is probably not an option, but maybe someone can hack it!

I quickly sketched a very rough concept (sorry that is is hand drawn, time!):
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nagutron (author)  80$man3 years ago
Great notes! Thanks.
robotmaker4 years ago
good idea,but this same instructable was posted awhile back
by same person,any ideas why
nagutron (author)  robotmaker4 years ago
Totally unchanged. Looks like the site just featured it again or something.
would be good to see new instructables ,instead of seeing the same ones posted every few months
True, not everyone gets a chance to see ALL the gems on the site -- good to balance rotating new ones with old.
starfox3653 years ago
No wonder there are no women in the tub.
kwhit1902114 years ago
Can't burn for beans! Ought to be ashamed of yourself for posting a nasty cut hole like that!!
There was a floating version of this in a Popular mechanics Book i the 70s. Powerd by outboard and heated by gas.
menahunie4 years ago
Interesting.
I did similar except I used a on demand hot water propane heater.
Yes I know what your going to say; but in the end it was allot cheaper and I didn't have to deal with wood...
Amazon has them for 115. RV water pump ebay 20.00, large blow up wading pool 25.00, hose 30.00. Propane tank already have 40lb propane 30.00.
I dug a hole and put that black gardening plastic down and then lined the hole with leaves as an insulator. Put the blowup pool in and inflated. Adjusted hole until pool was supported on all side. Filled with water. Put the inlet hose in it that goes to the pump and to the inlet of the water heater. The outlet from water heater into the pool. run the pump on a deep cycle battery that is used in my truck. You can even control the temperature on the water heater and flow. It takes about 10 minutes to warm up the whole pool. Remember the number of people that get in will displace the water.
nagutron (author)  menahunie4 years ago
Insulation with leaves! Smart. That addresses my main issue with the in-ground approach: The thermal mass and conductivity of the ground are really high, and will suck the heat right out of a hot tub without insulation. I'd love to see a photo if you find one to post.
foxie trix4 years ago
nice car :D
Dark Solar7 years ago
A good idea...but tell Joe Hair-shirt to cover his junk next time.
You can't tell anybody anything in SF ...
LOL, true enough. Been a while since i looked over this -ible...the pic in question has been suitably photoshopped in the intervening time so Joe Hair-shirt (the guy directly under the camera but not looking up in pic #4) is no longer showing his crotchly meatery.
gravguy4 years ago
You could really concentrate and control your fire by building a fire box.. My thought is to find yourself a drum from a clothes drier and mount it vertically under your stove pipe. it would be well vented since drier drums have holes in them and the heat path you would gain would be substantial. you would have to put some sort of opening in the front to stoke the fire but not too hard since you have a torch
i don't see a pressure relief valve or anything on the copper coil? did i just miss it? am just curious about what keeps this from blowing up if the water becomes stagnant, which could happen since the pump needs that generator.

...or i suppose the point may be that being an open loop hose, pressure will always be able to release through the open end of the hose. in any event, watching water output temp and making sure it stays below 200 could be key...
It will stay at normal pressure. The end of the pipe is open, there's no tank on it.
rippa7004 years ago
Instead of the complicated heat exchanger/generater/pump how about spiraling the copper and laying in in the fire slightly below the hot tub. The heat will flow up to the tub and cold will return - I use that in my house for a back-boiler on the wood stove to heat the hot water. Should work and be really simple.....
timmccool4 years ago
i would have used plastic and just doug a hole to a larger size. could have even formed seats.
cheesehead4 years ago
amazing ... i think im gonna make a smaller version of this... pure awesomeness. . . in a can.
Azzurro4 years ago
Blimey.. Two guys and six girls. What a waste! :DD Cool instructable and a good idea btw.
Some friends of mine built a sweat lodge/sauna for Apogaea that was based on a similar principle, but instead of mounting the heat exchanger on a frame, he mounted it inside of a 55 gallon drum used as a burn barrel, which eliminates the issue of heat radiating out in all directions and makes the fire more controllable.
I attended a sweat lodge, a few times, and it was done an old fashioned way. They had a huge bonfire, with stones underneath. Fire heated rocks, rocks brought in at various intervals and placed in center pit, tobacco sage & sweetgrass thrown on, then water splashed around on the rocks for steam. Very much totally unlike a European sauna. Hotter than any sauna I've ever been in. This was rough, but awesome, all at the same time!
When I have a chance, I'm going to dig through this thread and the accompaning links. Thanks for the ideas everyone!
nagutron (author)  thebigwheeze6 years ago
That's a good idea. Bonfires are messy and inefficient.
codongolev4 years ago
I once saw a hot tub (not sure if it was real or concept) that had a coil coming off the side. you build a bonfire inside the coil, and when you flood the coil with water, the stuff by the fire expands, pushing the water out. it only comes out the top of the coil, because it's easier due to the water pressure being higher where its lower. this immediately causes a vacuum , which sucks more water in through the bottom (again, it's easier due to pressure) and starts the process over again. it's like a big version of one of those little boats that putts around in the water.

to finish my long-winded explanation, the fire worked as both a source of heat, a pump, and a design point. I thought it was cool.
Isn't that how the Romans heated their hot tubs?
found it!
http://www.xerxy.com/2010/09/20/coolest-hot-tub-ever/
robotmaker4 years ago
i only allow women in my hot tub besides me.i made one close to it to take to the beach
but at home i like a real good hot tub ,LIKE SUNDANCE SPA
lperkins4 years ago
The only thing I can think of to improve it would be to put a turbine in the chimney and make the pump also run off of the fire. :)
I already posted about this, but I saw a tub that did that.... if you look up those boats that run on candles, it used the same principle, except it used a bonfire and attached to the side of the hot tub.
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