Picture of Wood-Fired Ocean Hot Tub
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.

Step 1: Build the Heat Exchanger

Picture of Build the Heat Exchanger
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.
DoğanT1 month ago

Are you crazy? :)

ntense998 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?
Wyattr551233 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!):
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.
starfox3654 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
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!
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.
found it!
ARJOON4 years ago
judging from the title i thought that people was sitting on a big frying pan under which there was a fire
munkless4 years ago
How much did you spend on the copper piping? I wonder if old water heater radiators would work for this project. Very fun project!
ARJOON munkless4 years ago
ok course it will work
Vautikos7 years ago
Somebody's "wooden frame and a tarp" comment just gave me a possibly brilliant idea. Your at the beach right? Dig a hole in the sand, line it with tarp and then fill 'er up. With enough people, and time you could potentially make a massive hot-tub for little more than The cost of a tarp and some copper tubing. Hmmm... Also, awesome 'ible. I'm envious.
nagutron (author)  Vautikos7 years ago
That's a great idea. One disadvantage, though, is heat loss to the surrounding sand (which conducts heat quite well.) The inflatable hot tub was a comparatively insulator, on the other hand. If you have a really hefty heating system or are building this in a warmer climate, though, the tub-in-the-ground idea is fantastic. Perhaps there are tarp layering techniques that would also provide some insulation...
Layers of bubble wrap, maybe?
Inspired by your hot tub and the above comments, I build a tub-in-the-ground variant.  I wrote a bit about it with some photos here: The Hundred Dollar Beach Hot Tub 

Great instructable!
nagutron (author)  nagutron7 years ago
"comparatively good insulator," I meant to say :)
Hmm, a good point. Working at a camping store I have found the possibilities of blue closed cell foam mats to be limitless however.
gregjd50004 years ago
If the copper tubing becomes "work harded" aka work hardened, you can heat it with a torch and when it cools it will be annealed and much easier to bend. I love the idea tho
tinymuscle4 years ago
this all seems so great...but am i missing something?...how does everything hook-up together?
gerjanw4 years ago
Remembers me when we build a Hot tub in the woods and someone called the police, it was or own wood so no problem.

We used 6000 liters water, delivered by the local fire department and heated up with old radiators.
james913755 years ago
Ok guys I have an idea for you How about a hot tub that floats in the ocean, All you will need to inflate is the top ring as the rest of the inflatable will be pushed out by the water inside the pool I recommend finding a pool at least 3 to 4 feet deep. If you boat has power you can add bubbles via an air pump and plastic ring at the bottom with holes in it. weight it down with something non sharp. do you heat exchange off the coolant on the motor. if it is a motor boat. etc
luvit6 years ago
ok. but in my hot tub, no men allowed. other than me.
TIB7 years ago
Hmmm.... just had an idea, but not sure of the feasibility: Your generator probably produces 6 times the thermal energy as it does electricity. I'm wondering if it's possible to modify the generator such that a heat exchanger is placed in the exhaust-stream (would quiet things down a bit too) to heat the water, and the electricity generated used to power both a pump and a resistive heating element, also in the same loop as the heat exchanger? This would eliminate the bonfire, and the manly pleasure of creating such, but things would be much cleaner/controllable. The reason for the resistive heating element is twofold. It loads the generator and thus increases the thermal output, and also acts to directly heat the water. Actually, I suppose if you had a variable load, you could modulate the heat output quiet well. I'd leave the cylinder cooling untouched for obvious reasons.
brokengun TIB6 years ago
I think it might actually be possible to make this whole thing work just based on the temperature of the water. It might not be as fast as the pump but if it was designed correctly the water should boil in the exchanger and rise back into the tub.... It might take some clever moves though.
nagutron (author)  TIB7 years ago
The manly pleasures of fire-building were pretty key for us. Also, I think you'd be thermodynamically better-off by simply setting your gasoline on fire rather than trying to capture the energy from both resistive and exhaust heat.
Coffee bean TIB7 years ago
bmw modified a engine so that it would generate steam to help power the engine. Here is alink linklink.
A great idea. But the execution is a little off. 6 dudes in a hot tub is not the ideal use for such a romantic device.
AGREED! I saw actual women in the other pictures!
zomfibame6 years ago
nice instructable; and the comment by KPM about "do it redneck style". reminded me of a few years ago when i was living in a trailer park, and i had a pickup truck.... I made a redneck pool of sorts..... i put a blue tarp in the bed of the truck and filled it with water.
nagutron (author)  zomfibame6 years ago
Ha! Awesome. That must have been hell on your shocks, though.
lou6 years ago
hey this is psychofish from flickr. nice use of the photo :)
rub-a-dub-dub, six (WTF?!) guys crammed in a wee tub. why ain't there any WOMEN in the tub. like one guy, ten women. preferably naked. and pretty. (jk, six guys is okay too, if you're into that kind of thing)
nagutron (author)  lou6 years ago
Hey, thanks! It was the perfect thing.
Bad-dawg7 years ago
I built one a while back using a radiator for the Heat exchanger. might I recommend a Rule 360 360 GPH bilge pump? It's 12 volts so no genny needed, just a big battery, and it handles sand and all kinds of crap in the water very well. Only drawback is the pump needs to be submerged in the tub.
wolfy027 years ago
where are the women?
Baron A wolfy027 years ago
Exactly What I thought!!!!!!!!

And using Wood fireed in the title can also be a little errrrr.....
wolfy02 Baron A7 years ago
misleading? HAHAHAHA
Baron A wolfy027 years ago
Yep and seriously the title picture does look a bit crammed with that many guys and a small tub.
sausage fest!
wolfy027 years ago
seriously though guys, good job on the tub!
nagutron (author)  wolfy027 years ago
Sigh. Thanks. I wish we had more photos of the women there. Actually, some of the better photos were taken by one of them. None of them wanted to get in until later, when the water had gotten up over 100 degrees. I didn't think that people would be quite so fixated on the guy/girl balanve, but I should have guessed.
Sweet! Nice and simple and somewhat portable! BTW, you should have included at least one hot chick in the tub ;-)
i totally agree! nevertheless i think i will use some of your ideas in future developments :D
abadfart7 years ago
how long dos it take to heat up
Trans_Am7 years ago
Why are there no women in your hot tub? The whole point of a hot tub is that it attracts scantily-clad women. Other than that, nice job on the pool.
nagutron (author)  Trans_Am7 years ago
Not everything has to be about chasing tail, you know. Sometimes it's just a guy's night out. That being said, there were a bunch of women there, but they refused to get in with all of us self-congratulatory men hooting packed into the tub. A couple (Nicole, Gia, and some random girl who just showed up) got in after we were done. But they didn't want us to take pictures :)
Note to self: If guy's night includes cramped hot tub, pass.
it's san fran bro
nagutron (author)  standupclothing7 years ago
j2bz6997 years ago
how did that sureflow pump work? what was the GPH?
nagutron (author)  j2bz6997 years ago
It worked admirably, putting out about 3 gallons per minute, which was enough to get the tub up to temperature (101 degrees F) in about an hour. It died an untimely death, unfortunately, since we didn't do a good job of filtering the sand out of the ocean water. The pump's internals were pretty chewed up by the end of the night.
stib7 years ago
I had hippie friends that live up in the mountainsup in the mountains above Hobart, and I remember fondly the old bathtub that they had connected to a chip heater (an old wood fired water heater, pretty much built as you did) which was fed from a mountain stream. In winter in Hobart you'd get a stack of firewood, some wine and a friend, and you could watch the stars and the city lights below. Perhaps you need to try it up in the hills using gravity feed. Or wave power..?
Gimpy stib7 years ago
Gravity feed!! I have been planning a similar project to this for an upcomming camping trip in the mountains of BC. The one thing I was worried about was the pump, as I didn't want to have to bring a gas generator, and I was slightly worried about the power I could get out of marine batteries. Thank you SO much for the idea of a gravity feed pump! The area I am considering has rivers everywhere, so I'll bring a few (hundred) feet of garden hose and make myself a low presure hose. I plan to post the project (along with a couple of others I've done) here. But this is what I LOVE about this site, me reading this comment has given me the idea of how to improve on my plan. Cheers!
bumpus7 years ago
hehe looks like a wicked sausage-fest 5/5
carpespasm7 years ago
Sounds like good times to me.
greatpanda7 years ago
So...how did the components handle the salt? This is probably the best thing you could possibly have done with an inflatable hot tub!
abnor7 years ago
actually, it would be better to take out a cauldron, and build yourself a fire, and THEN get in. no noise, more heat, and good smells (not really)
abnor7 years ago
complete sausage fest, with oven and everything xD
Joshld7 years ago
Very nice! About how long did it take to get up to temperature?
nagutron (author)  Joshld7 years ago
It took about two hours, since our fire wasn't quite big enough, and the lack of wind screens meant that a lot of the heat was blowing out from under the exchanger. Under ideal conditions, I think it could have been as little as 30-40 minutes.
This is a great instrutable and great idea, and I had a huge smile when I saw the picture. Looks fun!
sdtacoma7 years ago
Sausage Fest! Great Instructable though.
this is great. I've seen a similar idea with coils arranged like a radiator placed over the campfire. It's a godsend at winter camps when it's -30 outside. It's absolutely not when you have to get out. Well done.
shmacky267 years ago
You all seem pretty stoked about having 6 dudes crammed in such a small space, and why is that one guy wearing a sweater?
hooloovoo337 years ago
Rub-a-dub-dub 6(?) men in a tub. Very nice
I was gonna say that!
great job... Trans_am has a point about scantily clad women but... Featured, great project, I've been meaning to make something like this, but It's damned hard to persuade one member of the family that a hot tub is a great idea...
Do it redneck style! Build a wooden frame, add a tarp, fill with water. I'm sure there's a couple of tibles in that style.

Beware, though, of the Simpsons Pool Effect- one of my friends suddenly became the automatic host for parties when his parents got a hot tub installed at their house. Now we have had to instate the "no tubbing before midnight" rule to stop the aforementioned parties getting fragmented because 8 people are dans le bassin chaud and the rest are indoors.
Umm I was planning a big one, using cut a attached together water tanks for the bottom, and about ten' by ten'... The whole party could get in it...
That sounds like a small swimming pool... seriously, 10'x10'? If it was, say, 3' deep that would be 8,495 litres, and a few hundred kWh to heat it to body temperature, so if you leave your entire house's electricity connection wired up to it for four days it will be nice and toasty warm :P As always, if you ever do it, be sure to post pictures.
Well I was talking about lopping 1000L tanks in half and joining them... Actually it would be exactly 10'X10'... maybe four would do... Of course I was planning on making a fire that shouldn't be allowed to rage anywhere but the pits of hell... Of course the whole thing would probably have to use a low grade disinfectant instead of replacement all the time, since it would be so huge... Also It occurs that the ground under the stone bit in my garden may not stand up to 9 tonnes of water...
umm... thats way too many guys in way too small of a hot tub... i'll let you figure out this problem on your own
Chard7 years ago
this might not work for several reasons but...if the copper tubing went out the tub, in the bottom of the chimney and out the top before going back to the tub, the heat in the cimney should cause the water to rise in the tubing and cause convection current. this could see you do without the waterpump and generator! like i say might not work or be powerful enough but maybe worth a shot so u can be without the generator humming away so u can hear the waves and the crackling fire.
nagutron (author)  Chard7 years ago
We tried it; it certainly would have been nice not to have that generator going. Unfortunately, the flow rate seemed really low, so we were worried that the exchanger would overheat. Note that to make this work, we needed to get the output hose to be higher up in the tub than the input. If they're at the same level, you won't get any flow at all... We were also talking about making a steam-powered pump, but that probably wouldn't help with the noise issue:)
LinuxH4x0r7 years ago
Great idea, looked like fun!
dsman1952767 years ago
um, interesting first picture....
nagutron (author)  dsman1952767 years ago
Well, I hope you're not looking too carefully, but we're all clothed. No public nudity, after all. Especially with that ranger hanging around.
i was not really hoping to find out. ok, never mind then.