Wood-Fired Ocean Hot Tub




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False Profit Labs built itself a wood-powered on-beach hot tub as a side project. After all the blowingstuffup 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

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!

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

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

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!

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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    115 Discussions


    Question 1 year ago on Step 1

    Awesome job! Thanks for adding your instructables post.

    With my heat exchanger I've used recycled copper pipes of various lengths. Unfortunately I've had to join my coils 3 times using solder. The coils will be exposed to high temperatureschool and naked flames inside my burner.

    OK my question is- do you think the soldered joints will melt and leak ?

    I've added a couple of images so you have a better idea of what I'm trying to explain.

    Cheers for Australia.

    1 answer

    Answer 1 year ago

    So, the main thing that kept our pipes safe was the water inside. The theory is that the water would be boiling/exploding out of the outlet/way too hot for comfort long before the melting point of the solder. So, if the pipes are full of water, the solder joints should be safe. Ours stayed solid, in any case!

    That's a beautiful stove/pipe btw :)


    10 years ago on Introduction

    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.

    2 replies

    Reply 10 years ago on Introduction

    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:)


    4 years ago on Introduction

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


    6 years ago on Step 7

    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?


    6 years ago on Step 4

    dig a hole down to the water table and use a shirt to filter out the sand


    6 years ago on Step 5

    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.


    7 years ago on Step 7

    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?


    7 years ago on Introduction

    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!

    3 replies

    Reply 7 years ago on Introduction

    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.


    Reply 7 years ago on Introduction

    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!):


    Reply 7 years ago on Introduction

    would be good to see new instructables ,instead of seeing the same ones posted every few months


    Reply 7 years ago on Introduction

    True, not everyone gets a chance to see ALL the gems on the site -- good to balance rotating new ones with old.