Instructables

Wood Fired Hot Tub

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Picture of Wood Fired Hot Tub
Wood-fired Hot Tub
Perfect for 6-10 friends.  It can be carried in the back of a small pickup truck.  It takes about 3 hours to get up to temperature and cost about $650.

Materials
-55 gallon closed steel barrel ($30)
-Barrel stove conversion kit ($50)
-14 2" x 4" x 8" firebricks ($30)
-2 2 foot sections of 6 inch black stove pipe ($13 each)
-50 feet of 1/2 inch flexible copper pipe ($80)
-10 feet of 1/2 inch rigid copper pipe ($15)
-10 1/2 inch straight pipe fittings ($5)
-10 1/2 inch elbow pipe fittings ($5)
-plumbing solder and flux ($5)
-Rubbermaid 300 gallon stock tank ($300)
-1/2 horsepower centrifugal water pump ($40)
-10 foot garden hose ($15)
-1 foot 3/4" CPVC pipe ($5)
-assorted fittings to connect tub to pump and stove with garden hoses ($50)
-ground fault switch to turn on pump ($15)

Equipment
-jigsaw with metal blade for cutting holes in barrel
-hacksaw for cutting pipe
-pliers
-slip-jaw wrench
-drill with sharp drill bits
-propane torch for soldering pipes
-shop vacuum with blower attachment (optional)
-screwdrivers
-socket set

 
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Step 1: Build barrel stove

Get a 55 gallon steel barrel with a closed top.  I got mine from a guy in Bradford, VT who calls himself the "Barrel Man," but if you don't live in the area, you can usually find them on craigslist.  Make sure you get a barrel that hasn't been holding anything toxic, since you'll have to spend a fair amount of time crawling around the inside during the assembly process.  This barrel formerly held palm oil, and there was a small puddle of it on the inside (it had the consistency of crisco) and every once in a while I accidentally stuck an elbow in it.

I bought a Vogelzang Barrel Stove Kit, Model# BK100E from Amazon.com.  Assemble the stove following the directions in the kit.  The kit comes with the door, the chimney collar and the legs.  The stovepipe is not included in the kit.  I bought mine at Home Depot.

When cutting the holes for the chimney and collar, make sure you use a blade designed for cutting sheet metal, and when drilling holes, make sure you have a sharp drill bit, otherwise it will take forever to drill your holes.

After assembling the kit, I added firebricks to prolong the life of the bottom of the barrel.  I used 2" x 4" x 8" bricks there were 3 rows of 4 and a final row of 2 for a total of 14.
enielson4 months ago

This is an awesome instructable. I am dying to give this a try.

ArtificerMade6 months ago
You can braze the copper just fine using something like stay-silv or (way cheaper) a phos-copper brazing filler. In fact, using phos-copper brazing rid means you won't need to use flux.
kwhit1902119 months ago
I would try to solder the joints using silver solder for a higher temp. As far as brazing copper that's a impossibility, as the brass won't stick to the copper. I found that out long ago when I was making a grill out of schedule 80 copper pipe..
bmaust1 year ago
could you use an old hot tub for this instead for the Rubbermaid tub?
dan268 (author)  bmaust1 year ago
Sure. Any kind of tub would work. The Rubbermaid tub is nice because it's easy to transport, and already has a hole in the bottom for connecting the pump. If you're not planning on moving the tub, though, there are a lot of good alternatives.
danzo3211 year ago
wonder if rope in the tube could work.
dan268 (author)  danzo3211 year ago
I'd be worried that it would get stuck. If you try it and have success, let me know.
wax or sand works great
dan268 (author)  ibarnett521 year ago
I used sand for this project, but getting it in and then getting it out was a huge pain. Wax sounds like a much better idea, though.
iMartijn1 year ago
When you turn the pump off, how do you keep the water inside the coil from overheating?
dan268 (author)  iMartijn1 year ago
You need to leave the pump running all the time. Otherwise the fire will cause the solder to melt and everything will fall apart. If the heat exchanger was just a single piece of tubing (or perhaps if the solder joints were all outside where the heat is lower) you might be able to just let the copper heat up.
Aha

The I don't understand the part of the shop vac. I'm not even sure if I know what a shop vac is. (English is not my default language)
dan268 (author)  iMartijn1 year ago
The shop vac blows air into the stove to make the fire hotter.
mr fat1 year ago
Thats such a great idea and its something I had thought of doing for a long time!
zawy1 year ago
If anyone remained confused and in the dark like me even after reading everything: water instead of air is being pumped through the copper coil and the coil is inside the chimney, not in the tub. If you want to geek out on the heat conversion, assume R-factor of 0.1 for air flowing over a surface, 50 ft times pi*1/2 inch / 12 in/ft = 6.5 ft^2 exposed surface, assume average 700 F stove output temp above water temp, and use heat loss equation for the R-factor: Watts = degrees F difference times ft^2 / R-factor / 3.4 which gives Watts = 13,400 which is what you measured. 1/12 cord of wood is about 50 kWh (nearly 4 hours at 13.4 kW) which is about what you got out which would mean 100% conversion efficiency instead of what I would have expected to be 20% so I would have thought nearly 1/3 cord was used.
dan268 (author)  zawy1 year ago
The way I did the calculations was to look at the energy put into the tub (mass of water multiplied by change in temperature). The heat value of wood can be found on the internet. I think I ended up finding about 20% efficiency when I did the calculations that way.

If you're still confused, let me know and I'll post the spreadsheet I used for the calculations.
zawy dan2681 year ago
I agree with your energy calculation as received by the tub as I showed the design of the coil should provide that much. My confusion was that I made a mistake on converting the energy in a cord of wood. 1/12 cord of wood is about 500 kWh instead of 50 kWh, so my calculations based on your heat exchanger design agree with your measurements.
Could you run tubes straight up, then use a 180 fitting and come straight back down and arrange the tubes in a circle? That way it could be taken in and out to clean if needed.
dan268 (author)  Dantheplasticman1 year ago
One of the reasons I decided to go with a vertical coil was to make the tubing easy to drain in the winter. If you don't have to worry about freezing, vertical tubing might work.

Since there's no air vent in the top, you have to jiggle the stove a bit to get everything to drain out.

There was a lot of creosote build-up on the heat exchanger. I'm going to try cleaning it out with a chimney brush and see how that works.
scristian1 year ago
this is realy well explained gj :)
jmcgaghey1 year ago
I haven't personally done it, but I have seen it being done with small diameter tubing and it works.

you could have also gone and got a set of tube bending pliers I have used those for making sharp angles for brake line tubing which is pretty much the same diameter.
WimG1 year ago
expensive counterpart, but great for stealing ideas: http://www.dutchtub.com
lordzamiel1 year ago
For bending the heat exchange coil, you can buy basically a long spring in the plumbing department specifically for coiling copper tubing - no kinks, fast & easy.

http://www.homedepot.com/p/BrassCraft-Tube-Bending-Spring-Set-T073/100158906#.UYKBzrXP3ko is an example - Hope it helps somebody =)
jmcgaghey1 year ago
a better way that would save you a lot of time would be use water instead of sand and then freeze the tubing. Gives even coverage within the tubing and easy clean up after.
dan268 (author)  jmcgaghey1 year ago
Oh man, I wish I had thought of that earlier. Have you actually tried it?
triumphman1 year ago
Hey who made those bubbles ?
Moerdith1 year ago
I was talking to a guy who was working on making one, and his design actually got it to percolate, similar to a coffee pot. I really liked the idea because then there really isn't a need to have a power source. Last I spoke to him it was apparently working, and he was just trying to get a more efficient heat transfer rate.
dan268 (author)  Moerdith1 year ago
Cool. I'd love to see some pictures. Is percolation different from thermosiphoning? I like the idea of not having to use electricity. From what I've read, though, thermosiphoning seems to be a bit finnicky. The stove puts out a lot of extra heat, and your comment got me thinking about using it to circulate the water. Maybe some kind of steam engine.
Kozz1 year ago
"That's so hot..." </p.hilton>
But seriously, that looks like fun.  I can't help but think what you might be able to do with a rocket stove mass heater.  I saw an article on one recently and have been reading more and watching videos obsessively. 

I wonder if you could create the mass ( the "mass" in rocket stove mass heater) out of a body filled with water.  You may get some serious gains in heat retention (and combustion of ... well, combustibles).

Best wishes, and keep 'tubbin.