Instructables
In this instructable I will describe how I built a hot tub for less than $100 NZ ($75ish USD) from an old bulk liquids tank, a gas bottle and some stainless tube and junk that I had laying around.



This tub is cheap and relatively easy to build and is also useful as a plunge pool in hot weather too. Oh and you can cook on the top of it too!
 
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Step 1: Stuff you will need

Parts:
- 3 metres of 25mm (1in) stainless tube
- 1 old LPG gas bottle (the kind used for barbecue grills)
- 1 bulk liquids storage tank (the 1000litre kind with a steel palletised cage)
- enough old corrugated iron to cover the outside of the tank
- scrap polystyrene packaging
- 1 closed cell foam camping mattress
- a roll of soft narrow gauge binding wire (or equivalent - you could use cable ties at a pinch)
- scrap metal parts for the legs and decoration
- a canoe paddle or something similar

Tools:
- arc welder (and the safety gear to go with it)
- angle grinder with metal cutting and grinding discs
- pipe bender large enough to bend 1 inch stainless tube
- tin snips
- heat gun / paint stripper
- box cutter
- strong pliers with a sharp cutting edge
- the trusty knockometer
- various clamps
- magnetic welding clamps are handy
- hacksaw (omit if you're deft with the angle grinder)
- handsaw/drawsaw or equivalent
- narrow nail punch/set or a strong nail
- cordless drill with a 2 - 3mm bit
- a vivid or permanent marker pen

- a gas axe is an optional extra here if you have one and aren't afraid to use it
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peets3 months ago

that louks liek my cuozzinn

nhill52 years ago
I'm just done building mine except instead of using an old IBC tank i managed to find a second hand build-in plastic (stay away from fiberglass, they break easily) hexagonal tub from trademe for $80 NZD. It also came with a pump, filter, electric heat (i'll still be using a fire) and a blower for free. I built a frame around the tub from wood, i can seat 10 people and it looks the part. Just another though as it was only $80 plus some wood from bargain boards in Thames. Still trying to perfect the heating system. At the moment i have an old open fire place with a wet back system, but it is a poor conductor so i'm thinking of throwing an old all copper car radiator in its place. Any ideas on this would be appreciated. Thanks, Nick

Any idea where I could get a tub like this? (I'm in NZ too) What size is it?

Just thought I'd include that you can add bubbles quite economically by using a ShopVac to send air thru an underwater ring of holes made of PVC, copper, or whatever. Obviously some sort of an enclosure is needed to lessen the noise level of the motor. The effect is pretty impressive for a $30 vac.

awesome! did you solve the noise problem? Where did you get that nice big rubbermaid?
How is this heated and what size is the PVC? I would think that the shopvac would burn itsself out after not too long.
pmaag3 years ago
Thanks !
I used your basic idea but modified the water heater :D takes approximately 2 hours to heat the entire thing! :D I am also using a pump as the pipes in a radiator are too small to sustain natural circulation in an effective way :D

but how do you keep it clean ? cause the water turns bad really fast !
IMG_4242.JPGIMG_4243.JPG
How's that radiator holding up? How long did it take you to burn through it (or have you not yet done so)?
i just fashioned an oven with some aluminium ventilation ducts, cause thats what i had on hand.
I have not yet burned through it, but I do think that the steal tube might be better because there is a lot of residual rust and oil and other dirt in old radiators that doesn't seem to disappear.
i am planning to re-make it in some fashion as it desperately needs some sort of filter as the water turns bad and smells of rust quite fast
lrushton pmaag3 years ago
what did you use insted of the 25mm steel tube for the heater??
pmaag lrushton3 years ago
a radiator ;) and a thin one at that. it is about 15 mm but you need to build the oven to the specs of the radiator. it isn't as pretty as the original but it is effective.
Why don't you contact some manufacturers to FIND OUT yourself...
omegaton2 years ago
SOOO awesome. I am planning a one-person version out of a light-weight wall-tent type stove and a disused bathtub. The advantages I see for using a bathtub are that it will heat more quickly, take up less space in my modest-sized back yard, will be easier to remove when I move, and can be drained between uses so I won't have to deal with maintenance. Being able to accommodate only one person at a time is fine with me, as I will be using it for peaceful relaxation rather than socializing.

Aside from this utopian vision, I'm pretty clueless about carrying the project out, as revealed in the following questions for those of you who are handier than me:

1- Someone mentioned concerns about superheating the water if you use a rocket stove, because they burn so hot. Should I be worried about that possibility with the stove I am using? (see picture).

2 - I haven't acquired the bathtub yet, and am trying to decide between fiberglass (I can get an awesomely gaudy giant pink one at a used building supply place for $70) or cast-iron (smallish used ones starting at $200). Aside from price, I think the fiberglass option would have the advantages of being easier to move, easier to cut the outlet hole, and easier to find one that is sufficiently deep. The downside would be that fiberglass is more fragile. Thoughts on how I might mitigate the fragility of fiberglass? Are there other pros and cons I should consider for fiberglass vs. cast iron? Other tub materials I should consider?

3 - If I run the tubing partially through the chimney (assuming I can get the tub high enough for the water to flow properly), will that help capture some of the heat that would otherwise be lost? Is there any reason I shouldn't do that (e.g. the risk of superheating)?

4 - Some non-metal tubing is rated for quite high temperatures. Am I right in thinking that, nonetheless, it would be unwise to use PEX or other non-metal tubing in place of any part of the metal tubing because it could melt? I hope I'm wrong, because PEX is so much easier to work with, I have bunches of it lying around, and would be much easier to configure with respect to connecting to the tub because it is somewhat flexible.

5 - Aside from having additional steel plating welded onto the rather flimsy bottom of the stove, what can I do to prevent it burning out? Would putting a layer of fire brick in the bottom help? Other material?

Thanks in advance for any advice!
IMAG0390.jpg
Just make a frame for a fiberglass one, the way the make them in houses and insulate it with insulation foam.
About your second question. You could do all of your plumbing that connects into the bathtub, if you use fiberglass, and then basically flip it upside down and then fill it with something like foam so that it is light weight, sturdy, and also very good at holding in the heat. I'm not sure how much it costs but it would work well.

Also, if you heat up none metal tubing, most times it can ruin the PSI rating on it, I believe you would be beter off using metal tubing. And then in the heating system, toward the end of the pipe have some sort of removable filter to catch anything in the water from the metal reacting with your chemicals. While you want it to be cheam and effective, have to fix it repeatedly wouldn't work so, go with metal tubes.

To your last question, look at ideas from blacksmithing, buy a cheap fan and have it blowing into your stove, it will make the flames hotter and harder to go out, just make sure you stock your stove well.

Good luck.
milesnorth2 years ago
Great example and thanks for the pics. Can you explain to me how you keep leaks from happening around the inlet tubes into the tub? I have a regular "standard" hot tub that I got for free cause it has no pump. I want to make it in to a woodfired one. Anything you can think of that would be a problem with that? I wondered if the fiberglass would melt or something having the metal go directly through it (That might be incredibly silly, but so be it). I am also in Alaska, so i want to try to somehow insulate the heater and coils going to the tub to cut down on losing heat. Any thoughts on doing that? I love your guy. I have been looking at him for years now and finally scored a hot tub to try to make my own. Thanks for the inspiration.
veloboy (author)  milesnorth2 years ago
Hi there,
Those flanged fittings in step 14 are what stops the tub connection from leaking. They have rubber grommets that are sandwiched between the fixed and threaded flanges on the fitting. These are standard farm plumbing connections. I used threaded ones but you may be able to find some with barbed ends. That makes it easier to get a seal on the flexible hose.

I haven't had any problems with heat damage around the inlet. Remember - the tub is full of a lot of cool (by comparison) water that easily soaks up any excess heat around the junction. Which is partly why you can put a soda bottle full of water in a fire and it won't melt.

I use standard foam pipe lagging to insulate the coil pipes but they can end up mleting if they are too close to the fire.

Anyway - thanks for the feedback and good luck with the build!
jrauch22 years ago
mint dude.

im thinking sheets of ply stainded black instead of the iron might look good. (extra cost tho)

one question regarging heat flow?--------
you have your inlet and outlet pipes at only about 300mm apart?
wouldnt this work more effictivly if they were placed further apart?
how do you still get thermosiphon if theres people in the pool and water movement means theres a constant temp ( no hot at top and cold at bottom?

im also thinking for mine ill setup a 6l, p/m califont to heat the water.
ive looked into the cost. its about NZD$3 to fill and at 6L pm (25 deg above tap temp) it will fill to 40 deg in 1.5 hrs... for $3!
califont is 200-250 on trade me

my thought with the intex pump (40 on trade me) means (we) could now add chemicals to the pool so it doesnt need to be empitied?
i'd rather no chemicals but the empty/ refill may become a hassle?
do you get 2-3 days before it need a water change?

lastly, have you had any thoughts on the lid?
as you now have the flat pool edges, perhaps flatten the lid to strech across the pool edge, attach a layer of the same poly and a ply layer? making one solid lid piece.

anyway gerat buid and cheers for the inspiration
veloboy (author)  jrauch22 years ago
Chur for the props bro!

I haven't noticed any lack of oomph with the 300mm height differential. Thermosiphoning occurs due to the heat differential in the coil rather than the whole of the hot tub. When the heat source is cranking the water temp in the top of the coil will always be hotter than the water coming in the bottom - that's where the driving force comes from. Kill the heating and the thermosiphon will stop. Eventually it may even reverse!

Good luck with the build - post some pics when it's up and running!

Cheers
tocatech2 years ago
hey guys,

I hear alot about not using copper pipe in chloronated water and rather to use 316 stainless steel, it's just the price difference between copper and that grade steel is vast...

so is there anyone that tried copper in chloronated water?
And is their any other alternative tubing to copper and steel.

Another design i was thinking about was maybe using a barrol, fill the barrel with water, heat the barrol to get water to near boiling, have coiled polypipe in the barrol where the pool water will run through, what you guys think?
cool! i make this tomorrow (there finland clock is 23:24 or 11:24 pm)
How does the water circulate
I may be teaching you all to suck eggs, but may I suggest you use 3 legs instead of 4?
A "fouth" leg makes it wobble, and it's a pain trying to level a 4 legged stool, table etc.

Stunning projetc Veloboy, well played sir
smy13 years ago
so i am curious how is the how circulating in and out of the heater coil??
arhillbilly5 years ago
Very nice! I have an old therapy whirlpool that I couldn't afford to run using an electric water heater. But there are plenty of dead trees in my woodlot that need to be cleared for fire prevention!

Just a word of caution to those who don't know about these things. Common sense would tell you that if you emptied the LPG bottle and even triple washed it with detergent and hot water that there couldn't be any gas left in it.

THIS IS NOT SO! With both gasoline tanks and LPG tanks, the steel can and does become impregnated with the fuel and can leach out for MONTHS after it has been emptied and even triple washed with detergent and hot water!

I have two personal friends that fortunately lived through the experience, though not without scars to prove it. In both cases it had been less than ten to fifteen minutes since the last wash and the time they began cutting/ welding. They both woke up in the hospital!

If you must use an LPG tank, it is best if you can do it with the tank FULL of water with an Oxy/Acetylene torch AFTER triple washing. It is dangerous to cut with an electric grinder when it is full of water for what should be obvious reasons.

It can be done safely, but if you don't KNOW what you are doing, it is better to either find a different container (a 10, 15, 30, or 55 gallon drum would work very nicely or an old hot water tank) for your heater or take it to a professional to have them cut it open for you, IF you can find one that is willing to take the risk.

Please feel free to copy and paste the above warning to the beginning of step 8. It could save someone's life. I know of others that were not so fortunate.

Again, Very nice! Awesome artwork!

Thanks for sharing it!
 Buying a new empty gas bottle is an option
veloboy (author)  lukeyj154 years ago
The trouble with new 'empty' gas bottles is that they are not empty! CMIIW but they're full of nitrogen. When you take them to be filled for the first time they purge the nitrogen first. Cutting into a new bottle without purging the gas could result in a nasty bottle rocket.
but nirogen is inert. how will it do anything?
veloboy (author)  lukeyj154 years ago
Even though its inert it's the pressure that's dangerous in that it could turn the gas bottle into an unexpected projectile. Maybe I'm being overcautious, but when I watched the guy filling my tank for the first time, there seemed to be heluva lot of gas in there at some impressive psi...
Well it never hurts to be cautious, but opening the valve for a couple of minutes should be sufficient.
If you don't have a recepticle on the tank nothing will leak out when you open it. If you don't understand propane, best not to mess with it!
and yes, gas tanks have a lot of gas at a lot of psi
I agree... these steel containers that have had highly combustible liquids/gasses in them, can react to heat by exploding.

If you fill them with sand though, they cannot get enough oxygen to explode, without the risk of electrocution.

The project looks very interesting and much cheaper than other options on the market.

I would think 1½" - 2" pipes would be better for natural circulation of the hot water. You risk the heater actually boiling, if the circulation is too slow.'
veloboy (author)  fred bloggs4 years ago
If you wanted to use larger pipe I would recommend flattening it a bit first (ovalising it) so that the heat can fully penetrate the the water in the pipe. Otherwise there'll be a cold core to the water which would lower the efficiency.
best way to deal with flammable gas/liquid containers, wash out afew times, then just fill with water and grind the top off. it cant explode when its full of water. Yes i am speaking from experience. youre going to be lighting a fire in it eventually anyway so once youve actually cut it open you may as well do exactly that, theres not going to be any more combustibles in there once youve set the damn thing alight.
Putting some dry ice in the tank works as well.
don't use water, use sand. i used to solder gas tanks and we filled them with sand AFTER washing. Sand won't react badly with electricity. I would NEVER EVER cut into a propane tank. Find a water heater liner. Everything else is wonderful and I plan on copying you on this. thanks for sharing! I do wonder about filtration etc ...
wow thanks for watching out.
tim132113 years ago
What does the coil in the tub go?
Annatar24 years ago
Love this project. Definatly going to build this at some point once I get back from Iraq. For those of you looking for the container, check 'IBC Tote' on Ebay. You can usually pick them up for under $100, though due to the size its generally local pickup only.
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