Off-grid DIY wood-fired stock tank hot tub 

Just a heads up for future builds. Not trying to be one of those people who know it all. Just a tip, when you're bending piping you should fill it with SAND First that way you won't get any creases in the piping. :D None the less, great video. Take care!

@@KrunchTastic I was aware of that tip when I bent the coil but was unsure how to pack it into the original coil. If I recall ice was also an option.

Very easy sir. Just add water to the mix and everything will come together, it helps compact and keep the kinks out. :D Great Build! Each rotation will compress the previous bend. Have a Great Memorial Weekend.

......how long does it take to get hot in the winter?

There are stoves that work submerged in a hot tub. That might work for you.

How would you insulate the fire chamber? I’m thinking about doing this for my pool

Which type of fish tank pump would you recommend?

Those are both great ideas. Thanks!

@@bogamanz Use a closed cell foam. Also coat tank with Elastomeric first.

That would help, but dirt has a low r-value. This approach may be disappointing. If you don’t use a pump to drive the hot water, your heat source will need to be placed below the pool for siphoning to work. I would be afraid to run this setup if the water was forced through the coil, as failure would have some serious consequences.

bogamanz I was planning on making the fire pit in ground rocket style so the coils would not be above water level. They’d be parallel like yours

bogamanz I also used to spray foam insulation for a company. That stuff would be perfect to spray around the outside, maybe an inch at least before putting it in ground.

Great idea. This should work well.

I only use this on demand and use simple bleach as a sanitizer. It is never full for more than a few days, so I can't really help with your question.

The thermal action in the coil will cycle water on its own as long as there is heat. When the fire goes out, the action reverses as the coil becomes cooler than the tank, so I plug the coil to stop the flow once the fire is out. I connected hose fittings on the inside of the tank so I can hook my hose up to the coil and fill it directly while starting the fire. You wouldn't want to start a fire without water in the coil though. With a well designed system, the water will enter the coil as one fills the tub.

With thermo syphon the water feeds to the coil need to below the high water mark....ideally intake at the lowest point and out let just below the highest water point.....but in reality it will work as long as one pipe is higher than the other.... the trick is not to heat the water in the fire too much too quickly as it will boil...the way it works is hot water is lighter than cold so it naturally wants to rise in so doing allowing cooler water in behind it.....simple and effective....and no moving parts

I've added a sample parts list to my video description. I'll be posting some updated info on my new blog here: www.tinkletinker.com/blog/uncategorized/wood-fired-hot-tub/

You could reduce the volume of water you heat. A two person hot tub might contain half the water of my large tank. Insulation might help too for regular use. To use on demand, you might want a modern store bought tub that is always warm. Be realistic with a home-made solution. Just starting a wood fire takes time. This is more of a lazy weekend/holiday event for my family.

I actually did create this solution. Many of these heaters are not designed for continuous operation and have auto shutoffs. I can warm the pool, but not get it hot.

When we are not going to be in it, we certainly cover it! Good tip.

You might need to special order some hardware here. I used brass through hull fittings to connect to the tub. These have rubber gaskets to create a seal. You will likely need to coil the tube yourself for find a shop that can do it for you. The copper I bought came in a "coil", but it was 3-4 feet in diameter so I had to carefully bend it into a tighter coil. You could solder a bunch of those 10 foot pipes together. The water in the coil will keep the solder from melting under heat. Your questions indicate that you may be new to plumbing. Caution, water under heat/pressure inside pipes can cause deadly explosions. This project might not be best for a beginner.

@@bogamanz you answered another question for me in that if the copper is already coiled would I be able to make it smaller, thank you ~ I am wondering how could I create an explosion if my set up is the same as you have here where the water circulates itself without any kind of pump attached? Also, I can't picture what you meant by soldering 10' tubes together . . that would be really long ~ thank you for sharing your thoughts ~ an explosion would get me kicked out of my living space so I really need to know what you meant by that ~ thanks!!

@@womenswellness1577 If the water flows freely, the risk of explosion is low. If your system is obstructed, the pressure generated by the heat has to go somewhere. Under no circumstances should you plug either end of the system while heating. Research boiler explosions to get an idea on how catastrophic this force can be.

This build did involve soldering.

I do get sputters now and then. That is trapped water boiling, perhaps with explosive force. Steam power can move a train, so be careful here. Remember, boiler malfunctions were deadly back in the age of steam. I fix my minor issue by leveling the coil with a rock. You want to verify that at no point is the water forced downhill once it enters the heating coil. When my fire is super hot, the system will still sputter now and then. What is the smallest diameter opening in your system? I'd keep it over 3/4 inch, more is better (but I am no expert).

I don’t have the calculations you need, but a 800 gallon tank holds over 2x the water as this setup. There are versions where a wood stove is submerged in the pool and heats water by direct contact. That might be a more practical approach for a tank that size. Good luck!

The hot water circulates on its own thanks to the thermal siphon. We also keep a paddle on hand to stir the tub to a uniform temperature. There are no air jets or bubbles with a traditional hot tub.

I'm not a thermal dynamics expert, but would the pump actually speed up heating (assuming the same hand-formed coil was used)? My flow is slower but hotter. Not sure if a faster, cooler flow would impact the overall time significantly. The beauty of this design is that it is relatively safe (the coil won't over-heat if I loose power), it required only simple tools like a wrench, a blowtorch and a drill to build, and it can be rolled anywhere on my yard. This thing drains quite easily thanks to a lower plug. I've used a simple mouth-primed siphon to drain it from an enclosed area as well.

That's what I want to do. There's a few options on Amazon, try searching "hot water circulator". I'd imagine this would go in between the output from the tank and the heating element where the water's cooler. Just not sure if this is unsafe in any way, not much talk of it.

Not sure I still have the list still, but the rubber hoses shown here were a bad choice as the heat warped them. The setup today uses all metal connections.

Try this: www.amazon.com/RAINPAL-BBF020-Bulkhead-Fitting-Threaded/dp/B01C9A386Q

drain after your done and refill when you want to go in

Can you grill a steak on a camp stove though?

@@bogamanz si:) (thin steak ) and i cook my homemade tortilla on it too some onions and seranos

@@bogamanz i have the same fire pit ! And i love making corn and t bones :)

Getting the coil to rise at each point is harder than it looks. If the coil dips down in some spot, you will have problems. It seems like many who tried to duplicate my video have not been successful.

One thought I just had after replying: my coil (in the video) is hard to do right since each layer of the coil sits on the previous layer. If people trying this today added a way of spacing each coil vertically (so that there is an inch or so between layers to create more "rise"), I think they would have fewer problems. Than again, I'm a non-exert, and should be ignored!

It's gradual enough that I simply stop adding as much wood at some point.

Now I have a pump and filter. Here is the latest version: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-AW-uP_rUvS0.html

You can tell my wife that we are digging a gopher hole in the yard. I'll hold your beer.

Maybe 60 degrees. I think it would work lower than that, but haven't tried. You could always find a way to insulate the tank.

If they were empty, you would have a problem! The constant flow of water cools the copper considerably (since the heat transfers to the cool water).

Copper melting point is like 1000 degrees C. you're not gonna generate that sort of heat in a fire pit.

Yes, you are creating a thermal siphon. Warm water rises and sucks in cold water. Good luck!

@@bogamanz Amazing! One last question: Let's say the desired temperature of 104 degrees is achieved. If I install a shut off valve will the copper pipes melt due to lack of movement? Did your copper pipe that sits in the fire degrade over time?

Please do not install valves. This could cause an explosion. This video is for entertainment use only and should not be replicated by anyone.

Some advise: concrete may not hold up to these high temps. My understanding is that you should never build a fire directly on it. Why not just dig a pit in the earth and place a steel sheet under the tank and over the pit? If the underground pit was small, you might not even need the support. 300 gallons of water weighs a lot more than you and friends, so the rolling support function might not add much value. Good luck!

I started a fire on ground then put hot tub tank on four concrete blocks around fire and got a shop vac connected to a tube and that got it SUPER hot within an hour. spray fire down if too hot or add log and put shop vac back on if too cold

Here is the exact product: Coil, Type L, Pipe Size (In.) 1, Length 60 ft., Material of Construction Copper, Inside Dia. (In.) 1.000, Outside Dia. 1-1/8 In., Wall Thickness (In.) 0.050, Max. Pressure (PSI) 494, Temp. Range (F) 100 to 400, Standards ASTM B88,B306,B280, & B819, Compliance Certified For Use In Potable Water Applications, Package Quantity 1

add more water

I'd use a small swimming pool pump like you get with those cheap inflatable Walmart pools or any little 110 volt water transfer pump. I'd put the pump on the line from tje tank to the woodstove so it has cooler water running through rather than being on the hot water side. Then you can just put a ball valve on the upper hot water return so you can just reach over and adjust the flow of hot water coming in. Also, I've wondered about using small pump like I mentioned but wire it with a dimmer switch so you could vary the pump motor RPM to turn the flow down.

@@weneedmerch To the fire.

aproximadamente 3 - 4 horas.

The thermal siphon action takes care of circulation in the coil (warm water rises). To keep the pool the same temperature, I stir the water with a paddle.

I don’t. Is there a store in the countryside that sells goods to farmers? I’d look there first.

I've added an intex pool filter and a clorine dispenser. These additions keep the water clean.

Folks on RU-vid that use 1/2 inch report really long heat times. Depends on how much water you heat of course. It might work better if you create some sort of stove or way of getting the pipe closer to the hot coals. The short hoses were a bad idea here, they restricted flow and melted on my 2nd try. I've replaced this with 3/4 pipe and the setup works much better. It can get so hot that I can't enter the tub. This is perfect for heating during the day and using at night long after I put out the fire.

I have not had a problem, but the tub is empty most of the time. I only use a little bleach.

Where I live water is plentiful. Even if not, salt is better

I hear there is a shortage on these with the virus (folks are using them as pools), but Tractor Supply is a good bet. I had to make a special frame on my truck to carry this home.

I've commented this before, but make sure the coil is continuously rising. The water must not be forced downward at any point.

Does your setup have a steady rise at each point once the water is heated? The water wants to rise. Are your pipes too narrow? My setup does sputter every now and then when using extreme heat, but not constantly.

bogamanz im experiencing this same problem. What do you mean 'a steady rise' ??

@@Thousand.cranes.bonsai 1st, locate the heat source well below the pool. Use a hill if possible. You want to heated return pipe to rise significantly. 2. Once the water starts to get heated (starting at the bottom of the coil), every inch of your pipe should be higher than the last inch. If not, heated water can get trapped as it tries to go up in the wrong direction. If trapped, it will turn to steam.

Yes, this one is 2 feet.

@@bogamanz thank you for the speedy reply. So do you find that 2ft is enough depth for you to be fully submerged if you want too? I'm definitely going to make one of these, I just have to source a tub in the UK.

@@stevejohn01 Since this tub is 6 feet round, yes. You can simply lie down. My wife and I are tall people and we fit nicely. In this follow up video, you can see me up to my chin: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-AW-uP_rUvS0.html

it's funny because in Brazil we call them australian water tanks

In New Zealand it would be know as a water/stock trough. You could make enquires through livestock companies like Elders, Wrightson, Wesfarmers etc.

in america, a "stock tank" is a vessel intended to hold water for livestock to drink water from. it would be expensive to ship a galvanized steel stock tank to australia so that is why you are having trouble finding one and the only ones you can find are made of plastic.

I don’t use it in the winter or cold weather and have lots of wood, so that hasn’t been a problem. I’d caution that few have been successful at bending a working coil like I did.

@@bogamanz oh I didn’t know you made them yourself. I see some expensive ones and then cheaper ones. I was wondering what was the best option.

@@SaintJames40 I haven't priced these out, but the larger diameter the coil, the better. The stoves that sit the tub would be the fastest way to heat a tank--but take up space and I wouldn't use these around kids: www.alekoproducts.com/internal-wood-burning-hot-tub-heater-equivalent-to-10-15kw-electronic-heater/kitstovecmy2-ap/?srsltid=AfmBOoqWkK8Frgq-AEK3d1s8s5C0MYa3yLyDQ7Oic33Rid_Kg9O1WqQP9TM

No pool pump at all? Everyone’s saying I should do one but I really don’t want to drill into the side. Just trying to figure out my best option. I have a 300 gallon poly tank. Was wondering if one of those waterfalls that clip onto the side would be enough?

@@SaintJames40 A proper thermal siphon moves the water through the heat on its own. All you need is a paddle to circulate the water in the tank once every now and then.

He probably wrapped it around something round he had.

Yes, the short hoses were a bad idea. They restricted flow and melted on my 2nd try. I've replaced this with 3/4 pipe (metal to metal connection) and the setup works much better. It can get so hot that I can't enter the tub. This is perfect for heating during the day and using at night long after I put out the fire.

bogamanz “metal to metal” what do you mean by that. I’m planing on buying a 25feet 3/4 cooper pipe and attempt to do this. Will that work?

Nothing much better to do these days...

@@bogamanz true

Early on I used bleach, now I use one inch chlorine tablets in a floating dispenser. This tub is going on 3 years now and has been holding up well.

A chain called Tractor Supply. Any farm supply store will have these.

Rural King, Tractor Supply, you can even order one at Lowe's. I would hook up a pool pump to it too with multiple inlets

That would be one big rocket stove to deal with the larger diameter copper coil (I expect one would need) to heat a tub this size in a reasonable time using thermal siphoning. I have piles of firewood on my land, so this solution works great for me.

bogamanz rocket stoves produces what, like 10 times more heat then non rocket stoves for the same amount food? And it's a faster process. Just saying brother !

Dan de Angeli pressure

Some nerd will explain it better later I'm sure, but same principal as a hot air balloon. Warm things rise. The rising warm water draws in the cool water, with the process ejecting hot water into the big tank through the top pipe. Notice how the two connections to the tank are at different heights.

Ya, the water is boiling inside the coil causing rising pressure which escapes out the top pipe as it's pulling in new water from the bottom. It's a heat operated syphon

@@hillbobaggins The siphon effect doesn't require the water to boil, only to heat up and rise in the coil.

Convection ... en.wikipedia.org/wiki/Convection ... "Natural convection, or free convection, occurs due to temperature differences which affect the density, and thus relative buoyancy, of the fluid. Heavier (denser) components will fall, while lighter (less dense) components rise, leading to bulk fluid movement. Natural convection can only occur, therefore, in a gravitational field. A common example of natural convection is the rise of smoke from a fire. It can be seen in a pot of boiling water in which the hot and less-dense water on the bottom layer moves upwards in plumes, and the cool and more dense water near the top of the pot likewise sinks. Natural convection will be more likely and more rapid with a greater variation in density between the two fluids, a larger acceleration due to gravity that drives the convection or a larger distance through the convecting medium. Natural convection will be less likely and less rapid with more rapid diffusion (thereby diffusing away the thermal gradient that is causing the convection) or a more viscous (sticky) fluid. The onset of natural convection can be determined by the Rayleigh number (Ra). Note that differences in buoyancy within a fluid can arise for reasons other than temperature variations, in which case the fluid motion is called gravitational convection (see below). However, all types of buoyant convection, including natural convection, do not occur in microgravity environments. All require the presence of an environment which experiences g-force (proper acceleration)."

They mention "microgravity" but that is BS. Gravity itself is BS. It is buoyancy and density that makes things go up or down.

You can tell my wife we are digging a giant hole in the yard. Hopefully we don't hit the septic system. Also, where would I cook my steak? A hole is not a bad idea, but this setup has worked really well for us. This solution has the benefit of moving the smoke further away from the swimmers.

I disinfect with bleach, but only use the tub for a day or two before draining. Water is plentiful and cheap where I live.

It's not that hard. You add more wood to increase heat. You let the fire die down to lower heat. You throw a bucket of water on the fire to turn it off.

It is going on 3 years of working without a pump. The video shows it working.