So I’ve had a plan to build a natural inground hot tub at my off grid cabin.
I had planned to just build a fire and run copper or steel pipe through the fire in a coil and let thermodynamics move the water for me…
Similar to this. 
Now that I am on the WG bandwagon, I figure there must be a better way.
So…use woodgas to run a generator and use electric elements or is there another way?
Piping inside a gasifier?
Thoughts?
Sometime ago we did have a discussion about heating water
http://forum.driveonwood.com/t/i-was-amazed-at-how-much-water-was-heated-so-fast-with-this
Rindert
I had started reading the thread but stopped before I got to the good parts. Lol. It was all over the place.
So can you explain the spinner concept?
To me…this design seems like it would be efficient.
Remember…I have to heat up somewhere around 400 gallons of water that is going to try to cool off fast because its open to air.
I can cover it I suppose to bring it up to temp faster.
Also if this is going to be a rocket design that reaches 1500°, wont the copper melt or will the flowing water keep it cool enough?
Maybe I’m getting C° and F° mixed up here…
IC engines are not very efficient, most of your heat is going to be going out the tailpipe - now if you could catch that exhaust heat while also doing some other work, you would get a lot of likes around here. Another popular idea is to use the heat from making charcoal, which is otherwise just wasted.
Try soldering (let alone melting) some copper pipe with the water still in it, and you will quickly discover the answer 
I played around with a simple copper coil system for heating a little hot tub (only like 100 gallons of water). It was not very effective - probably in part because i tried to keep the cost down by using 1/2" pipe, and its hard to get enough flow. If I were to do it again, I would build some sort of water jacketed boiler type of arrangement, or reverse engineer a Snorkel unit The Snorkel® and Scuba® Stoves.
It’s so funny. As soon as I read that I remember trying that exact thing.
I’m a little confused with the snorkel. How do you keep feeding the fire?
It says only the air intake and exhaust are out of the water
You must feed it like a rocket stove…stick it in the air intake.
I really like this idea.
Hi Barry,
Just a few random thoughts on the unit. Yes, there’s a lot of heat “in” the unit. How
that heat is directed is the answer to the efficiency of the unit as a whole.
I know there’s a lot of square feet of copper in the coil as shown, but the coil separated
by “air” spaces doesn’t direct the transfer of heat efficiently.
Also, the chimney will pull the heated air towards the exhaust side of the unit. A negative heat
event for the shown coil, I feel.
I think a copper coil wound tightly around and “in direct physical contact” with the
core (chimney), would be a better heat transfer configuration. Wrap the coil with copper band
to insure good physical contact with the chimney. Then you could reduce the diameter
of the main core somewhat, lessening the surface (cooling) area of the “radiating”
shell. Hope this helps.
Pepe
@KristijanL built a pool heater unit for a neighbour. It should be an entry on one of his threads.
The issue isn’t going to be how quickly you can heat the water. 400 gallons for a pool is not a lot of heat. A couple of home made solar thermal panels would probably do it cheap made from pex or simply get a used flat plate panel from craigslist you will probably have to disassemble it and fix some leaks. But if you want to heat it with wood I would look at the thread recently about building a wood gasification boiler with a storage tank. You could simply make the pool one zone on your home heating loop. I would go for an outside boiler or shop boiler though as I wouldn’t want to put that heat in the house all summer. You can often find used Tarm wood gasification boiler on craigslist for short money just avoid the coal comb boilers I have read that coal smoke is hard on them and shortens their life.
I wonder if building fins on the outside of the chimney and putting copper tubing between the fins and in direct contact with the chimney and the fins would facilitate a higher heat transfer.
Also, I wonder if running the exhaust horizontally for a couple feet under water would transfer some additional heat and not affect the operation of the stove?
Can you explain? Nor sure what Copper Band is?
I thought of this, however the cabin and hot tub are in dense woods so without cutting away enough trees to get the sun in, it wouldn’t work.
I bet the roof gets sun
It’s not built yet but the chosen location wont get much sun at all.
This is by design, as sun would make the cabin hot as hell in the summer.
In the winter, the wood stove will be going for heat and cooking.
Consider a snorkel stove. The ready-made version is aluminum from snorkel.com. Years ago I heated our above ground pool with a 100 # propane tank immersed in the pool. I calculated the buoyancy and poured cement in the bottom for ballast. Just welded on a chimney pipe sticking up and a filler/draft tube down into the tank.
How much solar heating you get all depends on the design of the building. If you haven’t started building yet I would seriously recommend you read up on passive solar design. I could recommend some books if you like. There should be a discussion on here where I did that a while back it didn’t seem to get much intrest at the time. But avoiding the sun will also avoid labor free energy. If you include solar in your plans you can get you electricity and a good chunk of your heating from it. I am unfortunately in a situation where I don’t have the luxury of building new. I was planing to do just that when my family situation changed.
Thanks Dan, I already have my plans for electricity generation. At least 1 4’ overshot waterwheel with a generator and battery bank. Makes power 24x7 rain or shine.
Solar is excellent if you can’t do a micro hydro project but I have plenty of water and enough head to build as many water wheels as I like.
Not to mention I like the sound…lol
I am considering it Bruce, the only question in my mind is temp control. With a pipe I can simply shut it off or divert the flow and the temp wont rise.
Not sure how the snorkel would work with the temperature. I know you can adjust down the intensity of the fire but there would be residual heat in the stove and the fire still burning at all will still increase the heat of the hot tub I would think?
If you have no shortage of electricity it might be easier to use an electric water heater off your hydro electric. Honestly you won’t need batteries for anything more then in rush power buffering.
Around here it is next to impossible to actually get permits to build in or around water. But if I could build an old fashioned water wheel house I would be in heaven. Well atleast until it started freezing in the winter because I am sure there are issues. My advice on that type of system would be to get a solar or wind MPPT charge controller. It will be much easier as you won’t have to worry about getting the frequency correct on the AC generator.
Barry,
Water takes a lot of energy to heat and there are significant heat losses even if well insulated. Unless your immersed heater is massive or your hot tub is tiny, you should have no problem controlling the stove output to match the tub losses. 25/75. 25% good design, 75% operator skill.
I will have an mppt charge controller for sure but the mini Hydro generator will be 12 volt DC.
I will have a fair amount of power but a 2 or 3000 watt water heater elements might strain the system a bit