Rocket Stove Hot Water Heater for Outdoor Wood Fired Hot Tub

Hello Ladies and Gentlemen -

First, awesome forum! Just what the world needs. I've built passive greenhouses, have a hugel bed, studied sustainable development as an undergrad and am hoping to soon build a passive house, so this is a place I will be visiting often. With any luck, you folks may help our species survive all this destruction.

In the works is a rustic device of leisure - an outdoor hot tub. The stove will be a drum with perlite/cement insulation and duct pipe riser with a copper heat exchange. Here's my plans... I owe thanks to many other's research and tips on youtube and other forums, so thanks!

Overview:

55 gal drum shell
Burn chamber - 7" ID (inner diameter) brick
Air/Fuel mixture - 4-5" ID hole in drum connected to 7" duct pipe insider
Heat riser - 7" ID x ~30" duct pipe
Insulation - 4 to 1 perlite to cement
Heat exchange - 50' 1/4" ID copper tubing (filled with frozen water for smooth bending into coil)

Bottom 3rd of drum is filled with perlite/cement insulation (Ins.) around all duct pipe and 90 degree elbow. Remaining heat riser has 2" Ins. jacket. 2" Ins around interior of drum (remember we want this heat to go into the water, not the air). 6" exhaust hole ~15" from bottom of drum.

Copper heat exchange runs up from tub, into drum, into the top 12" of the heat riser where it forms a 6" ID x 12" coil and then runs down out of exhaust hole (in a large coil if there is excess tubing). I may also just have a small tub of water on top of the barrel that drains into the hot tub.

The tub (6'Lx4'Wx3'H) is approximately 500 gallons totally full, maybe 400 allowing for people, so a good bit of energy will be needed.

Questions:

1. Should the heat exchange be inside the heat riser (near the top) or will this degrade the rocket stove performance?
2. Should the hot gases be in direct contact with the copper tubing or should I heat a sleeve of water and put the coil inside (like a double boiler) - a question of both efficiency and durability?
3. Is my general stove design/dimensions sound?
4. Other comments/concerns/questions?

Thank you very much for this excellent resource! Any and all advice is welcomed!

First, the 'double boiler' method is much safer. In your pic I see a couple things "less than optimal". The inside of the heat riser is not the place to take heat from. Let the combustion complete or you'll get soot and/or creosote in the rest of the system. And your water flow is backwards, you want the cooler water in contact with the cooler gasses - flowing hot water past the cooler gasses is not productive. But exposing the piping to the intense heat makes things critical. Pump failure is not an option, the water will flash to steam and likely blow a pipe apart. Mineral buildup may become a problem, depending on your water.

This is the best setup I've seen: http://donkey32.proboards.com/thread/1096/hot-water

Something like this:

could work with a coil in the pot, maybe put a J tube under the barrel and use a deeper barrel inside if you need more capacity.

Interesting. Thanks Bob!

That picture is very helpful. I'll scratch the coil at the top of the riser and just put a pot ontop of the drum with a contained coil as you suggest. Seems like most of the heat energy should conduct through the metal and into the water. I'd like to keep the design as simple as possible, so I'm not sure about dropping the pot into the actual drum.

Actually, is it even worth having the copper coil in this case? Is it more efficient? I could just circulate water through the pot, which would save some money.

Is the chimney in that design a critical component or just to move exhaust away?

If you do away with the coil you're moving away from the double boiler. It could be OK if the pot is not closed - pump water into it and let it overflow. If the pot is closed and it gets low you might have a problem (bomb).

Having your flue gasses in contact with the sides of the pot gives a lot more area for heat transfer. The chimney probably improves draw but you could also set it up with an open top (with proper gap) instead. Go to
http://www.aprovecho.org/lab/rad/rl/stove-design/category/1 and look at items 5, 8, 19, and any others that may catch your eye.

Thanks so much for the aprovecho resource (turns out I have some friends interning there - a pleasant coincidence!) I had to drop the project for a while but I'm back at it.

I attained a free 50 gallon gas water heater. I took off the bottom to expose the natural gas burn chamber, which funnels straight up through the water tank.

It is this design:

http://jimblog.me/wp-content/uploads/2014/09/DSC04887.jpg

Seems like an inefficent design to me, but they mass produce these things, so you would think they are fairly efficient. If I were to put my J-pipe below the flue and seal it all up, would that suffice?

The aprovecho water heater filters hot exhaust around the entire water tank which seems better (more surface area). In the existing gas heater, there is a gap around the tank filled with insulation which I could remove and then filter exhaust around. Then I would need to re-insulate the outside of the unit however. Anything I should watch out for?

Either way, I plan to dig the J-pipe/burn chamber into the ground and surround it with perlite cement mixture.

Zane Bridgers : Thanks for the Kind words, We try very hard to give the very best answers we can, Heating water with any solid fuel heater is very problematic!

The more 'safety devices' we pile onto the simple heater you want to build, the more potential problems for safety of the build we create !

At the Top of this page right hand side, below the Permies Banner and above the Permies video of the week, is the Permies Toolbox. Find and click on the [SEARCH]
space! This will take you to another page where you can enter the words Boom-Squish in the search engine, then scroll and select at forums search the
all available option. Then make sure you have selected a Permies wide search and click on Search again !

The over 300 articles that you will receive will convince you we have considered this option before, and the extreme seriousness we treat ALL enquires in this
Area!

I also must caution you to view any thing you see on you-tube with ultimate careful observation, plus a review of the tone of comments positive and negative
in the comments section, if the builder has blocked comments or there have been no recent comments this is a bad sign !

There is a LOT of Steaming Crap on u-tube For the Good of the Craft ! Big AL

I greatly appreciate your warning! While it makes intuitive sense that phase changes in pipes could result in a pipe bomb, I'm not sure I grasped the severity (just watched the mythbusters to convince myself).

Now as for this design, the existing gas water heater has a burn chamber and vertical flue (heat exchanger). I expect the risk with a direct rocket stove conversion is the increased fuel/hotter flame. If enough of the water turned to steam too quickly without an escape outlet, that would mean a big explosion.

However, there is one blow-off valve built in (likely inadequate for increased burn temps/solid fuel mass) so could't I simply drill a number of large diameter holes on the top of the unit as extra precaution? So long as it does not hold pressure, there is no immediate risk, correct?

Now, as for the actual rocket stove design, it needs to be far enough below the unit for the exhaust to heat and re-burn before coming in contact with the water. I suppose that's a calculation I can do with some searching.

What about the heat exchanger? Any thoughts on directing the hot gases around the exterior of the tanks verses through the existing central pipe?

Many thanks for all the support and wisdom!

Zane Bridgers : Help is coming, I just have to make you pay for it

1st a word about all the crap on U-tube, Because U-tube is hungry for content, if Someone will shoot it, u-tube will make a place for it. No checking
to see if its, safe or Anything. Most of the So-called Rocket Mass Heaters/Stoves are little better than 'Flaming units of Death', and within 4 months
of us saying This is the worst one yet, there will be a new crop of Frankin-clones out there even worse !

This has long ago convinced us that we have to be extremely careful about what we put out there, Something that I freely share with a plumber
qualified to wrk on high pressure boilers gets misunderstood, or a critical part gets installed backwards or upstream from a valve instead of down-
stream, or a hunk of rust goes for a swim, And you can have the Boston Marathon Bombing, only worse !

So Kirk Mobert, one of the original geniuses who were trapped in slavery to Interned with Ianto Evans has an open/non-pressurized system
he thinks is near fool proof. He Recently did a workshop where this model was installed. After being in service for a year or so, Kirk Mobert Might
consider sharing some detailed plans. In the meantime you can go to

:::--> permies.com/t/39243/rocket-stoves/Rocket-Mass-Heater-Workshop-emphasis<--::: Hint highlight the BOLD part and Right Click,

It should open in the Address Window, or as a Google Search ! The pictures are in todays Thread extension from Kirk

I hope this is timely and you find it useful. For the Good of The Crafts Big AL

Thank you kindly Allen!

That design does seem pretty foolproof. Ironically, it reminds me of Geoff's design on youtube (I know I know):



Just to clarify the copper coil in water bath design. Is this for efficiency or safety? I know it's a big no no to have copper coil directly exposed to high heat, but in an open system like this, it seems just as logical to keep the boil-off tube, but just circulate water directly through the tank. I suspect therefore it's water-to-water heat transfer being more efficient than air-to-water...

Bump ! Big AL