Hello I am new to the sight and am about to undertake a boiler project. I have an idea on how we were thinking of how we could make a tank into a boiler may be the wrong direction.
Would it work if we took the 500 gallon tank and cut it in half. Then fabricate a face for it to install a door along with a fan and barometric damper. Cut a flue stack in the tank.
Then to pick up the heat take copper tubingand wrap around the tank.
I'm in the mid stages of a similar project. Learning a lot here.
I'm less than enthusiastic about copper tubing as a heat exchanger. Great heat transfer material, easy to work with, but has a lot of potential for corrosion with dissimilar metals. Nonetheless, there are 100 youttube videos of guys doing just that. Copper tube loosely wound around the tank will have terrible heat transfer. You can increase it by A. insulating the copper with high temperature insulation and B. Adding some material like plaster of paris or furnace cement around the copper tube to get good heat transfer to the hot surface. or C. increasing the heat transfer surface by using more pipe.
There is a great youtube video of a guy *freezing water* inside a copper tube, then easily bending it around a mandrel without crimping, then using it in a wood stove heat exchanger. You can do the same with sand, but it is harder to get out of the pipe. Using more 1/2" copper pipes is easier than less 3/4" copper pipes (which have twice the flow and half the pressure drop of a 1/2" pipe)
Lots of people here are enthusiastic about the Garn which is similar to what you describe - a wood stove going full bore in a big tank of water. The Garn uses 30 feet of welded pipe inside the tank as a flue, that and the firebox are the heat exchanger. Flue is accessible and cleanable. Plan for that flue to get fulla crud, and be cleaned. You could build one you know.
I've been playing with scaling up a rocket stove, then adding a flue heat exchanger see
this thread
I'm looking at separate heat exchangers - so-called fire tube (flue gases in tubes within the water tank) or water tube (water in the tubes, flue gases around). If you have fire against the tubes in any way, copper won't work - won't take the heat, corrodes fast. You'll have to use steel or better yet $tainl$$ $teel tubes if they are directly in contact with flue gasses.
Eventually you'll need to know about water treatment -
this site http://www.woodboilersolutions.com/ has excellent support for the DIYer wood boiler owner, and has good tutorials on the subject and sells chemicals and test kits. If you have a simpler atmospheric tank, you'll need more treatment chemicals. If you have a more complex closed system tank, you'll use less water and thus less treatment, but have more gizmos and overtemp valves and expansion tanks against a steam explosion. Choose your poison wisely.
Buffer Tank size is simple - it has to absorb all the heat your stove will produce in one firing, regardless of the heat load in your house.
This post goes into the geeky details about sizing things relative to stove output and your house's heat load, with references.
That's about all I know, and I'm like you trying to learn as fast as I can (winter's comin' you know). I am determined to be using a new outdoor wood boiler before the snow flies.
