I've put off responding to this thread, mainly 'cause it needs some thought. Heating water has come up more and more lately. It’s taken a few days of rumination, then a few days more to write some stuff down, think about it, write more, etc. Please excuse if I ramble, oversimplify or assume.
The major bug to avoid is steam explosion. It's a REAL hazard that deserves serious consideration. If not for this ONE issue, heating water would be a fairly simple matter. So, I think it' makes sense to separate (the task of heating water) into two basic categories:
1) The Bucket-'O-Water (or) the open system, pressure CAN'T build up but when you want to use the hot water, you gotta take it out manually.
2) The Closed System, one that relies on or uses pressure for delivery, without relief valves and other safety measures, you've got a bomb in yer space, but it will deliver hot water TO you via piping.
What they need in COMMON, is good heat generation and transfer. The general desire is to get as much energy into the water as possible as quickly as possible and eliminate a lot of waiting around. Fortunately, the concepts (and science) behind heat transfer are relatively simple and well proved. In a nutshell: make as FEW conversions of medium (fire to pipe (walls) to water, or fire to pipe (walls) to oil through pipe (walls) to water) as possible in as short a distance (pipe runs and such) as possible from the source. Provide a lot of surface area for maximum transaction between whatever medium you place between the water and the fire (coils, radiators, the tank itself, etc.), everything should be as conductive as possible. Free heat can dissipate fast, grab it up!
You're intending to heat a LARGE amount of water (55-250 gal.) so don't even bother with less than an 8 inch rocket stove system, maybe even a 10 inch one! My question would be, “Why do you need such a big tank, what’s all the water for?”
Trying to heat large amounts of water takes a LONG time, whereas heating a small amount takes VERY little. Rarely is 55 or 100 or 250 gallons of water used, or if it is, it's usually used slowly. I've done workshops where after a hard day, 10-20 people all wanted showers. My system holds 20 gallons of water which takes a half hour to get hot (on average) one tank full obviously won't shower them all, BUT! We've been able to CONSISTENTLY do 'em all back-to back. It's a usage pattern thing, no one passenger uses all the heat, the volume is easy to reheat, if the fire is running all the while you gain AS you use though mostly during short down times. While the last passenger is dressing they pop in another stick and give the fuel a shake, by the time the next passenger is in the tub, you got blazing water again. We've made TOO MUCH hot water during this process before. Showering 10, back-to-back, we've popped the safety.
This leads to another disadvantage of wood fired water, REGULATION of fuel and water flow. Coil systems (maybe I should say “exchanger systems”?) can be divided into two basic categories (IMHO), passive, thermosyphon and active, pump systems. With thermosyphon systems, the MAJOR advantage is complete off-the-grid use without electricity or moving parts with the exception of safety valves. The tank should typically be above the exchanger or coil by a foot or two to avoid reverse-thermosyphon and EVERY foot of horizontal pipe run needs 2 feet of vertical for proper flow. Pumped systems can have thermometer activated pumps or not and have the advantage that the tank can be ANYWHERE and there can be active heat regulation with bypass tanks and other safety features. But, when the power goes out, yer outta hot water.
Personally, I like to avoid coils and such; they can kink, break, or glop up with hard-water deposits. They tend to have (at least the home-made ones) limited surface area for good heat transfer. I have seen what I consider to be effective use of coils and other heat exchangers, so I'm NOT saying don't do that, I just like to steer clear of 'em. I think they add (unnecessary) complexity, a point of possible weakness or failure in the system, potentially dangerous failure!
I grew up here on the North Coast of California, during the Back to the Land Movement of the 70's a whole lotta folks moved into the woods and started working on their off the grid systems, the "Blazing Showers" company was based in my home town. Their solution (indeed at the time, THE solution) was to retrofit a woodstove with a coil. This system worked but it had some shortcomings, water coils IN the fire cool it off, lowering the fire temperature, causing smoke. Many chimneys developed creosote problems and there are dangers of running a little bit of water (at a time) through a LOT of heat.
Fortunately, with rocket stoves, it's easy to place the coils downstream from the burn zone. Be careful though as these stoves can produce VERY high temperatures, if you place a coil too close to the direct exhaust heat and/or use too small pipe diameter, it's REALLY easy to flash the water in the pipe to steam, which gets you MOST of the way towards blowing yourself up. A possible fix for that, it has been suggested that coils can be buried in a cob bench after the first 5 feet of run. That's great if you're not heating a LOT of water AND you don't expect TOO much from it. A nice side-effect or secondary benefit of living with an RMH, BUT that won't be adequate if your PRIMARY need of the system is lots of hot water. 'Nuff said about coils.
I tend to favor placing the tank directly in the heat; my personal preference is to remove as many parts, ESPECIALLY moving ones, as possible. Usually, the tank itself has more (or at least as much) surface area as a coil would and there's a LOT less fussing with pipe, sweating copper and all that business. I’ve been using cast off electric water heater tanks, our local dump attendant lets me strip the tanks at the dump as long as everything gets to the right bin.
What seems to work best (for me) is to build the tank into a bell-like system; bell stoves are VERY good at even distribution of heat. Water heaters, most often look for all the world like a pot sitting on a fire.. Yeah, it works but I feel that this isn’t really the best way to go about it. Almost whatever you do, the thing acts like a chimney and generated heat will flow up and away very quickly. Too much heat just slips past and contributes little to the water. It’s the reason why Aprovecho put so much research into pot gaps and it makes a difference. It seems easier to me and probably more effective to use bells. Not only do bells distribute heat very evenly but they also fraction temperatures, with the hottest gasses remaining in the bell at the top and the coolest leaving (for the chimney) at the bottom of the system. This will effectively prolong contact between heat and tank, making more available to the water WITHOUT fussy tolerances in the system.
I built one this way out of adobe with a straw/clay plaster-like insulation (inside the adobe bell walls), a 20 gallon scavenged electric tank, used the unrolled sheet metal cover (from the tank) as the lid, insulated that with woodchip/clay, the tank sets BESIDE the rocket heat riser and heat flows into the bell from the top, the safety valve pokes out the top and is plumbed away.. It’s an 8 inch rocket stove, though it works best with a 6 inch chimney (from the bottom). This quite primitive little beast makes for a hot shower in 15 minutes and can be carefully (and judiciously) fed for practically unlimited turnaround. We HAVE popped the safety valve on these, which is a mixed message.. On one hand it tells me that the thing works well, on the other it says that it’s not behaving safely. Though I’ve got to say, while these systems are NO LESS dangerous (in their own way) than coils, I’ve found that they are “self-training”. You learn very quickly how much wood it takes. In the 7”x7”x12” firebox (8” system) in my bathhouse stove (at home), two loads of small(ish) split oak will get you a hot shower, three a scream’in hot bath.
It seems to me that an open topped system could be fabricated pretty simply. It would feature a bell chamber surrounding (or welded to) a 55 Gal. barrel or something like that. Placing the barrel beside the stove keeps all that hot water down low in a safe place, instead of up over the fire (and your head), it’s easier to tend that way too. The top of the barrel would clear the bell and a tight, insulated lid would cover. I imagine the thing could be all welded, filled with pearlite, wrapped in kaowool or other lightweight insulation and be portable.
