IBC tote rocket water heater?

Okay guys, I'll start with a quick introduction, I've been interested in rocket stoves/ rocket mass heaters for a number of years, and google yields a lot of results from this site, so I figured it's time to join and ask the experts for some input prior to attempting my first heater.

I part time off grid, I've build a wood barrel sauna next to the river, and next I'd like to add a plunge tank. I've got a couple IBC totes I received for free and I'm willing to cut one up to make a plunge tank.

My issues being off grid, is keeping it from freezing.  

Design: what I'm thinking is build a self feeding rocket heater housed within a propane tank or similar metal vessel, but really, it will be whatever is cheap and available, so I'm leaning towards that. Of course I'll take all necessary precautions prior to cutting or welding.

I know it's mentioned that metal for the burn chamber is subject to extreme temperature will burn through eventually and isn't ideal, and should properly be insulated by fire brick or clay. Unfortunately I don't think that will be possible inside a propane tank due to its size. So perhaps adding metal plates surrounding the burn chamber might buy it some time? It also won't be running daily, the intentions are only to keep the IBC tote from freezing, so maybe weekly burns, or maybe the odd extended burn to get he water up to hot tub temperature, if it's even possible. Again, primary goal is to keep the water from freezing, and it would be a bonus if I could get it to hit tub temperatures!

I would like to have the burn chamber and riser within the propane tank, and the heat pass around the outside of the riser and exhaust out a chimney from the side of the bottom of the tank.

So up until this point this heater would work above water, what I'm proposing is to have the angled feed tube extend through the side of the IBC tote by means of a gasketed metal flange bolted in place that surrounds the feed tube as well as ash clean out/ air feed. Hopefully the water in contact with the metal "flange" coupled with some distance until the plastic tote would prevent it from ever melting. And of course have the chimney from the unit extend above the water line.

What in hoping this would accomplish by having the top of the heat riser submerged, with a jacket around it before passing exhaust is to extract the maximum amount of heat from the fire while also keeping the burn chamber and heat riser hot (as opposed to the water pulling heat directly from an unjacketed heater such as a commercialy available hot tub snorkel stove).

By means of the gasket and bolts, the entire heater could be removed from a drained tank come time when the burn chamber burns through, or should a weld spring a leak and flood the entire system and require re welding.

This design would also allow the heater to be positioned at the bottom of the IBC tote and housed under a wooden bench, heating cooler water and theoretically allowing for more volume of water above the heater to be warmed up, addressing an issue I've found with my parents snorkel stove hot tub.



What do you all think about this proposed design?

Anything I've overclocked?


Can't wait to hear your valued input,
Cheers!

This is the type of design I'd like to try, except an angled feed tube that meets up with the riser inside the tank, instead of vertical and horizontal. And also house it all within a 20# tank.

It's from a thread in here: https://permies.com/t/18110/Welded-Garage-Rocket


Maybe the size of a 20# tank won't be large enough to get a proper height riser?

https://permies.com/t/154112/Pocket-Rocket-snorkel-stove-pool

And I think having the heater inside a vessel will solve the problems in point 4 and 5  from Eliots response in the thread above.

The bell as he calls it will be completely submerged, and the riser won't be in contact with water, but surrounded by hot exhaust gases before venting out the chimney from somewhere around the base, similar to the photo I attached from another build thread.

Karl:

Interesting!  Welcome to Permies and thanks for sharing your plans.

This approach does indeed solve the riser problem.  I'd encourage you to consider an insulated riser (a 5 minute one?) just because that really helps it get going when cold.

I'm uncertain about submerging the bell.  I'm not an expert so I won't say it will or will not work, I can just say I'm concerned!  Please, prove my concerns wrong!

I wouldn't worry about the steel feed tub getting hot enough to melt the HDPE of the IBC - as long as there is water surrounding it on all sides, its going to be very effective at drawing off the heat, and the feed tube area is generally one with low temperatures anyway (a lot of air movement there too).  I'll point out that, IMHO, nothing sticks to HDPE so you'll need a gasket, bolts, flange to seal that.

Don't place it directly on the bottom!  Use bricks, rocks .. anything really ... to make sure there is a good amount of water between the bottom and the IBC.

In terms of insulating the chamber vs using more steel ...  the right ceramics are thermally inert and don't care about the temperatures (they may care about friction...).  Steel is ablative, slowly losing thickness under the assault of temperature and friction.  My observation is that steel rocket stoves that hold up are probably not working well and not getting hot enough... so a steel one that lasts is a bad stove, and a steel one that eats itself is a "good" stove.  With all that water it may never get hot enough to damage the steel.

Also ...

I don't remember my propane tank sizes ... around here a 4" round cross section is generally considered the minimum size for an RMH.  So make sure that there is enough room for that diameter of pipe.  

If one tank isn't tall enough, just weld another one on?

Thanks for the reply and welcoming to the forum Eliot.

I've got a few questions, and some answers to what I'm thinking.

What is a "5 minute riser"?

What are your concerns about submerging the bell?



I would plan to make a gasket as you say, and bolt the flange up to the tank, I was leaning that way to be able to remove it if need be, and wasn't aware about the difficulty of having something like an RTV gasket stick to the tote.

And I was visualizing the top handle of the tank to rest on the bottom of the tote or perhaps a concrete paving stone with anchors to prevent it from floating, as well as being held in place by the flange. That should hopefully give it more than enough of a barrier with the water surrounding it.

I'll try and look at a way to insulate the riser/ form it so the metal could wear away leaving only the riser. Would a heat clay be able to be packed in and hold its form if the internal pipe wastes away? My concern is using a 4 inch pipe as the inside, going up to something like a 6 inch for the outside will not only be much harder to source, but also leave very little room inside the tank after that.

I also would like stay within the size of a 20# tank because of the small space of the tote, I'd like the unit to be able to rest under a bench. So hopefully that size allows for a proper height riser in relation to the feed tube.

Thanks again for the reply!

I'm sorry to have to tell you that a 20# propane tank is not going to be tall enough to enclose a functional riser. Angled feedtubes leading directly to the base of the riser eliminate one of the features of a rocket which is the wood gases encountering sharp bends which promote mixing and better combustion. If the wood is burning in the feed before it reaches the bottom, the flames will need to be sucked down and you will need an even taller riser to keep the draft going the right way. Starting such a beast will also be problematic - you may never be able to get a fire that wants to go in the right direction. You would need to insulate the feed at least near the bottom, or the water around it will steal too much heat from the fire.

Recommended proportions for a J-tube core are in the range of 1:2:3 or 1:2:4 or 1:1.5:3 (feed tube to burn tunnel to riser), and in any case the riser must be much taller than the height of the feed opening from the base, or you constantly risk the fire burning backwards. A 4" system will consume its fuel so fast that you will have to feed it every ten minutes at the least. Can you have someone sitting outside feeding the fire while someone else enjoys the tub?

Given the size of an IBC, I don't think you can fit an effective heater in it and have room for people. I think the way to go would be to make a standard J-tube core outside, heavily insulated, and run its exhaust in and up through the tub. It could go through a propane tank for a place for gases to slow and transfer heat. You would need a metal plate at the penetration to avoid overheating the plastic, but with water cooling the plate, a couple of inches clear around the flue should work.

Karl Harasyn wrote:
What in hoping this would accomplish by having the top of the heat riser submerged, with a jacket around it before passing exhaust is to extract the maximum amount of heat from the fire while also keeping the burn chamber and heat riser hot (as opposed to the water pulling heat directly from an unjacketed heater such as a commercialy available hot tub snorkel stove).

I just want to revisit this  - its a perfectly understandable approach to put as much of the hot area into contact with what you want heated.  Burn wood, make heat, move heat.  The hard part to understand about RMHs is that the gain in efficiency happens during the burn - that the combination of feed tube, burn tube, riser (and bell) are a finely tuned engine for producing heat.  Altering a part of the engine can have huge consequences ... so too short a riser, placing the whole thing in water, too much cold back pressure - these are like the potato in your auto exhaust, or some some of intentional de-tuning of your engine.  And you can't just have a free-burning stove in a tube and expect it to perform.  I do wonder how the proposed idea - inserting the entire bell in the water - would work.  It seems to me like all that cooling would act like a brake dragging on the engine, but maybe not.

So to paraphrase Glenn, put the engine outside of the IBC.  Wrap it in insulation, then pipe that heat into a stratification chamber in the IBC.

Again, its hard to wrap your head around.  I'm going to throw out some random numbers for illustration.  The snorkel style stove is (say) 80% efficient at transmitting heat into the water, but because it doesn't burn efficiently only 50% of the potential heat is created.  So 40% of the potential heat gets into the water.  The RMH "engine" placed outside of the water , even if insulated, is losing heat to the atmosphere and its mass - so let's say that its only 60% efficient at putting heat into the water.  BUT its something like 100% efficient at converting potential into actual heat - so 60% of the potential heat gets into the water.  That's 50% better than the snorkel, and its all because the "engine" is so much more efficient.  Again, numbers are imaginary but the conclusion is, I think, real.

Note that the numbers are very different for heating a house - comparing a standard wood stove to an RMH is slightly unfair b/c the standard stove doesn't try to store heat in a mass.  But it does allow Paul to boast about how little wood they use!

So Karl ... building a J tube OUTSIDE the IBC means you can do it with brick/ceramic and it will last longer.  Use the propane tank as a stratification chamber in the IBC (I'd suggest at least two of them).

If I were heating an IBC with an external hot air source, I would run the flue in horizontally near one corner and around the walls to maybe the opposite corner and then up. Maybe around three walls, if the exhaust after a two-side run was still quite hot. Holding the flue about a foot off the bottom would allow feet to stretch underneath for more roominess. A bench built a few inches above the flue would protect skin and give a needed function of seating. I would give the flue a slight constant slope with a weep hole outside the IBC so any water that gets in has a place to drain. I would make sure the flue leaving the riser top slopes down an inch before meeting the IBC so that water cannot get back into the core.

For best draft and general function, I would try to have the J-tube sunken relative to the IBC, so that the exhaust could go (ideally) horizontally or else down a bit and then horizontally from the riser top to the IBC inlet. A hillside location, digging down a bit for the J-tube, or raising the IBC on a platform, or any combination, would ensure easiest function.

Okay I don't mind the idea of having a J tube heater outside of the tote, and I could flange the exhaust flue into the tote, around the base, and then out another flange.

That's just a lot of fabricating and material to do so.

I would also like the unit as a whole to be somewhat portable, so building a larger, heavier rocket heater on the ground and pumping heat into it isn't my top choice.



So back to theorizing having a submerged rocket mass heater... If I had a taller tank to accommodate a larger heat riser to stay within the suggested ratios, managed to insulate the riser within the tank, maybe then the riser would be able to stay warm enough for proper combustion?

Karl:

Just so you understand, your use requirements create a bunch of conditions that are generally considered non-optimal.  But the relative portability, ease of manufacture, etc are all totally reasonable requirements - they may just result in the stove itself being less optimal while the overall package is just perfect for your needs.  You're just bumping into the fact that most folks have been optimizing these little beasts for heating a house and so you're in that area of the map where we just don't know enough.

I think so long as the riser is long enough you'll get a pretty clean burn.  An insulated riser will really help with the start-up draw.  We're all pretty worried that if the unit is trapped in a cold mass that is also super thermally conductive (water) it may be really difficult to start - there are assorted techniques for improving cold starts, but I don't see that many of them could work in this case.  Oddly enough it might be useful to have the exhaust exit the IBC and enter a T before rising 6' or so.  The bottom of the T could have a closeable opening, and you might have a little fire there just to help suck air through it ... consider it an option if it proves otherwise difficult.

And a weird thought after using my awesome battery powered Stihl blower to make burn piles burn ... If the outside air is warmer than the water, clean out the ashes and then stick a blower in the feed tube for a moment to shove out a cold air plug.  I'm not sure that your design won't immediately create a new cold air plug, but its worth a try?

Please do let us know how it goes so we can map out this use of an RMH.

Again thanks for the in depth reply.

I'm not opposed to venting the exhaust out the side of the tote, I could go through the same flange for the feed tube, and I would be able to play around with different configurations.

Would insulating the exhaust perhaps help add some draw to the system? rather than having it exit under the same cold water.

I'm not personally opposed to using external fuel to help tings get heated up, a splash of used motor oil, bad diesel fuel?

If worst comes to worst, I seal up the flange hole, I've got an open tank, and I can maybe the heater would work better freestanding in air. And I'm not out a while lot of money, and it will be a fun experiment.

If you want the heater in the tub and fed through the side, I see no reason to run the exhaust out the side of the tub rather than straight up. Straight up, perhaps insulated, offers the least amount of drag on airflow, and would allow for a startup bypass between top of riser and chimney. Then once the fire is going well, closing the bypass routes the flow down through whatever bell or duct you decide on.

I would locate the riser enclosure (propane tank(s)) as close to the tote wall as possible while leaving some water space for safety, so the feed tube can be as vertical as possible, and allow a bit of flat burn tunnel. I think this will help the combustion effectiveness. For that matter, you could even make a larger (say 12" w x 20" h) metal plate in the tote wall and have a proper J-tube, with all insulated vertical feed outside, burn tunnel through the flange, and riser in enclosure a couple of inches inside the tote. Dimensions like 12" feed, 18" burn tunnel, and 36" riser (measured along outer edges of flow channel) would probably work very well.

Another idea would be to mount a "bell" into the floor of the tote and put the insulated J-tube under that, so the hot core doesn't need to pass through the tote wall at all. Run the chimney from near the base of the bell, either straight up or around the floor first and then up. Elevating the tote by a foot, or digging down a foot for the core, seem like trivial tasks.

Glenn - I'd like to clarify your advice here so that I understand it!

Regarding the exhaust and cold starts... Are you suggesting installing an exhaust pipe (with gate) directly on top of the propane tank bell?  This would effectively be an extension of the riser  - and yea, that sure would help a cold start.  But once that is closed the rest of the exhaust run presents a challenge for the new fire.

I was thinking that the exhaust would exit the bell at the bottom and - like in a bench system - move laterally.  Running that pipe through the water affords additional heat exchange - as well as more cold air.  My thought on an external T is that by opening the bottom two things could happen: 1) the additional resistance of cold air in the vertical exhaust is removed.  The vertical cold air can literally fall out of the exhaust, the cold plug in the lateral faces less resistance in being expelled.  2) if necessary a heat source can be applied at the bottom of the T.  This would forcibly expel a vertical cold plug and should establish a small venturi vacuum in the horizontal run, helping the draft on the other end.

Its really just a question of where to provide draw assistance to the fledgling fire.  You're in a MUCH colder place than I so I'd like your judgement/experience on where to apply this help.

Also, I like the idea of raising up the IBC and installing a bell in the bottom - this is similar to my thinking for a hot tub.  Practical concerns ... having the core underneath a big hunk of splashy water means that the core may be surrounded by wet dirt - not a problem if the steel version is used, but an eternally soggy ceramic construction seems a problem!  Also, I'd think its easier to penetrate and seal the side (of an IBC) instead of the bottom - also easier to see if its leaking!

It's a worthy experiment.

There are multiple threads on here talking about how poorly a RMH runs before cob is applied.
A couple of photo spreads with pics of what looks to be pools of creosote accumulating.
Your proposal has the RMH doing a dead lift (complete with whatever condensation forms) every time
The condensation will surely shorten the longevity I think.

There was an earlier thread last week about using eutectic salts and this is roughly what I proposed for that application
If the salts were maintained at 200f as the lower set point with a top end of 400+ then condensation would not form methinks.

One other option is making it a true monotube boiler, install coils inside the bell and eliminate the potential for air pockets via gradients, then locate your IBC tote nearby and simply pipe to said coils. (keeping the elimination of air pockets as a priority).
Prewound steel coils as repair parts for steam cleaners are available

http://www.farleysinc.com/coils/Alkota_Coils.php

This coil could be used as is with a pump circulating, or cut in half and the inner and outer layer separated for thermosiphon applications

Mounting the core beneath the IBC would definitely require water-shedding or diverting treatment. Positive drainage so that water cannot accumulate near the core, and construction separated from the earth so damp soil cannot touch it, would be needed. I would want a large enough space that you could reach and look in when it is not running to check for leaks or other issues. I would put the bell in a corner of the IBC which I think would give the most spacious feel; this would make the underside pretty easy to get at for inspection. I think the initial construction/installation of the flange in the floor would be no different than in the wall. Maybe caulk would be easier to keep in place during installation.

If you exit with a duct from the base of the bell and run all around the sides of the IBC, the chimney could rise right next to the bell and have a bypass connecting the top of the bell to the chimney. Gradually closing the bypass should be able to encourage flow through the duct.

Some interesting points being brought up.

I never thought about condensation forming inside the heater when the tote of water cools back down. I wonder if that will really be something to worry about, or maybe when the mass of water is cooling down, the heater will be cooling down at the same rate and won't condensate. I'm really not sure.


And with regards to mounting the heater under the tote, in order to achieve that I would need to be considerably more confident in the gasket for any leaks. It would also mean I need to fabricate a stand for the tote to sit on, and full they weigh 2500# I believe.

I'd really like the whole thing to be as self contained as possible. And empty tote is fairly easy to move, and isn't going to tip over even if the ground is slightly off level.



I'm also off grid, I don't have the ability to add circulating pumps to any type of coil, and I also want to stay away from a coil or external piping which could freeze between fires. For me the whole point of this is to warm the water up, and then be able to let it sit for a few days to a week without having to worry about it freezing.

So it seems what you're looking for is a top fed underwater stove as used for hot tubs.

https://snorkel.com/hot-tub-info/snorkel-and-scuba-stoves.php

Bill Haynes wrote:So it seems what you're looking for is a top fed underwater stove as used for hot tubs.

https://snorkel.com/hot-tub-info/snorkel-and-scuba-stoves.php

Well, no.  Karl is trying to use the idea of an immersed wood heater but with the efficiency and clean-burning benefits of an RMH.  Its a challenge because of constraints imposed by location and materials.