Rocket stove water heater for showers. Advice needed.

Hi all,

I plan on building a rocket stove water heater to run a couple of outdoor showers, I'm a plumber and I have some experience with solid fuel heating systems. I would appreciate any advice from you guys.

My idea is to use an old oil boiler like a Firebird popular 90, cut a hole in the bottom and mount it to the top of my heat riser. Basically I want to make use of the burn chamber, baffle system, water jacket and flue outlet already built in to the oil boiler. From the boiler I want to run two 1" copper pipes to an indirect hot water cylinder as a gravity thermosyphon circuit.

The boiler will be filled from a header tank and will have an expansion pipe and safety blow off valve.

The cylinder will be duel coil (to allow for a solar circuit later on) 200L unvented, fed by mains water with non return valve, pressure reducing valve, pressure vessel and thermostatic mixer valve.

For my burn chamber I'm going to use either 4" or 6" box iron, insulated with ceramic fiber inside a 40 gallon drum, J tube design.

Is the oil boilers burn chamber and baffle system going to be suitable for transferring the heat to the water?

Would 4" or 6" or even bigger be suitable for the burn chamber?

Is it going to be safe?

Any advice or suggestions welcome.


Thanks.

Damian Murphy : Welcome to Permies, and a big Welcome to the 'Rocket' and 'Wood Heaters' Forum / Threads, you will long remember your 1st few posts here at Permies.

With over 19,00 fellow members you can always come here and find someone who wants to talk about what you want to talk about !

There are two other places to go to from information within this site. At the top of the page, just below the Advertisers Banner is the Permies Tool Box ! If you click on search
it will take you to an area where you can do a Google search here in the mass data of Permies ! Also check out the ''Similar Threads" box at the bottom of the page where
some similar topics have been picked for you !

You may also wish to check out the comments in the Forum / Thread '3 different rocket stove water heater designs, input requested'!

As you can see you have a fellow Permies member Paul Jones who is familiar with Solid Fuel Burners and their codes and regulations, Perhaps you could reach out to him-
much of what I can quote to you is out of date and not relevant to your Situation !

I would strongly recommend that you try a conventional Rocket Mass heater in a safe location out of doors 1st, to gain the experience to modify A rocket burner so radically !

For the Craft, think like fire, flo like gas, Don't be the Marshmallow! As always Comments and questions are solicited and welcome Big AL

This is an ongoing experiment in Rocket no mass water heating. It is built out of stuff I had about except for the firebricks. I do not recommend anyone tries it.



This is sited outside, it's all in the name!

4.25" J tube with a Baxi Bermuda gas boiler cast iron heat exchanger. It runs about 60c after about 1 hour. After a couple of hours it will run central heating but it is too warm at the moment to be sure how effective it will be. A 7 hour burn did give me 30 gallons of stored hot water and raise the house temperature about 3.5 degrees. At 60c you can put your hand on the top (an old meat tray) above the heat exchanger, and it is not uncomfortably hot. You won't leave your hand on the flow pipe! This suggests a fairly efficient conversion of heat to water.

It has already undergone some changes so if there is any interest I can do a thread on the build and what I got wrong/right.

For the record I have no qualifications in solid fuel. I do have a fair bit of experience from the days before such qualifications were required, but I am qualified in another fuel.

et al : On this side of the pond the only people using solid fuels in boilers or who have had experience in solid fuel-fire boilers are commercial or maritime experienced people.

Having multiple boilers and a whole ocean to dump heat energy in does not exactly prepare anyone to operate a system where the only way to Shut it down in the middle of a
run is to grab your fire tongs to extract the burning Fuel.

Any water jacket that tries to extract heat too soon from a rocket mass heater will find that it has extracted so much heat energy that the rocket mass heater is running sub par
due to there being not enough Heat Energy left to keep combustion temperatures in the zone where near-complete combustion can occur, If the needed heat energy is taken off
at some point further down within the Thermal Mass We would like that temperature to ether be a near match or higher for TheTemperatures we are trying to extract !

Even if we use large enough tubing to create a Thermo syphoning system ( which means the storage tank is located somewhere in our building above the level of the occupancy)
We have to have a need for that heat at that location, or or rely on a pump to pump water to that location, if we have a 12-14vt pump with back-up batteries for say 48 hrs, we
will have solved our plumbing/pumping needs for normal operation.

There is still the problem of the unplanned event, With ether an electrical failure, or pump failure we have to have a second tank above the 1st that is plumbed to drain down the
system, to prevent a water flashing into steam event, the system must be capable of alerting the Owner-operator to the need to remove the wood fuels from the system, as at
some point the absence of water within our piping will case a failure !

In the case of a weather event the inclusion of a water supply system into our Thermal Mass may create a situation where in order to not damage the Hot water system we can
not continue to use our Space heater, Not an plus situation for any number of common cold weather events !

The only suggestion I have is pie in the sky, I am in hopes that certain research will produce a commercial pump that can work on the heat content of the fluid being pumped !
Keep your eye on this site! For theGood of the Craft !!

Think like fire, flow like a gas, Don't be the Marshmallow! As always all your comments and questions are Solicited and Welcome PYRO - Logically Big AL

Al.
Firstly, may I apologize to you and all for a basic assumption that further research has shown to be wrong.
In my innocence I took it that wet central heating systems are the norm. In the UK warmed air systems are in the minority as a means of heating homes. I am not suggesting that wet is right, or the only way, to heat your dwelling, but it is pretty standard here. These systems are powered by wood and coal/coke, oil, but mostly gas. As a result of this we grow up, and live, with very hot water. A controlled gas system would not unusually deliver 80c (176f) to every room. Because the coal/coke/wood options are almost uncontrollable heat sources, avoiding boiling problems by system design is second nature. On the rare occasions there is a problem people just run a bath.

The J tube differs in that it is totally uncontrollable. It requires more care to avoid overheating the water. This is why I don't think a combined mass and water system is a good idea. That isn't to say I think it is impossible, but you raise very valid issues that can result from this combination. So for me it is one or the other. I have chosen to explore the water route purely because it suits me. Attempting to build any type of instantaneous water system into a mass, in my opinion, takes stupidity to a new level. The potential dangers of this cannot be overstated.

You have raised a couple of questions:

'Any water jacket that tries to extract heat too soon from a rocket mass heater will find that it has extracted so much heat energy that the rocket mass heater is running sub par
due to there being not enough Heat Energy left to keep combustion temperatures in the zone where near-complete combustion can occur'

I have pondered at length on this statement. Could you expand on it? The reason I ask is because surely the barrel in a standard rocket, with mass, is extracting heat at a high rate with a gap of only a couple of inches from the riser? Mr Evans suggests, if I have not misunderstood, that the riser provides the push, and the cooling of the barrel, a pull. Does this relate solely to extracting heat for water whilst still trying to heat a mass?

'If the needed heat energy is taken off
at some point further down within the Thermal Mass We would like that temperature to ether be a near match or higher for TheTemperatures we are trying to extract !'

Does this not require an exchange rate of 100%?

Your input is always very thought provoking - caused a few sleepless nights as I've chewed it over, and this might be another one!

Hi,


Thanks for the feedback Allen and Paul.

Great forum, lots of fascinating and inspirational stuff on here. I've been checking out some other threads and have seen some great ideas.

For the moment I am just interested in heating water for showers. I think a mass heater will come later. I have already built a prototype portable rocket stove powered pizza oven, turned out to be under powered with only a 4" heat riser, so back to the drawing board with that one.

My rocket stove water heater is going to be outdoor, mounted on a trailer for portability and using no electricity. This rules out the gas boiler heat exchanger Paul, as I dont think you would get a thermosiphon working through that much restriction and it would require a pump.

Here is a link to a rough sketch I made. http://flockdraw.com/gallery/view/1780342

As you can see, the water in the heating side is open to the atmosphere, if it overheats it can expand into the feed tank or run
through the pressure release valve. This eliminates the chance of nasty, steamy explosions. As the water heats it rises up to the storage tank, where it moves through the heat exchanger coil, giving off heat and then returns to the burner to be reheated.

The water jacket in the boiler holds 19L, with the pipework and heat exchanger coil I would guess around about 24L of water in the heating circuit, with between 120L and 200L in the hot water storage tank (depending on what I can get my hands on).

I'm hoping to have a heat riser of about 36" high and as i dont think heat transfer is going to be anywhere near 100% i was going to oversize to 6" diameter.

I just have no idea how much heat I'll get out of it.

I realise it's not going to be the most efficient and after reading some other posts I'm not even sure if its a proper rocket stove as it doesnt use a cooling downdraught and perfect donut shaped whatyamacall it.

I' m pretty close to putting this thing together and I'd be really interested to hear your thoughts on safety and performance.

Thanks again.

Paul Jones : I didn't chicken out, It took me a long time to marshal my thoughts, So here is a first try at it, Initially we want the rocket stove to be cool, if say the Heat Riser
was an uninsulated metal pipe, as some of the earliest models both here and in Oz were, we would need an additional source of heat from some secondary source, otherwise
the temperature of the gases rising within the heat Riser would stabilize with the Temperature of the outside of the Heat Riser (and within the barrel) and our Rocket Mass Heater
R.M.H. would Stall*. While this can happen with an oversized Rocket in a too small room, or a barrel with too much Cob on it, the Several layers of upgrade in Heat Riser Tech
and wrapping it in Mineral or Ceramic Wool, does several things, It has proven to increase Temperatures to where we are seeing sintering of sand and of mineral wool and it is
reaching these higher temps much faster !

Apparently what we have been seeing and hearing is an interesting transference of forms of energy immediately after the the hot exhaust gases leave the Top of the Heat Riser
and slam into the inside of the 'lid' of the barrel, here they form a Turbulent tumbling shape sometimes called a Toroid, this doughnut shape is roughly formed by the continued
upward flow of hot exhaust gases, the lid of he barrel and the gap between the sides of the barrel and the top and sides of the Heat Riser! It was pointed out to me by a
countryman of yours that when you increase turbulence in any flow you lose some flow and always gain noise and heat, He was concerned about it, as anyone trained in fluid
mechanics would be but, I just saw it as a non-issue. More heat ? more noise ? in a rocket stove ? I gave it no further thought !

I am now proposing to you that this shift in energy from one form to another, coupled with energy released in the final burning of hydrocarbons freshly mixed with Oxygen in the
Toroid is why we were able to make the original R.M.H.s Work, even if there was a stabilization of heat energy acrost the bottom of the Heat Riser, there was still a difference
in heat energy to create the heat differential engine that 'drives' the R.M.H.

Allowing for the large number of problems associated with trying to take heat off of our barrel safely, I am now convinced that we must allow the R.M.H. to perform at its peak
temperature and efficiencies until at least past the Toroid ! There can be little expectation of finding any usable amount of unburned hydrocarbons past this point !

So if we were seeking only hot water, and zero space heat, we would want a large diameter spiral/coil placed within the space between the Heat riser and the (insulated?) skin
of the barrel, this arrangement would create turbulence and more heat and reduce the distance the hot exhaust gases could flow, and that is a design question left to someone
with less scar-ed grey cells!

I think that the size and style of your R.M.H. would have to be changed to a batch loading model which would make the function of the Toroid as a secondary/final burner
absolutely critical! Again this is pure research and outside of my purview.

I do want to sneak in one final thought, when a fluid pump is created that will work off of the heat energy of the liquid being pumped, start working when the fluid is hot, and
Stop when the last of the fluid is pumped, Then and only then will we have a reasonable chance to escape from the Dangers of solid fuel-fired hot water! 4 the Good of the Craft!

Think like fire, flow like a Gas, Don't be the Marshmallow!! As always, your comments and questions are Solicited and Welcome ! PYRO - Magically BIG AL !

AL.

Thank you for taking the time and trouble to post this detailed explanation. The penny, as we say, has dropped. Well almost.

When you say 'the final burning of hydrocarbons freshly mixed with Oxygen in the Toroid' where does this oxygen come from? Is there sufficient oxygen still left in the riser after the initial burn or is it created in the burn? I can see that when the fire is first started this oxygen will exist.

Refuelled the experiment continues!

Paul Jones : For a long time it was assumed that one of the reasons for the R.M.H.s barrels long life was due to the fact that the inside of the barrel was an oxygen deficient
environment, mostly the presence of almost no Carbon monoxide in the exhaust was a first clue that there had to be 'some more' O2, but while I have no facts at my fingertips
I do believe that 'some' levels of Oxygen have been found ! This of course begs the question- 'Oh yeah'? 'How do you explain the barrel lasting so long'? I think we need a
chemist who understands highly excited Molecules at the electron level or a really clever Metallurgist ! I was up and checked my email I am going back to bed !
Big Al (then i will be refueled )

reading through all the theory is interesting, i am currently operating a rmh with a watercoil wrapped around the outside top 1/2 of the barrel, and from the discussion it appears that i may be sacrificing some efficiency by extracting heat from an area where a final burn is taking place.

One thing is true, there is not much heat left for the room or thermal mass, but the rmh is working perfectly other than the possibility of loss of efficiency

after installing a more insulated heat riser i noticed the top of the barrel glowing cherry red which sort of scared me, so i now keep a big tub of water over that spot to extract heat there as well and hopefully prolong barrel life.

do rmh barrels glow red like that continuously and still last for 10 years?

i have really enjoyed reading this discussion even though it was difficult to follow some of the reasoning.

it seems that removing heat outside the heat riser should simply increase the fall rate of the cooling gasses once they pass the top of the heat riser,,although i would hesitate to put the coil inside the barrel because it might interfere with the exhaust and in turn put a back pressure on the heat riser,, seems like no good could come from that

thanks again, sorry i didn't see this thread sooner

Hi all.
I want to start with saying thank you to everyone posting all these very interesting ideas and suggestions in this marvelous forum!
I am also planning to build a water heater rocket stove and just found this very interesting thread.
I already have a water based heating system in my house based on regular firewood and solar water panels.

Since I'm a rookie that haven't even built a rocket stove yet please do try to tolerate some maybe not so enlightened questions.
For the purpose of just heating water, as mentioned in this thread, is it necessary to include the barrel over the heatriser?
Could one not just add a meter or so of chimney with a coppertube wrapped around and insulated above the heatriser to extract the heat?
Or would this somehow kill the draft/suction, up/down flow of exhaust gases that is created with the barrel.
And how long does the heatriser have to be to ensure good efficiency?
And what sort of temperaturs can you expect in the exhaust exiting the heatriser?

My plan is to use a fairly strong water pump to push the water through the copper spiral (about 1" thick tube) and circulate it in an open system so that pressure buildup should not be a great danger. I will then circulate this water through another spiral in my accumulator watertank (1000 liters) that heats my house. Or I will circulate my small outside tub through this setup depending on the occasion.

Any suggestions and explanations will be greatly appreciated.

perhaps one of the original posters will see this activity and offer suggestions, but no one responded to my post from a month ago, so i'll offer couple thoughts.

First, since you're already heating water with a woodstove and copper coil(?) i'm going to assume you have some relief valves somewhere in that loop--pump failure, electricity failure could create a very dangerous situation


I guess in a sense your proposal would still be considered a rocket stove, and i would definitely try it out outside in the summertime (with the water coil extracting heat) to make sure of the dynamics.

but i would first say you have superheated gases(1,000+ F) at the top of the heat riser which you will be cooling rapidly with the copper coil of water which to my mind will start to put a back pressure on the system, so you are no longer using the rmh dynamics where the cooling air flowing down the outside of the riser creates a suction or draft, in this case it will be inhibiting the draft


i suppose depending on pipe diameter and amount of copper coil you could possibly extract all the heat from the exhaust into the water over 5-10 feet of length of stack, but would probably need to add a fan of some sort to keep the colder air moving up and out-- of course if you installed that fan then maybe you could do away with a tall stack

the stove itself will put very little radiant heat into the room,

the highly insulated heat riser will probably warm up a little, but nothing like the heat you are getting from your wood stove, and the wood feed gets hot down in there, and some heat radiates upward, but again, nothing like with the wood stove

and now for the biggest discovery i made--a 6 inch dia rmh takes a whole lot of attention, if you are feeding a regular stove, you can go an hr or more depending on the stove with it burning less efficiently with big logs,, but you'll be feeding your little rocket fire box every 10-15 min, an 8" system would be better, almost twice the square inches so more heat for less attention, but still not like a wood stove,, mine puts off some heat from the top of the barrel, a little from the lower half of the barrel, the mass barely warms up at all and of course a little heat from the firebox

take some time this summer and stack some bricks outside and try it out, i'd be interested to know what you finally figure out

Thank you so much for the quick respons Bob.

I now see that the best way of doing this will be to make a regular RMH and extract the heat from the barrel somewhere. Sounds like the setup you have is a good one.
Have you insulated the copperspiral on the outside of the upper halft of your barrel?
I plan to set up this rocketstove just outside my basement where my technical room is, so that I can run the hot water in there if it works out good.
My woodburning stove has several security measures installed. Cooling spiral connected to the mains with a overheating thermostat in the stove that open should the stove reach any critical temperatures.
Also I have pressure valves connected to a expansiontank. And then I have an unbreakable power supply that can run the pumps for a couple of hours should the electricity fail.
So I'm not worried about putting heat from the RMH into the system. Everything is also connected to a 1000 L accumulatortank which won't boil in minutes. I have actually never had the tank above 70 degrees celcius.

As you were saying about size I would like to go as big as possible. I saw another thread here that talked about loading long poles of wood and thought I might try that. Not with an open slide as suggested there but rather with rollers to be sure nothing jams.
How big can one make a firebox, and what kind of firewood can you use in it? Can one use regular split wood as one uses for a woodburing stove?

Thank you again so much for the reply and I will update you on my progress when I get started.
Any advice is again greatly appreciated.

I'm not sure about the automatic feeds, as i have never investigated them.

my idea was to set up a thermosiphon between the water tank and the heating coil so that pump failure was not an issue,

i am using a direct feed into the water system, not a heat exchanger coil. this allows me to depressurize the system just by turning off the cold water feed and opening a hot water spigot-

also, next time the coil will use the entire barrel instead of being compressed around the top half, i believe that will give the coil better access to available heat. also having the intake lower will improve the quality of the thermosiphon

I do have some aluminum foil currently around the copper tubing and in the final i may take the skin off an old water heater and use it as an extra safety measure, i thought about wrapping fiberglass around it, but the foil stayed pretty cool so i never bothered --

i have actually pushed my system to failure several times and learned a few things--at least about my system--first the system never failed as long as it was unpressurized, steam buildup found it's way through a maze of twists and turns in the plumbing to exit through the showerhead, but never blew out the pex
second, using pex pipe connected to the copper, when pressurized and pushed to failure the pex always blew out (very close to the connection point)-- my best guess is that if somehow the copper were plugged at both ends forcing the copper tubing to burst, i believe it would simply split open (no shrapnel)releasing live steam which could be contained in an emergency by the metal skin , it would be a mess, but should provide protection for anyone nearby. (please note, this extra layer should be unnecessary if there is a tpr valve in the system, preferably very close to the coil with no valves in between that could inadvertently be closed)

and it is very important to have the coil as tight to the barrel as possible, that metal to metal contact extracts heat much more efficiently, it's difficult to keep those windings tight, but it is worth the time and effort it takes

please, take some time to verify these findings, my setup is far from the quality of a real safety lab and just a preliminary report of experiences

Oh, the throat or the tunnel between wood feed and heat riser determines all the sizes, but you would find this when you buy that rocket stove book--

basically all the cross section square inches need to match or be bigger from the throat on--ie heat riser and actual exit from the heat riser (inches the barrel is above the riser times the circumference) and then all the exhaust pipe through the cob to the exit

and the feed box will work best (less smoke) if it is the same size or a little smaller than the throat, but you do need to make sure it is getting sufficient air so not too much smaller--

also, there are other relationships between wood feed and riser height, length of pipes in the cob etc, but please buy and read the manual carefully it's not that much and a great investment

also there are lots of ways to build the core of the stove, paul and the gang have a lot of details on their development of the shippable core,, and broaudio has a bit on utube about casting the fire core and heat riser using fire clay and perlite on utube