rocket hybrid boiler stove?

hi forum
i'm contemplating a bit of a project, & would value comments/suggestions from some of you more experienced rocketers before i begin...
what i want to do is a boiler for a central heating system, a big insulated water tank is acting as my thermal mass, but the stove is my main subject here. i'm starting with an old jotul woodstove, & what i want to do is
insulate the firebox inside with vermiculite or ceramic fireboard or similar
partition it part way up with more insulation
make a riser of (possibly) standard flue pipe surrounded by more insulation which goes through the partition, & up through the hole where the hotplate would normally sit
cap this off with a metal plate, onto which are welded 2 concentric circular metal sheets, sealed at the bottom to form a round, top mounted version of a backboiler. water enters this by a pipe at the bottom & leaves through another at the top, & this arrangement sits with its inner edge flush with the outside of the hotplate hole.
so the plan is that the fire in the insulated box is smaller but more intense, the riser keeps the gases hot & burning in standard rocket fashion, then i can use the plate on top as a hotplate, & my boilers inside surface transfers heat from the gas to the water & the only place the exhaust has to go is back down the hotplate hole into the stove, where it loses more heat to the area of stove above the partition, then exits through the flue in the normal way, hopefully retaining enough heat to make it up the chimney.
hope i've explained that adequately. a diagram would've helped, but i don't know how i'd do one on here.
so...whats going to go wrong with that? haven't decided the dimensions yet but the hole i've got is just over 9 inches across so i reckon if i allow for a 5 inch inner flue & an inch of insulation all round that just leaves me a slightly wider area of downward 'flue' inside the boiler. i have no idea how high i should make the stack, nor how thick the boiler should be to heat the water effectively without boiling it. current plan is to experiment with some mild steel, which i've got, then try & learn from the results before doing something more durable. any thoughts...?

Welcome to Permies Rich,

There are several ways to create a drawing & post to this forum. Some people just make their drawings on a piece of paper, and take a digital picture of them with a digital camera, it is fairly easy to upload a digital picture to this forum by using the 'Attachments' tab which is next to the 'Options' tab both of which are just below the typing area that I am using right now...when posting a response. Once you click on the 'Attachments' tab, it will say: Filename with a button next to it marked 'Browse', click that 'Browse' button to transfer a picture from your computer (like a picture of a drawing) to upload it into this forum and post. There is even provision for a description of your upload file, such as 'old jotul woodstove'. Keep your picture file size small so that it is easy to upload, something around 25-68 kbs is a good size to upload, don't try to upload a digital pix that is 1.5 mb in size, it would take forever to upload, and waste a bunch of time for everyone trying to view the picture, from the internet. Once you hit the 'Submit button' to post your message and picture to the forum, wait about 10-30 seconds until the thing actually uploads/posts. Don't repeatedly hit the Submit button, one time is enough. If you need help resizing a picture to upload simply ask. Read this thread: https://permies.com/t/31696/tnk/posts-simple


Now if you don't have a digital camera, maybe you have a program that comes with your computer, it would be called Paint, and if it is on your computer it is often listed in your list of seldom used programs under the heading: Accessories. Using the Microsoft Paint program it takes a little time getting acquainted using the thing, but you can make simple line drawings, fairly quickly, great for beginners drawing digitally. There is another program you can download from the internet that is called 'google sketchup' but it is 3D drawing and somewhat complicated and difficult to use for a beginner that has never used it before...but it can make really fantastic design drawings. I think you have to have the latest Windows 8 or newer to download/use the latest program of sketchup. With either program Paint, or Sketchup once you have created a drawing then you have to 'save' it, somewhere on your computer and give it a name. Remember where you saved it, typically I save my Paint designs in 'My Pictures' on my computer, so it is easy to find. Then it is fairly easy to upload that design when you use the directions I gave in the first paragraph. Hope that helps.

james beam

thanks, thats going to have all sorts of uses. makes me wonder what other clever things this computer can do that i don't know about...anyway, for the moment i've done a little top & side view sketch of the thing i was trying to describe, which hopefully is now attached.
cheers
rich

Rich Tetlow : This is going to be like the blind men describing the Elephant ! Your Sketch needs a little work, it is almost backwards, We speak of the guts of our system


as having a 𝐉 Bend shape ! The short leg of the J is the wood feed, In your sketch this should drop down to much nearer the bottom of your J shape
where it meets with the Burn Tunnel were the hot pyrolyzed wood gases flow from left to right to the base of the Heat Riser where the Hot gases rise up its center
Reaching sustained temperatures Over 2,000ºF before hitting the underside of the Barrel, there giving off about 40% of its heat as prompt Heat! Then much cooler and
dense, the gas flows down the outside of the heat Riser/ inside the drum to its exhaust port near the bottom 1/3rd of the Right hand side of the longer leg !

In the Picture looking down from above you have drawn the barrel, and your proposed water jacket in blue, There should be a smaller doughnut shape in the center.
At its heart is the Doughnut Hole, it is the center of the Heat Riser that the hot burning Wood gases flow up through, The doughnut shape represents the Heat Riser itself
usually constructed with fire brick and wrapped in additional insulation !

This is where I strongly recommend going to Rocketstoves.com to Download a PDF Copy of the brand new 3rd edition of Rocket Mass Heaters

For the good of the Craft ! Big AL

Late note : This is a Better link to a good working picture/image where I don't have to worry if i have the creative commons right to re-post it 2nd and 3rd images!

http://www.richsoil.com/rocket-stove-mass-heater.jsp

A. L.

sorry, that was a pretty poor diagram, done in a rush as well as in an unfamiliar medium. let me try again;
i've now drawn the stove in black, & my proposed layers of insulation in thick red, this should be treated as forming the whole 'doughnut' of the riser. the hatched area represents insulation face on to the view, hopefully elucidated by the top & end views.
boiler in brown/blue, with pipes not shown, & flame flow in orange. if anything is drawn in proportion, its accidental!
that richsoil site was the first place i came across the rocket stove idea, & i love it, but i don't really have the option to do the whole thing; this is my attempt at a compromise & its intended to be a true hybrid rather than a proper rocket built inside a stove box. my layers of insulation are thin, partly for space reasons but also because i want them to be imperfect- i don't really want gases hitting the boiler at 2000 F, or it will actually boil...i think.
the key problem i want to solve is of having an effective boiler stove that doesn't have the water sitting right by the fire leaching all its heat away before its got started.

Rich Tetlow : A Late Welcome to Permies.com / Richsoil.com, and A Big Welcome to the Rocket And Wood Stoves Forum threads! With 28,000 + Fellow Members

You should be able to come here 24 /7 and talk to someone who wants to talk about what you want to talk about ! O. K., Now that we have a slightly better look at your

thoughts, you are trying to think outside the box - and we prize that greatly here at permies.

A couple of things, I rather think that you have not really got a good idea of thee true size and dimensions of the Rocket Mass Heater, Especially with your double
horizontal passage of the hot exhaust gases before they flow into the Heat Riser, It will be next to impossible for you to build a smaller version of A Rocket Mass Heater
RMH using identical materials.

The double horizontal passages with be very difficult to get to draft when your system is cold and will almost certainly need a diverter built into the system to allow the
system to come up to operating temperatures before the gas flow is reverted to the double horizontal flow you have sketched ! This is a common feature on many wood
stoves This will increase the time the unit is burning inefficiently !

You did say that your drawing is not to scale, allowing for that, As drawn you still are exposing the Water tank and its contents to temperatures easily 1000ºf above the
boiling point of the water, This creates the potential for the water to flash to steam, We call this the 'Boom-Squish' scenario, Think Boston Marathon Bombing with More
fatalities and Full body, full thickness burns !

The sweet-spot you are looking for with exhaust gas temperatures closer to 250ºF is actually several feet further down Our exhaust pipe, AFTER the wood gases
have all been consumed.

Your system would probably still contain wood gases high levels of Carbon monoxide and other unburnt hydrocarbons which would tend to collect on the cold walls of
your water jacket !

I am going to repeat my suggestion to go to Rocketstoves.com, for a PDF copy of Rocket Mass Heaters, this book and the Forums here at permies will
help you more clearly see what is possible without reinventing the wheel, and we will all grow from the new ideas you will bring to these pages !

For the good of the Craft ! Big Al

Rich Tetlow : I am sending some eye candy your way, while the unit in the pictures is a 4'' and a little small for your needs, a 6'' system will only be close to 1/3rd
bigger ! Size will not be your problem

Link below !

http://www.permsteading.com/viewtopic.php?f=6&t=601

When the barrel goes on the final vertical chimney must go up !

For the good of the Crafts ! Big AL

hi there big al
thanks for your input. i've had a good look through 'rocket mass heaters' (only 2nd edition though) & started trying to put things together. on the subject of dimensions, my stove is 10 inches across (widening to 11 at the top, which i love), 17 deep & 14.5 high, & the riser, which i drew quite short on my diagram for space reasons, is 20 inches high. i'm hoping this will be enough to burn up most of my gases, if not i guess i can extend it up, which given your comments about temperature might also be a good time to let the extension go clear of the water jacket so a bit of heat is dissipated there. my problem here is that i really want as much heat as possible transferred to the water rather than the surrounding air...but without boiling it. i guess the volume of the water will be quite significant here, haven't made the outer part of the water jacket yet but i'm currently looking at about 1.5 inches of water between inner & outer, which feels like quite a bit to just flash into steam. i guess i'll probably put it all together & fire it up in the garden where i can run for cover easily if things go wrong! then proceed based on the results from that, i'm hoping the reduced size of my system will mean a bit less heat than the somewhat daunting temps most rockets seem to hit, but i'm changing so many parameters its hard to estimate, specially as a complete amateur!
also, thanks for the heads up about diverters. i'd kind of wondered if i might need something like that & based on your opinion have decided i do. fortunately the way things work out, as seen in diagram 2, the top of the horizontal bit of burn tunnel just comes up above the centre of the rear flue hole, so what i'm thinking is to cut a slit in the insulation at the back there & a hole in the top, replace the flue blanking plate with a suitably cut piece of steel (so i don't have to cut the blanking plate itself) & poke a steel plate through the slit so it just comes up to the edge of the hole. that way my exhaust starts by going straight up the chimney, then when things have heated up i can just push the plate in, it goes across the hole & the gases are diverted round. leaving a break in my insulation where heat can still pass from just above the firebox to the cooler part of the flue, but not to a significant extent hopefully.
by the by, the stuff i'm using for insulation is an inch thick ceramic fibreboard sold for lining kilns & things. absolutely horrible to work with, like a dry, dusty version of wet mdf but apparently very good at resisting erosion by hot gas. just about structural enough that i think i can just stick 4 sheets of it together to make the riser as well. i mention this for information & interest. thanks again, i'll now carry on, watching this space in case anyone wants to tell me if i'm about to do something dumb, & report back on any results that seem worth it.
cheers
rich

rich tetlow wrote:hi there big al
thanks for your input. i've had a good look through 'rocket mass heaters' (only 2nd edition though) & started trying to put things together. on the subject of dimensions, my stove is 10 inches across (widening to 11 at the top, which i love), 17 deep & 14.5 high, & the riser, which i drew quite short on my diagram for space reasons, is 20 inches high. i'm hoping this will be enough to burn up most of my gases, if not i guess i can extend it up, which given your comments about temperature might also be a good time to let the extension go clear of the water jacket so a bit of heat is dissipated there. my problem here is that i really want as much heat as possible transferred to the water rather than the surrounding air...but without boiling it. i guess the volume of the water will be quite significant here, haven't made the outer part of the water jacket yet but i'm currently looking at about 1.5 inches of water between inner & outer, which feels like quite a bit to just flash into steam. i guess i'll probably put it all together & fire it up in the garden where i can run for cover easily if things go wrong! then proceed based on the results from that, i'm hoping the reduced size of my system will mean a bit less heat than the somewhat daunting temps most rockets seem to hit, but i'm changing so many parameters its hard to estimate, specially as a complete amateur!
also, thanks for the heads up about diverters. i'd kind of wondered if i might need something like that & based on your opinion have decided i do. fortunately the way things work out, as seen in diagram 2, the top of the horizontal bit of burn tunnel just comes up above the centre of the rear flue hole, so what i'm thinking is to cut a slit in the insulation at the back there & a hole in the top, replace the flue blanking plate with a suitably cut piece of steel (so i don't have to cut the blanking plate itself) & poke a steel plate through the slit so it just comes up to the edge of the hole. that way my exhaust starts by going straight up the chimney, then when things have heated up i can just push the plate in, it goes across the hole & the gases are diverted round. leaving a break in my insulation where heat can still pass from just above the firebox to the cooler part of the flue, but not to a significant extent hopefully.
by the by, the stuff i'm using for insulation is an inch thick ceramic fibreboard sold for lining kilns & things. absolutely horrible to work with, like a dry, dusty version of wet mdf but apparently very good at resisting erosion by hot gas. just about structural enough that i think i can just stick 4 sheets of it together to make the riser as well. i mention this for information & interest. thanks again, i'll now carry on, watching this space in case anyone wants to tell me if i'm about to do something dumb, & report back on any results that seem worth it.
cheers
rich

Rich Tetlow : I am glad that you have had a chance to look at ''The Book ", it least we will be on more common ground ! In order to Get Your version of a Rocket
Mass Heater to work, the total height of the Heat Riser needs to be 1.5 Xs to 2Xs the total length of the Burn Tunnel which you have made to be more than 3Xs
the length of a more traditional Rocket !

With such a short Heat Riser You will not yet have consumed most of the wood gases which will then be exposed to your water bath which will be below 212º F
This will cause your incompletely burned Wood Gas hydrocarbons to precipitate out and adhere to the metal skin of your water tank !

About metal components, our very earliest versions of the RMHs Heat Risers ran at Temperatures around 2000ºF, Todays Rockets reach 3000ºF, for that reason
The use of any Ferrous Metals Before the Heat Riser is problematical, Please take the time to do a Google Search for the Term High Temperature Hydrogen
Attack, also of some value to your investigation are the terms, Hydrogen or Steam Embrittlement, Hydrogen Blisters, and Dicarburization.

Part of your further learning curve is to get an Understanding of the need to comply with the "Keep a Constant Cross-Sectional Area'' Throughout
the entire build, the Feed Tube, Burn Tunnel, and Heat Riser, and the connecting ductwork throughout the entire build both in the Rocket Burner Base and The Thermal
Mass need to be the same size

Short version is your design needs a little Rework !

Hope this is timely and helpful ! For the Good of the Craft ! Big AL