A design idea for your consideration...

I am considering building.... no, preparing to build, a rocket mass heater in my home in southern British Columbia, Canada.  I currently have an indoor catalytic reburner insert plus an outdoor wood-fired boiler driving radiant floors and a couple of forced-air radiators.  I bought the whole "Better Wood Heat" 8-DVD set (kudos to Erica, Ernie and Paul for some great info from truly intelligent people) and Ianto Evan's "Rocket Mass Heaters" 3rd edition PDF book, and I have been reading through various threads here.

I thought I'd best toss in some cheap personal credentials here, for Paul and Erica and Ernie to consider, before they tell me how dangerous this all is for normal people.  I fully agree.  But then, I've had a colourful life, with stuff like, I'm a decades-experienced professional touring musician and audio engineer/producer, subjects I also later taught at college for nine years, along with stuff like psychoacoustics and synthesizer design, so critical thinking, extrapolating cause and effect scenarios, tinkering, working in insane circumstances under stupid time pressure with at times limited resources, troubleshooting, highly-creative problem-solving and extreme preparation for pulling off very public events to perfection in front of far too many people watching, are built into my soul.  I've also done a great deal of hard, manual work in an underground mine my father once had, so I've run bulldozers, loaders and hoes, diamond drilled, blasted, run cyanide leach systems, done lab testing of various outputs, and had to build things like redneck gravity-fed water systems running from natural sources.  I now live rurally with wood heat that I log to supply the fuel for, and I live and work alone, so I am used to having to be smart and not kill myself the first time.  I'm used to looking, before leaping, and thinking about it first, too.  Yes, Ernie, I'm going to build it out in my field, first, and there's no one here for me to inadvertently kill off with my reckless and haphazard experimentations... : )

So... I saw some creative and interesting ideas in the DVDs, and one of the things that caught my attention, was the installation of a manual A/B vane to direct the manifold output to either run through the mass, or flow directly to the outdoor exhaust.  The reason given for this in the video, was that the shorter exhaust routing might make the heater easier to cold-start in certain temperature near-inversion scenarios where there was only minimal draw on the firebox.  It was also later suggested by Paul that the feature had not necessarily worked as well as they might have liked for that purpose, and that it was overall not recommended, because there was too much "human" intervention factor in it, meaning that absent-minded or otherwise unthinking people had been demonstrated to not pay attention to the vane and guillotine controls, and they had caused more waste and trouble than they were worth.  However, the concept got me thinking...

A part of my interest in this whole RMH thing, from the beginning, was the thought of having the RMH drive my radiant floors and radiators as well as being its own standalone hot rock, if that were possible.  Enter Boom Squish and various other commentaries on the horribly fatalistic journeys of careless or unintelligent people toying with obscene physical forces that nature never intended them to come into direct contact with.  Overall, these sources have painted a very dismal, graveside kind of image of "the unfortunate ones who dared to apply a little bit of knowledge".  It's a pretty clear message yelling, "Don't try this at home!"  Noted.

The issue that seems to be the overriding one, where rocket stoves and water collide, is that the stove can certainly heat the water up... but then, it won't stop, either, until it creates another Boom Squish case history to laugh sadly about.  Squished men tell no tales.  The old conventional-stove concept of choking the input air supply is not a great answer for a rocket stove, either, because the system's entire value lies in it running fast, clean burns that heat the burn chamber wayyyyy up and then distribute that heat to the mass, in a rocket mass heater format.  Excessively choking the stove eliminates that function, may cause a reverse-burn and smokeshow, and can deposit more creosote and moisture inside the assembly, which will, as a minimum, rot it out faster than God intended, if metal stove pipe is used in the mass flow assembly.

So... this got me thinking, on two levels.  For one, I don't want to build an RMH just for heating in winter.  It's a big project.  It would be intelligent to build it so that I could also use it as a secondary or emergency cooking appliance, if I'm ever actually out of propane here, which I run a range, a barbecue, an on-demand hot water heater and a clothes dryer on.  A full tank lasts me about three years, but still.  Cooking on the RMH in winter isn't really an issue, but running it in high summer here would be suicidal, heat-stress wise... but then, I thought, what if I didn't heat the mass, in the summer?  I started thinking of maybe perverting Erica and Ernie's cold-start vane idea, into a mass yes/no switch, more or less as they had made theirs, but applied to running the stove, only, in warmer weather... fire the core, heat the barrel and exhaust outdoors immediately, so the heat trapped indoors is emitted only from the barrel head, really, which cools quickly afterwards, and the indoor heat can be quickly vented outside from the intended location of the unit.  Might give some all-season utility to this behemoth.

This idea then spawned another one.  If a radiant heat water/fluid were located inside the mass, and the manifold output were able to be switched to and from the mass and thus, to and from the water tank, then it would be possible to maintain or control the tank temperature, regardless of the state of the fire in the box and without sacrificing the barrel's heat output.  Once the water heated up, the mass would charge with and then maintain that acquired heat, so the water would essentially act as just an interchanging liquid mass, stored within the cob mass.  Upon firing, the tank would heat the fastest - I was thinking of a stainless steel tank with an 8" cylinder passing through its centre, so the fluid was heated almost directly by the manifold output passing through the pipe inside it - and this fluid would then heat the outer tank walls, which would heat the cob in contact with them, and the full mass and living space, from there.

The location of the tank, in terms of its distance down the exhaust tube from the manifold output, would help determine what the maximum temperature that might actually be applied to the tank would be, as whatever length of cob before the tank would pull off some consistent value of heat. From what I've seen of Ernie shooting temperatures at various points along the pipe pathway during test burns, it seems that allowing maybe a foot or two down the tube from the manifold, would prevent the tank from ever being directly heated with more than about 300 - 350 degrees F. in the pipe.

Meanwhile... here's the cool part... a probe in the tank would register the fluid temperature and tell a control unit about it.  That unit would, in turn, operate a relay to a simple solenoid or similar, which would operate the A/B function of the control vane, directing heat out the stack when the water was at temperature, and topping up the heat when the probe started feeling the chill.  The solenoid would be rigged so that the default passive setting was to bypass the mass/tank, so a loss of power to the system could not allow a runaway condition in the absence of thermostatic or personal oversight.  My current boiler damper control uses this default.

I'm thinking of a thermostatic shutoff at ~180 - 190 degrees F., so that there's decent play space below boiling for any kind of brief overrun, and so the output fluid hits my Pex hose at or under 180, which is its rated happy place.

A manually-operated vane with the same function placed upstream of the thermostatically-controlled one would allow total override of the mass for summer use, without even bringing any of the rest of the rig into play at all.  It's also possible to rig the automated vane with a manual override.  Either/or, though separating the two seemed safer and smarter to me, despite the added system drag.

I would have an open 1" breather to the outdoors from the tank, as my boiler now has, so there is no inherent internal pressure in the system, no opportunity for buildup and no pesky relief valve to fail and give me a birthday surprise.  I would use ~480 litres (120 gallons) of tank space, and fill it to ~400 litres/100 gallons, so there was breathing space designed into the system.  Shaping the tank like an "L" to follow the bench contour would allow the breathing space and probe to be located up top at the back, so there is no cold spot in the bench seat.

Yes, I'd build easy access to the tank top and probe, up at the breather area, so I could service that aspect of the system and inspect the tank as needed.  Running the probe lines inside conduit would prevent them from ever fracturing under expansion/contraction stress.

There may be a few ways to preset the steel tank to have the least expansion effect on the cob around it... maybe use a highly-heat-conductive but more flexible substance in a thin layer between the tank and the cob... maybe lubricate the steel and then just cob it in and fire the system to capacity during the curing and leave it that way for a few days... maybe modularize the tank area by segmenting the cob bench on each side, so the expansion concerns are limited to only one specially-engineered segment... maybe build that segment with a "bottom and top" configuration, so the top layer of cob is a fitted piece sitting on top of the tank and base cob bench, with a small gap of wood veneer or similar between the two components, so the top can simply rise up a fraction of an inch when it's hot and settle again without chipping... essentially, an engineered crack in the bench... other ideas... ?

A circulation pump would send and return the tank fluid flow to/from the radiant system, after passing it through whatever floors or radiators.

There are some further thoughts on some things I'm wondering about in this.  First, the intended space to build this is currently outdoors, under a sunroom.  I intend to take in that area below it, so the sunroom will now have enclosed space and an RMH under it instead of mediocre insulation and -20-something open air in the winter.  The outdoor space was initially blacktopped, likely in the 1970s.  I have seen how the RMH builds use small rocks as a "breather" space underneath them to prevent moisture wicking into the cob and dissolving the heater.  Is there any reason why I reaaaaaally need to remove that asphalt from the area, or can I just put the heater on it, on a small rock base, and put interior floor over the rest of the space?  Putting it another way, it's ancient blacktop that isn't really going to off-gas or anything, and it does seal the ground very effectively, there, but.... how much heat can I expect to flow down from the heater and get to the asphalt?  Just a question I figured I should consider before I build a self-destructing three-ton insanity on possibly suddenly-softening underpinnings.  Heat shield the area under the core, maybe?

I have wondered about the benefits of using ceramic or masonry pipe instead of metal for the interior flow through the mass.  Any comments as to durability or efficiency being improved, or is the steel pipe really the best solution?  I don't want to build this twice if a pipe rots out.

I made a cheap diagram of the idea, so that whomever could tell me I'm insane with accuracy.  Any comments, kudos or obituaries are welcome.

Hi Robert;
Lots of information in your post.
I'll try to answer some of your questions. But first I think you need to learn about batch box design heaters.
Here is a site from Peter Berg the innovator himself . http://batchrocket.eu/en/
Here is another site that discuses different type of batch box builds.  https://donkey32.proboards.com/

As you will learn B.B. are more technical to build, but have a much higher heat output. They also use larger horizontal wood and a door, rather than an open tube.

Using a bypass is now accepted in rmh builds as standard equipment.
Using other material rather than pipe thru your mass is completely acceptable as well.

It is becoming more common to forgo the piped mass all together and instead build brick bells (heat stratification chambers)  to hold your heat.

Your drawing shows the fluid tank to be a safe distance from the heat core.

The asphalt slab is a good place to build on BUT... you must raise your rmh  (flat clay bricks are commonly used) If you do not all your heat will sink into the asphalt and then into the earth...
No matter how much wood you burn, you will not succeed at heating the earth.

Check out batch box's and brick bells over at batch rocket or donkey pro boards.
Read about the same here at Permies.
You might find they would work better in your situation.

Last I will tell you about Matt Walkers site. Matt sells plans to build exactly the type of cook stove / mass heater you are hoping for.
Here is the link   http://walkerstoves.com/index.html
I can't recommend Matt' s products, and his support while building enough!!  Top of the line and fair prices to boot. You won't go wrong with his designs!
Check out his rocket water heater!

Hi Robert,    Your project sounds very inspiring to catch all your needs in one shot but if you have never built a rocket mass heater before, from my experience would be to consider starting a lot more simple and adding on as you go. There are so many things you will learn from a basic setup that will teach you about their functioning, what you really need/don't need and ways to improve it that you never would have considered without operating one first.
When you do build one, I would also recommend using clay/sand as a mortar so that it can be modified much more easily without having to destroy or throw away materials because they can't be reused.
I hope this doesn't bum you out in not getting exactly what you want right from the start, but think that it may be worth considering so it doesn't get overwhelming or make you frustrated if so many factors don't work out.
One of the links that Thomas provided is for Matt Walkers website. He has also been doing a Live Stove Chat every Wednesday for everyone to ask him questions and chat about rocket stoves. Maybe we'll see you there?

Gerry Parent wrote:Hi Robert,    Your project sounds very inspiring to catch all your needs in one shot but if you have never built a rocket mass heater before, from my experience would be to consider starting a lot more simple and adding on as you go. There are so many things you will learn from a basic setup that will teach you about their functioning, what you really need/don't need and ways to improve it that you never would have considered without operating one first.
When you do build one, I would also recommend using clay/sand as a mortar so that it can be modified much more easily without having to destroy or throw away materials because they can't be reused.
I hope this doesn't bum you out in not getting exactly what you want right from the start, but think that it may be worth considering so it doesn't get overwhelming or make you frustrated if so many factors don't work out.
One of the links that Thomas provided is for Matt Walkers website. He has also been doing a Live Stove Chat every Wednesday for everyone to ask him questions and chat about rocket stoves. Maybe we'll see you there?

Hi, Gerry.  Nice to see a local-ish post here, and thank you for your insights and concerns.  I also know your name, possibly from many years back in a town near here, though you may or may not be the same Gerry, but if you are, hi again.  I have actually already been and looked at Matt's site some weeks back, and listened to one of his recorded podcasts, and looked through his material and considered his ceramic cores.  He seems to have some good things going on.  He's now had two solid recommendations here.  Having looked, I felt his hot water heater design, as he was demonstrating it, was not producing sufficient heat to do my job, from what I saw, hence my decision to design my own customized rig.  My old experience as an audio guy and touring tech/performer, with some seat-of-the-pants underground mining experience, has made me good at problem-solving and getting outside the box on stuff, so I don't feel I'm in over my head, here.  It's just a matter of thinking through all the possible events and fallout from those, and resolving them as well as possible on paper, before testing my theoretical solutions on a mock-up in my field here.  I also have my existing boiler system to take cues from, as far as what it needs to function well, and how its water system is laid out.

I fully agree on using cob mortar (clay and sand mix) to build the rig, and possibly finishing it with a light layer of plaster, with old horse pucky blended in to make it more resilient.  The plaster would probably be junk if I tore it out, but the bulk of the cob can be dissolved and reused.  Fire brick or clay pipe on the inside, likewise, can probably be mostly reused, if there's a major surgery needed someday.  I've been reading up on the batch box, following the suggestion given here, and that may indeed be a better option for the heater, but the main exhaust vane/cob mass/radiant water/thermostat concept would remain essentially intact.

As far as doing the whole project or only a part, the thing that is upon me, here, is whether my slowly leaking boiler will keep me warm yet another season... it's kinda hanging in, but if it decides to put up the white flag in December or something, that might be, uh, disquieting.  I'm semi-confident that it'll be fine, but there's nothing like just resolving the visible issues and moving forward.

As I've said, I do intend to settle on a design and then construct a good mock-up of it outside, here, so I can fail gloriously if needed, without breaking my back hauling cob.  Maybe you'd like to come and share the pain... uh, I mean, glory?  : )  Might see me at the Walker chat Wednesday, if I remember to be there.

Good to hear that you have a football helmet and band-aids to put on those skinned knees when some of your experiments don't turn out quite the way you planned. A finicky dragon can bite, throw fire and smoke you out all in one breath! Perhaps you are a dragon tamer and get it right the first time though?

Do keep us informed of your progress and pictures of your mock-up when you get to that point. We'll help as much as we can.  

Gerry Parent wrote:Good to hear that you have a football helmet and band-aids to put on those skinned knees when some of your experiments don't turn out quite the way you planned. A finicky dragon can bite, throw fire and smoke you out all in one breath! Perhaps you are a dragon tamer and get it right the first time though?

Do keep us informed of your progress and pictures of your mock-up when you get to that point. We'll help as much as we can.  

I'm an alumnus of Saint George's School... seven years there... taming dragons is my thing... : )

So... after a couple of chats and a bunch more thinking on it and walking around the space, I have made a number of changes to the game plan, so I thought I'd post a revised diagram of the concept to this point, as it progresses.

Essentially, the changes from the first diagram are: the J-Tube is now an 8" Batch Box (thank you, Gerry Parent and Peter van den Berg), there is an addition of a labeled cooking surface, and there is greater refinement in the thermostatic control layout so it affects only the fluid tank and not the mass, making for far less wasted heat if the fire is still strong after the tank is up to temperature.

I may add a passive warm bench extension to this, depending on how it finally sits in the space, once I know what "it" is going to look like.

The diagram is highly simplified and not to scale, but it gives a representation of what's supposed to go where and do what.

I have debated making a more formalized cook area, similar to one of Matt Walker's small cook stoves/ovens.  I have also considered his idea of a longer and less-aggressive burn profile on his "riser-less" design, which may or may not be of benefit in my application, both in terms of the lower height, meaning, more accessible cooking surface height, and because of the longer burn creating perhaps a more consistent source for the fluid tank.  I'm still chatting with Matt and Peter and will probably change a bunch of stuff again, soon enough.

Thoughts, comments or eye-rolls, scoffing and muffled laughter are welcome.

Back and forth with Matt Walker, and got to talking about core builds using Ceramic Fibre Board versus masonry, versus castable refractory, and he essentially said there were "lots of tradeoffs", and too many to list there, and that I should do some research to figure which might be best for my needs.

So... I'm researching.  My "core" needs are mostly excellent durability and service life, and easy, minimal maintenance, but of course, ease of obtainment of the materials and easy build and customization are valuable attributes, and excellent stove operation, output and efficiency are the main goals of these rigs, anyway, no?

I'd rather pay money for durable efficiency than save money on a less-effective or less-durable core and firebox.  The system is gonna be here a long time.

Anyone have any thoughts about those various materials, where the core is concerned?  I know cob mortar is the least expensive and mostly durable, but heavy and labour-intensive.  Firebrick is excellent, but doesn't handle abrasion well, while cast refractory done with a vibrating table is awesome but harder to cast and work with to get a really good final product that won't crack, and CFB is much lighter and simpler to cut and probably won't crack much, but may have durability issues or other drawbacks in certain designs.

Comments, suggestions, guidance, or alternatives?

In Matt's last stove chat 14, he had a question concerning the differences between a high mass vs an insulated firebox, cf blanket and using clay tiles starting at around the 7 minute marker.

Also, looking through my notes, he also mentioned in chat 12 around 25:50 minute mark about IFB insulated fire brick (kiln brick) vs CFB ceramic fiber board comparisons.

Hope these help to answer some of your questions.

Gerry Parent wrote:In Matt's last stove chat 14, he had a question concerning the differences between a high mass vs an insulated firebox, cf blanket and using clay tiles starting at around the 7 minute marker.

Also, looking through my notes, he also mentioned in chat 12 around 25:50 minute mark about IFB insulated fire brick (kiln brick) vs CFB ceramic fiber board comparisons.

Hope these help to answer some of your questions.

Thanks, Gerry... I'll look those up.  I've done a bunch more watching, reading and chatting, now, and collected a fair bit of info, so I'm just posting looking for any further experienced opinions, just to hear them and round out the info I have.  Matt is definitely a storehouse of experience.

I like where you are going with this.  Very much like the system I'm working on.  I am building a drain back solar thermal domestic water heating system which uses a repurposed natural gas water heater tank for the heat exchanger.  In short, the sun heats my solar thermal panels outside, then that water is pumped through the inside pipe on the water heater, this in turn transfers heat to my 50 degree well water coming in to the 50 gallon water jacket that surrounds it.  Any excess hot water this system produces, (I'm only shooting for 110 degrees as this is primarily a pre-heater) is stored in 4 blue 55 gallon drums.  These drums are plumbed in such as way as that they can share their heat back to the heat exchanger if need be, such as at night.
Like your system, a temperature controller determines when things kick on and off.

Now, this is the relevant part.  My batch box heater will heat an oven, perhaps a cook top, but mostly it will heat a large bench constructed of solid cement block.  Embedded in a layer of the bench top will be several turns of copper tubing, probably inlaid in a layer of cobb between the blocks that will feed back to the 4 storage barrels by use of a circulator pump and another temperature controller.  In this way, my batch heater can heat my bench, but the temperature of the bench will be limited and prevented from becoming uncomfortably hot by the excess heat being absorbed and carried away to my hot water system.

All this and of course, there is no chance of a squish boom because the water lines are just not going to get that hot.