Design help for a cabin RMH/Cookstove

I want to heat and cook in a cabin. I am curious about building a rocket mass heater to meet this need. I have a lot of questions to see whether it's a good idea, and, if so, exactly how to do it.
I know there's a huge amount of info in this forum already, so if you are aware of specific posts that answer my questions, please do point me to them. I'm just having trouble pulling all the infomation I need together into one place.

Situation:

1360-1540 ft³ room volume, depending on how thick the slip-straw walls are. That range represents 12" thick (leaving a smaller volume inside) to 8" thick (larger volume). I'd welcome input on how thick (i.e. how much insulation value) makes sense given how small the footprint is.

Here's data for my climate (how it used to be, anyhow):

	Oct	Nov	Dec	Jan	Feb	Mar	Apr
Daily Average (°C)	8.5	2	-3.6	-7.1	-6.9	-2	5
Daily Maximum (°C)	13	5.5	-0.5	-3.3	-2.6	2.5	10.2
Daily Minimum (°C)	4.1	-1.5	-6.7	-10.9	-11.1	-6.5	-0.2
Extreme Minimum (°C)	-6.2	-19.5	-27.4	-31.1	-31.5	-32.5	-15.4

I am okay with a cool room in the winter, 14-16°C, as long as there's a warm spot to sit. That's one of the things that attracts me about the mass bench idea - keep me warm rather than the whole room. However, I want the capacity to warm it up to 20°C or so if I have company who likes it warm.

I am working on the land, so I can tend the fire somewhat, but don't want to babysit it all day. I am fine with it getting pretty cold overnight as long as the room doesn't freeze.

The floor is going to be softwood, the walls light straw clay, and the roof steel for now, hopefully switching to thatch within a few years. Support to ground for the mass would be challenging.
I can and will run calculations on the dead load capacity of my floor once I have a clearer idea of the shape of the mass (which affects where the weight lands relative to the floor supports).

Questions
Cross-section of system:
I understand that one can build a variety of cross-sectional area systems, with 6" diameter being the smallest that usually works. I imagine for such a small space it would make sense to keep it as small diameter as possible: 6". Does that make sense? I've heard rumours that you actually need to run quite long fires with some rocket mass heaters to be warm enough, which isn't all that appealing given the repeated feeding required. Would 6" be large enough to allow relatively short/few fires per day?

Stovepipe:
I got some insulated stovepipe for only $25, but it's 7" ID. Is it possible to increase from 6" to 7" at the transition to insulated stovepipe? I think this is considered a very bad idea with conventional woodstoves because the gas expansion results in cooling that may consense water vapour, leading to creosote adhesion, leading to a chimney fire. But I think the exhaust from a rocket stove is already below condensation temperature at this point in the system, so does it matter?

Mass sizing:
I imagine a 6' long bench would be nice so I can sit or lie on it, but I'd rather not make it bigger so it doesn't take up more of the room. I would like to do a stratification chamber design; would a 6' bench be enough internal surface area to absorb enough heat to be efficient? How thick would the walls need to be? This correlates to: how much does it need to weigh to provide enough thermal mass? I need to make sure the floor can take it. It would be nice if the mass is big enough to let me be away for a day or two without the room freezing - is that realistic?

Fire safety:
How can I install the rocket stove and mass so they don't set the floor on fire? My impression is the light straw clay wall should be fine being near or against the mass - is this correct? What about a structural post in the wall buried a few inches into the LSC?

Cooking:
How well does it really work to cook on the barrel? Anyone here using it as their only cookstove in the winter? Are there options that work better than a barrel? Is it possible/practical to build in an oven where the exhaust first enters the bench?

Materials:
I have a powerful ethic of living my life using low-embodied energy, low-tech, local materials. The things I don't love in some rocket mass heaters are: purchased firebrick, wool insulation, foil tape, the barrel, stovepipe, metal heat shielding, perlite/clay mixtures, hardware cloth or hog panel for forming the cob bench... that sort of thing. Perhaps you get the picture. I will use this stuff if I'm convinced it's the best overall option available (compared to much worse performance, or a different kind of stove), but I'd like to push the envelope: how localized can this technology get? How much of it can be made of cob? What options are there for effective homemade insulating firebrick? Are there other chimney options?

the masonry heaters association site featured the build of a stove ,conceived by Richard Jussel and developed by the Austrian Tile Stove Association ,built entirely from clay sourced and processed on the site, but i think it needs a lot  of maintenance work  on these ---daily use would damage it ---not impossible to repair ---most likely requiring a major rebuild over a years use ,  as to making your own high performance insulative type brick --in the above 1400 C range -no ones managed yet ---the energy embodiment and sourcing /processing the material for them is very high, perhaps minimise the use of  modern materials to gain a better stove that uses less and gets more out of its fuel and greatly reduces its emmisions maybe a compromise---but better for the longer term of the enviroment and your own health , good luck with a build , and lots of reading ahead of you .

Thanks, Tony. I'm having trouble finding the article you referred to on the MHA site. Can you link it for me?

I feel it is good to move in the direction of the materials ethics you mention, but it is a question of what kind of progress is made and how. RMHs come from a 'scrapper' ethic, making real world improvements for people that need something that works. Now.

For me, the priority is materials that are affordable and promise to be environmentally neutral or positive. I avoid cement because of embedded energy, but also because a large slab in the ground will remain an obtrusive part of the environment well after it's initial purpose is forgotten. Cinder blocks on the other hand can easily be removed and repurposed, so I consider them a compromise sometimes acceptable when alternatives offer little promise.

Of course, cement isn't an option for a RMH in any aspect that doesn't have an easy, preferable alternative. Firebricks seem a comparable situation. There is an embodied level of energy and cultural dependence to make and transport them. But I see no downside past that point. They present no threat of toxicity to the environment and break down into natural elements solely by physical means as far as I'm aware. For my ethical calculations, the good they can do in application far outweighs the costs of production, assuming the general manufacturing and transportation base exists in anything close to what it is at present.

One element of "local" is how to view the existing production and distribution networks. 'Scrapping' makes use of readily available materials that would otherwise become landfill waste. But if use of such materials becomes part of a codified, growing system, it helps reinforce production methods that produce waste and excess. It would seem a tough call at what point making use of 'waste' products becomes encouragement to produce more of that waste. Using scrapped materials though often makes an RMH more reliable and reproducible than creating a similar element from local, scratch material. I guess that is the trade off you are looking to find help deciding on?

This is just to reflect on the difficulty of innovating in the direction you mention without practical experience of working examples, knowing what change has decent promise of being part of a fully functional system and all the possible trade-offs a change may have on connected elements. I believe there are many innovators out there who have made systems that include incredible novel changes, but they work in such a way that it is difficult to communicate what they embody and how others may implement them.

So I have some experience to address your questions, but it is impossible to say if the would apply to something you construct if you plan to deviate much from common design elements my system is based upon.

I have an 8" system based upon the Ernie and Erica barrel/ducted bench model. I am in a yurt with limited insulation (my primary deviation from the assumed norm), and have found the need to have the system running to keep the room livable during the winter. As such, I've discovered it would likely have been better to install a 6" system. I do cook on the barrel as my almost sole source of cooking heat in the winter. I don't notice the lack of fine controls over temperature, it is something I find easily adjusted to through any number of creative innovations and the overall aesthetic just feels wonderful. I can boil water in the center of the barrel top, but it takes a good while running at full blast. The only other problem that comes to mind is that one of my go to dishes is a 'stir fry' that is more of an enhanced sautee' I'm told. Keeping frying temperatures going for close to an hour can make the space uncomfortably hot even when it is -10* outside. (I also found out that high temperatures can destroy the coating on plastic eyeglass lenses, need to find actual glass ones somewhere.) Not sure if a 6" system would be different. Then again, a little sweat in the winter season is probably an overall healthy thing.

For a situation with a well sealed and insulated space, an 8" system basically means you need to run the system less than a 6" one. I suspect the only reason to go with 6" would be if you wanted to save a bit of space with your mass.

I have to wonder what objections you have to wool insulation? Are you referring to rock wool/super wool? Or actual sheep's wool?

sorry i tried but no longer works ---could just be my old system and my lack of computor skills---Richard Jussel is from austria ---has a site ---feurmacher.com ---perhaps make contact and go from there ---the MHA article was from 2008, so might be archived ----heres some pics i saved --there was mention of a 20 page set of drawings and some documents plus a video for sale.

Coydon, you're in a pretty cold location for yurt living! Although I have run into a few in Ontario at Ottawa latitude and in central Vermont, both of which also get pretty cold winters.
Your reflections on the materials are worth hearing. Thanks for sharing them.

Coydon Wallham wrote: One element of "local" is how to view the existing production and distribution networks. 'Scrapping' makes use of readily available materials that would otherwise become landfill waste. But if use of such materials becomes part of a codified, growing system, it helps reinforce production methods that produce waste and excess. It would seem a tough call at what point making use of 'waste' products becomes encouragement to produce more of that waste. Using scrapped materials though often makes an RMH more reliable and reproducible than creating a similar element from local, scratch material. I guess that is the trade off you are looking to find help deciding on?

My own journey is a mixed focus on here-and-now impacts and modelling visionary alternatives. I sometimes follow the salvage mentality in areas where I am choosing to not give up a technology (e.g. some internet access: I got a computer from community computer recycling NGO), or where doing so has a positive footprint impact (e.g. a more efficient biomass cookstove from scrap metal makes less air pollution than a three stone fire - maybe important given how many people we have right now). I think the modelling of true alternatives is important too. As you say, depending on salvage ultimately requires the continuation of the present system. So I don't want to eat primarily on dumpster diving, or mulch my garden with cardboard, or get all my clothes at thrift stores, even though those things are possibilities in the wasteful context I'm in. We need people dancing to different music, expanding our collective vision for what is possible and how.

For this particular need, I'm not looking for help deciding which path to lean toward so much as putting the question out there about what options people are aware of so I can make a more informed decision about what to try. What I take from your writing is that you would suggest that I build a proven design so I know what good operation looks like, and then experiment from that baseline later.

Coydon Wallham wrote:As such, I've discovered it would likely have been better to install a 6" system.

Can you explain why 6" would be better than 8" for that situation?

Your notes on cooking are helpful. Sounds like it'd be okay, but not any too much cooking power. I'm a bit spoiled having recently switched my summer cooking from a bare firebrick J-tube to a TLUD based on Paul Anderson's Champion design. It boils water fast, and fries quite well.

Coydon Wallham wrote: I have to wonder what objections you have to wool insulation? Are you referring to rock wool/super wool? Or actual sheep's wool?

Yup, I was vague. I meant ceramic wool - high embodied energy and high-tech/centralized processing needed (very hard to localize). I have no problem with sheep's wool in reasonable quantities. I guess the latter is pretty fire resistant for an organic material - are you suggesting it could have a place in RMH building?

Tony, thanks for more info about that article. With the date I was able to dig it up in the Internet Archive. Looks interesting. Not sure whether it's what I want, but definitely will keep it in mind.
The Energy Saving Stove

yes not for most , but the original idea is to help those who can t source or afford the nicer materials we can get to play with , its going to be a challenge to use only natural material ---there are some types of rock that will come closer to fire brick in performance---the donkey 32 site has info on use of them ---but if you cant source local ---its going to cost.

Andre,

For the bell you can improvise and use local materials.
For the firebox, I would recommend the best NEW firebricks you can buy. Old ones may be worn thermally and close to their end, or come from industrial furnaces and be saturated with heavy metals, or come with old fire mortar attached that may have asbestos and you will have to remove it first.

I you are concerned with their embodied energy:
-assuming 1 firebrick weighs 3.7 kg it has embodied energy of around 11 MJ
-firebox and riser for BBR 150 mm can be built with 60 bricks (or less or more, depending on the brick laying design)
-so 60 bricks have embodied energy of 660 MJ
-such a firebox will have 26 l capacity and in theory can take 8 kg of firewood which is around 100 MJ
So after only 7 burns you already match the embodied energy of your new firebricks. If they are quality and new they will last long and the longer they last, the more negligible will be their embodied energy.

Andre Wiederkehr wrote:Can you explain why 6" would be better than 8" for that situation?

It is similar to something I learned recently about choosing an air conditioning unit. An interesting piece of advice I was given when asking about BTUs/sq ft cooling power of the units- don't buy a unit that is much larger than the space you want to cool. It will quickly cool the area and make it feel clammy. It is best to have a unit sized right because it will run longer and work to dehumidify as it does.

Now that doesn't exactly apply to the yurt as it isn't airtight to keep moist air out, but the dehumidifier effect is analogous to the heat coming from the radiant chamber- the barrel in my case. No matter how warm I get the mass in the bench, it will not keep up with the heat loss on cold days through the yurt fabric, even with the light but noticeable insulation I use. There needs to be some heat coming from the barrel pretty much any time it is below freezing or the air temperature inside drops fast. But running a full load in the 8" system heats the space nicely in less than an hour usually. I end up running the box with just a few sticks most of the time just to keep heat coming off the barrel. It coals out often when I don't pay close attention, and I've seen some creosote build up in the exhaust stack. If I had a 6" system, I think I would be running it close to full most of that time and have less problems.

Andre Wiederkehr wrote:Your notes on cooking are helpful. Sounds like it'd be okay, but not any too much cooking power. I'm a bit spoiled having recently switched my summer cooking from a bare firebrick J-tube to a TLUD based on Paul Anderson's Champion design. It boils water fast, and fries quite well.

Here's another design option that affects performance on this subject. With a barrel, there has been differing opinions on placing it over the riser. From the E&E book, they recommend 2" minimum clearance. 2 inches is optimal to create a hot cherry spot in the middle that would be good for boiling water quickly on. But it also runs the risk that ash could build up on the lip of the riser and start to limit draft. Other people say you cant have too much space there for system performance and recommend placing the barrel as high as practical, or simply to have a brick bell with no radiant chamber, both of which of course remove the utility as a cooking surface. I've gone with 3" to keep flow solid but provide a decent cook surface. Even so, If I focus on loading the box full with small, well seasoned sticks, I can get a boil going fairly quick.

And it is about perfect for breakfast- as I get up and start the fire in the morning to heat the yurt back up, it is hot enough to cook bacon, eggs and toast, and warm a morning beverage pretty much as soon as I get myself moving about. It works much better than my summer cook solution, which is an induction burner that seems to have a heating element that can't reach the whole 9" surface of the cast iron pan, and cycles on and off to maintain the programmed temp so it is actually always a bit high or low. With the barrel, pots and pans can be moved toward or away from the center for more or less heat with a constant gradient, and I also put a grill grate on the top some times for more options. Of course it is an adjustment coming from the fine, almost immediate controls of gas or electric burners, but it is free fuel if the space needs heating anyway.

Andre Wiederkehr wrote:Yup, I was vague. I meant ceramic wool - high embodied energy and high-tech/centralized processing needed (very hard to localize). I have no problem with sheep's wool in reasonable quantities. I guess the latter is pretty fire resistant for an organic material - are you suggesting it could have a place in RMH building?

Just saying wool makes me think of the animal type, though ceramic makes more sense in context. Animal wool is probably only relevant to an RMH as a material good for on top of the mass used as a bench or bed. The ceramic "superwool" is a compromise I chose for my RMH because it makes for a much easier and better performing riser than anything else I'm aware of. The manufacture elements don't make me hesitate much, but the irritant nature of it after it has reached refractory temps is one thing that needs to be kept in mind during future maintenance.

Well, winter's coming and I'd really like to get something built this fall. Still lots to figure out for that to happen!

Here's a specific proposal that I'd welcome feedback on:

7" diameter system to match the insulated stove pipe I happen to have.

The building is on stone plinths, with stilts to deal with the slope. I'd propose putting the stove in the uphill end, building right on the ground (on a rubble base?).
The mass would be built on the ground and form a section of the floor (flush with the rest of the floor). This would save space for other uses in a very small room, allow me to build on the ground rather than on footings or extra joists, and have the bonus of making the barrel a more comfy cooking height, I imagine. The under-floor space is not currently insulated, so the mass would be insulated with light straw clay walls 12" thick, with a 3-4" air gap between the LSC and mass to allow convection from the sides of the mass up into the living space. This gap would be covered with some sort of grate.

Space constraints under the floor indicate a maximum size for the entire stove footprint of ~3.5'x6'. I'd like to do a stratification chamber for the mass as it's a way to not need so much metal pipe. The end where I'd put the stove core is 20" from floor level to ground; the ground slopes down toward the other end, making a depth of 33" there. I imagine I could dig it down a bit on the short end to give more depth there. I know this is quite small for a RMH. Would I somehow need to add drag to the system to extract an efficient amount of the heat from the exhaust?

The flue pipe would go straight up and exit near the ridge of the roof. It's a little in the way of feeding to put it in front of the fuel feed, but unless it would be practical to run it right under the fuel feed and then up, I don't see any other option.

Details on what constrains the footprint:
The building is framed with six large posts. The middle two are tied by a pole under the floor. This would make it hard to extend the mass past the midline of the structure. The floor boards are to run the short direction (8' 8"'), carried on the long sills and two joists that run the long direction of the building. Since the aforementioned tie pole is not designed to be weight bearing, the simplest solution is to run them full length on either side of the mass. For distribution of the floor load between these four members, on the non-stove side of the building, I'd prefer to not put these joists farther apart than 1/2 the width of the span (that is, placed at 1/4 and 3/4 of the span). This limits the mass to the middle half of the width; including the air gaps, that leaves only about 3.5'.

Does that make sense? Does it seem like a viable plan?

If that mass seems way too small, I could consider expanding it to the entire half of the building floor, and find a different way to support the joists on the other half.

That looks like a good plan to me. The barrel also counts as bell ISA, so say 20 square feet for the barrel and 20 sf for the top of the bell, plus at least a foot down on all sides gives around 60 sf. An 8" J-tube is approximately equivalent to a 6" batch box in rate of heat output, wanting around 55-60 sf ISA, so a 7" J-tube could be served by a slightly smaller bell than you propose. I would keep the design, and have an adjustable plunger tube so the sunken bell active volume could be adjusted with experience to give good draft and good heat extraction.

Such a small room could be quickly overheated by a full exposed barrel, so I would allow for enclosing part of the barrel in cob. If you don't have a removable lid for the barrel, you can configure it so you can remove the whole barrel for inspection without damaging the cob.

I would move the feed/barrel unit a bit farther from the wall and tuck the chimney flue behind the barrel. Aside from greater convenience for feeding, the barrel needs woodstove clearances to combustibles.

About the spacing of the floor joists, consider that the omitted central joist would only bear half the load of the side joists, so would not be such a problem for the tie beam to carry. I think the best bet here is to have a header which ties into the two other joists and also sits on the tie beam, to distribute the load over several elements. This would give no point load at the center of the tie beam, which is typically the most stressful position.

Great feedback; thanks Glenn!

I'm new to all of this, so I have some questions to unpack what you shared.

ISA = internal surface area?

Adjustable plunger - I'm not familiar with this concept, but I am imagining you are talking about having the exit pipe be able to slide up and down so that it can extract exhaust from the bell at various heights, effectively bypassing part of the volume of the bell if needed. Correct? That is a cool idea.
How would this be plumbed in practice to make it sufficiently gas tight?

I like the idea of enclosing part of the barrel with cob if needed to adjust radiation vs absorption rates. Again, I can't quite picture how to make the barrel removable without damage to the cob. Most barrels I've seen have protruding horizontal corrugations every foot. And how does a removable lid affect barrel removability?

For both of those, if you can point me to other threads that illustrate how to do it, that would be great.

The walls will be LSC with earth plaster. I believe this is considered non-combustible, so I'm not too worried about getting more clearance unless someone tells me I should worry. And there are no wood members by the barrel.
I would definitely prefer to have the flue behind just for feeding and cooking access and aesthetics, however.
Moving it behind the barrel would mean piping it horizontally right under the riser or burn chamber, correct? If so, wouldn't I lose the option of adjustability you mentioned by having a fixed extraction height? Is behind the barrel better than sort of beside the barrel and feed, as in my new picture? This is more compact (but doesn't move the barrel out, if that's a concern).
I would probably need to jog the pipe forward again up in the roof triangle to clear the rafter (rafter's shadow is added to the new floorplan as well).

Glenn Herbert wrote:About the spacing of the floor joists, consider that the omitted central joist would only bear half the load of the side joists, so would not be such a problem for the tie beam to carry. I think the best bet here is to have a header which ties into the two other joists and also sits on the tie beam, to distribute the load over several elements. This would give no point load at the center of the tie beam, which is typically the most stressful position.

I think you're describing an idea for how to add a third joist on the non-stove side of the floor (so joists at 1/4, 1/2, and 3/4 of the width). Correct?
I think the solution you describe would be quite workable. For my expected loads, however, I think just having the two joists at 1/4 and 3/4 is actually sufficient. The spreadsheet calculator I built says at that span (4' 2") 2" thick white spruce boards should be able to carry 900lbs uniform load per ft of floorboard width, which is far more than I expect to pack into such a small area.
I could use your solution if I wanted to make the mass wider and move the outside joists farther apart, however, so I'll keep it in mind.

Internal surface area, yes. Only walls and roof above the height of the exit flue are counted in this.

Here is a good description of an adjustable plunger tube (they can be non-adjustable also, where the adjustment feature is not needed). Putting the sliding part inside the fixed part would allow the moving joint to be inside the bell and only a fixed pipe going through the bell roof. It may be tricky for you to do this, unless you can put a cleanout/access point in the base of the bell to reach the tube for adjustment. You should have a cleanout somewhere in the bottom side of the bell to pull fly ash buildup every few years.

I would move the feed opening farther from the wall, as that might in extreme circumstances have flames coming back out the top. It should not happen with a well built RMH, but better safe than sorry. You could probably twist the core so there is enough space behind the feed tube for exhaust gases to freely reach the exit. 4" clear would probably work fine in your case. Your core's floor would only be 16" below cabin floor level, and even accounting for 4-5" of burn tunnel floor thickness there would be around a foot of clearance below that floor. You can build piers to support the core and allow free airflow beneath it as well as some around the sides.

A 4" thick cob jacket behind the barrel (say a half cylinder an inch clear of the barrel and 8" taller) would protect the wall whether the LSC is technically "non-combustible" or not, and would allow easy barrel removal. A barrel with a lid can be placed with the solid floor largely cut out and the lid on top, so inspection is a very simple matter. Then the barrel does not need to be removable.

How thick are you planning to make the bell walls? I would expect your 7" burn tunnel floor to be about 24" long, plus a few all around for brick thickness. Maybe 30-32" long overall. How wide do you expect the bell interior to be? 3'-6" outside with 4" walls would leave 34" - workable if the core is square with the bell and pushed against one side. Or you could turn the core about 45 degrees with the feed out into the room (4" clear of the bell wall) and the exhaust behind it. With a good cob jacket behind the barrel and some airspace to the wall, you should be safe and convenient. You would obviously want the exhaust to rise straight from the bell interior for the adjustable plunger tube.

If you need to shift the exhaust at the roof to clear a rafter, I would suggest putting one or two lengths of stovepipe at a slight angle rather than a pair of 45 degree fittings. It's a good thing RMHs don't leave creosote, as I would not want to get on that roof peak in winter to sweep it

2" thick white spruce may well be able to carry that load at 4'-2" span, but deflection is the killer here. You really don't want the boards flexing against each other as you walk on them, not to mention the difficulty it could make in sealing the floor against drafts. If you don't add the center joist, I would plan to dowel the boards together so they can't flex differently.