Rocket mass heater with 8" J-tube and bell

I fired up my rocket mass heater for the first time last Friday. It had a slight draft while stone cold, and took off immediately. The built-in cooktop recess reached 350F within an hour, while the temporary stovepipe exhaust was just comfortably warm. It took the space (700 square feet on the main floor, plus lofts) from around 62 (adequate for a radiant floor while wearing a sweater) to 68 pretty quickly, and after four hours of burning ( more than half with a couple of big logs that burned slowly), it stayed warm overnight and was still comfortable the next morning, distinctly higher than the thermostat setpoint.

I have burned a couple of times a day since then, for an hour or two, getting the feel of the beast, and keeping the space very comfortable. This has been with the weather mostly below freezing at night and up to 40 or so during the day, some days cloudy and some partly sunny. (A sunny midwinter day can make the space comfortable without any added heat.)

I will be posting the complete build sequence here as I get time.

That looks awesome, I can't wait to see the build!

My house plans from the start included a custom woodstove built into a two story, 4' x 2' stone chimney. Once I discovered rocket mass heaters, I decided to switch to that mode, and make it a principal heat source instead of an accessory. Many iterations later, I settled on the design shown.

I had a massive footing below the basement floor to support the future chimney and hearth, so I could build up from that with concrete block filled with concrete and rebar, sufficient to independently support a two story chimney and the RMH mass. I poured a slab below floor level to be able to sink the J-tube and have a cooktop built in and hopefully get enough heat even though it could not be directly over the riser.

A layer of bricks on the slab made an air gap for circulation to positively ensure that the slab would not overheat under the J-tube. I put a layer of cement board on these, carefully leveled, laid out the component locations, and started building the base of the bell and core. I set a full course of insulating firebrick under the core and up around the space where the core casting would later be lowered in.

The design allowed a 16" flat apron around the feed tube to minimize the possibility of sparks getting to the floor, and give a convenient place to sit and tend the fire, or just enjoy it. This left room for a storage cubby in front of the core. I planned a secondary air supply from the back of this cubby, via a steel insert that fits down the feed tube to protect it and guide the air around to the P-channel at the front of the burn tunnel roof, cooling the insert while preheating the air.

hey glenn,
it looks beautiful!

how is the thickness of the perlite-clay between the burntunnel and the bricks?
from the measurements of your heatriser, you used 2 inches there. but for the rest?


how did you cob the bell? do you have more photos on that? how did you make the top/ceiling of the bell?


thank you very much
tobias

There was less than 1/4" between the J-tube casting and the insulating firebricks around it, barely enough to pour a bit of loose perlite into.

The core casting was 1 1/2" thick of refractory cement all around, except the burn tunnel roof which was closer to 2".

More photos to come - it is pretty much all documented.

Looks great glen Congratulations !    You will love it.

A bit of detail on the heat riser:
To get 12" outside diameter, I snapped two sections of 6" stovepipe together. The mathematics of this means that the circumference of two 6" circles exactly equal the circumference of a 12" circle. Any other combination works the same way, adding the diameters. A 6" plus a 4" would make a 10" cylinder.
I slightly flattened the stovepipe sections on an anvil to approximate 12" diameter before assembly, as they were too stiff to snap together otherwise.

To mix the perlite-clay, I used a technique suggested by others a few years ago. With the perlite in a mixing tray, I misted it with water, dusted fireclay powder over it, and mixed it a bit. I repeated this until all of the perlite grains had a very thin coat of clay all over, and it passed the standard "make a firm snowball that pops with finger pressure" test. I had previously tested this method, firing the result in my kiln, and it made a strong yet very light brick. This time I used less clay, and after firing the completed riser sections, they were still a bit crumbly in places, requiring some pre-installation patching with a stronger mix. I have a gallon of sodium silicate (water glass), but did not take time to brush some on the inner surfaces before installation. When I inspect the interior, I will pull the riser out and coat it if it looks like it needs help.

I had originally planned two 2' sections of riser to set on top of the 9" high core casting, but wanted more chance for heat to reach the cooktop, so cut one section in half, making a total riser height of 36 + 9 = 45". This is a bit less than 1:2:3 proportions, as the feed tube is 16" deep and the burn tunnel is 24" long overall. I was prepared to add another section if it appeared to need it, but so far it is drafting well with almost no smokeback (only in specific combinations of tall punky sticks which really wanted to burn).

I positioned the outer shell and the 8" sonotube inner form (actually 7 3/4" outside diameter) on a base, poured perlite-clay into the space, and packed it fairly tightly with a piece of 2x4 and a 2x2, both of which fit pretty closely between the forms, tamping gently all around until it seemed solid. It can be tricky to keep the forms aligned as you work, so you have to pay attention to the gap and adjust it if it starts to shift.

I neglected to put a sheet of paper under the riser before packing it, and the bottom stuck to the cement board base in some places, leaving gaps I had to patch. Using something to prevent sticking would make removal from the base much easier.

This documentation of your build is invaluable.
Thanks for taking the time!

Is the perlite mix a better compound to use than cob throughout the heater?

Not better overall, but better for certain functions. Perlite-clay is a very good insulator, but has almost no mass. It is exactly what you want for the heat riser, so it can get up to temperature immediately, but it is not very abrasion-resistant or structural. Cob makes good mass and load-bearing structure.

Nice build there Glenn! Is the stove still in the drying/curing phase? Looking forward to more details of the bell's construction, and its fully cured thermal flywheel performance later on this winter.

Have you decided on the type of finish plaster?

I started work on the foundation two years ago, and finished the cobbing (mostly) last spring, so it is thoroughly dry.

I am planning on a lime plaster finish. I have tested the technique on my dragon cob rocket oven already, but want to plaster this in the spring when it can sit damp and cure slowly.

Hi Glenn, great build !
From the drawings and pictures it seems you actively cool the upper part of the feed tube by channeling the air for the p-channel around the metal collar that´s sticking in the feed.
If I´m right, how well does that work, and does the plate next to the feed still get hot enough to cook some tea on it (as there is a small kettle there) ?

Looking forward to more pics,
best wishes
Ralf

I wish I'd seen your post before building my RMH for my greenhouse...  Among other things, I'm wondering about my core...

I mixed up perlite with Refractory Cement that was already rated for 3000 degree temps.  You can get 25 pound buckets of it from Menards.  But I didn't mix it the same way you did...   I mixed it with water to a crumbly, barely dry consistency, but a lot more water than you did misting it...  I was worried it was too dry while I was packing it.  

The core ended up very heavy and dense/hard like concrete: is that correct?

What did you do with the pink styrofoam after pouring: scrape out the big chunks then let the edges burn off at first firing?  Great idea for an internal form!

The metal insert does indeed guide fresh air to the P-channel. It seems to cool the feed quite a bit, presumably preheating the incoming air. The edges of the plate have occasionally gotten hot enough to be unpleasant to touch, but not dangerous. Therefore, there is obviously no cooking on it. The kettle was just an expedient air damper, since replaced with a piece of cement board which is quite effective.

My core is straight refractory cement (dry bagged material), and is quite heavy but not terribly strong. The styrofoam inner form was removable and did not burn; in fact, I plan to reuse it for more cores, as the method proved quite successful. More on that imminently.

The next step was casting the J-tube core. I used dry bagged refractory cement, cast in two pieces around a styrofoam inner form. For mold release, I taped thin plastic (shopping bags) over the styrofoam surfaces. This worked even better than expected. Next time I would use larger sheets of plastic, as getting the bags to fit without wrinkles or gaps was quite painstaking.

I built a box with self-aligning joints on the sides, open top and bottom and 1 1/2" larger than the styrofoam on all sides, and a 2x4 rectangle fitting between the styrofoam and box. I cast the lower 2/3 first with the forms upside down, using the 2x4 frame to keep the cement 3 1/2" away from the ceiling side of the form. The outer box was 1 1/2" deeper than the styrofoam so that the floor was 1 1/2" thick. After the first pour had set, I inverted the form, pulled the 2x4 rectangle out, added a separator layer of masking tape, and poured the top section including the burn tunnel roof.

I set the form for pouring on a board balanced on an ATV tire, which gave ample flexibility for vibrating the mix. I didn't have time to mount a vibrator on the board, so I and my helpers held a couple of oscillating sanders on it, which seemed to work well enough. I didn't have the form parts completely fitted before mixing started, so the first batch of refractory had started to set before pouring (or scraping) it into the forms, and it ended up with some honeycomb areas which I patched before proceeding with the second part of the cast. The second batch of refractory was wetter and poured nicely. Within an hour or two I could take the outer form off, and the next day I wedged the top and bottom apart. It was hard going at first because there was no usable seam at the joint, but as soon as it started to move, the rest was dead easy. I had given the styrofoam a tiny bit of draft, tapering less than 1/8" from joint to top or bottom, but it was enough for the styrofoam to slip out undamaged.

Hello,

I am super impressed with your RMH.  I do have a couple of questions.  I am concerned about the loss of integrity in the burn chamber by having it in two parts.  I understand why you did it, and I think the result looks fantastic.  Instead of using the foam insulation, couldn't you use a non-toxic material allowing the J-tube to be a single piece and then burn the form out?  I was thinking of using cardboard shipping tubes cut on a 45 degree angle to form the J-tube, and then securing the parts with glue and some paper mache.  I was also intrigued by a RMH design where an inline duct fan was placed near the end of the "chimney".  He would turn the fan on at the start to basically "prime" the system and turn it off once the fire was burning and drafting on its own.  Do you feel that such a fan is worth the expense?  

A one-piece core casting would have no joints for differential stresses to be relieved, and would inevitably crack somewhere. I theorized that the usual mirror-image halves of a typical two-piece casting would have different stresses at top and bottom, and possibly crack from that. With the top separate, either top or bottom can heat up more and potentially have a slip plane.

I did find that the top section has a hairline crack at one side of the feed, but as that is contained and relatively protected by the metal insert, I don't expect it to be a problem.

Peter van den Berg, who has done major research on batch box design and construction, has moved to a three-piece casting (two bottom/sides and a roof), because of the uncontrolled cracking of two-piece castings.

Cardboard tubes may not have the strength or stiffness to hold up to casting, depending on the cardboard character and the size/weight of cement. Also, one of the benefits of casting is the ability to precisely shape the interior. This is important for getting the best combustion. Sharp internal angles have been found to improve the beneficial turbulence during combustion.

If your installation has poor draft which needs assistance for starting, the fan you describe might be a good stopgap. Otherwise I don't think it would be useful.

Hi Glenn,

Thank you for your prompt and thoughtful reply.  I had not given any consideration to cracking.  It certainly gives me a lot more to think about.  

Troy

Awesome design and thorough explanations. I'm a visual person so all of your labeled pics are great for me. If I had seen the burn chamber build in the beginning the build would have been easier for my dumb ass to understand. Can't wait to see more.

With the J-tube core cast, I could proceed with construction. I built up a shell of insulating firebrick around the core location, and built up the brick bell inner shel at the same time. I built in an 8" x 8" opening at the bottom for a cleanout, and an approximately 8" x 8" opening for the exhaust to the future bench mass (it currently leads directly to the stovepipe chimney.) I added a hood in an attempt to make airflow follow a more circuitous path than falling directly to the exit. I have no way of proving that this works, but I can say that the exposed brick on the back of the bell gets hot all the way down to the floor, implying that  none of it is being bypassed by hot gases.

Before setting the core into its cradle, I filed gaps with perlite, and added a thin layer of perlite-clay on the floor, leveled as perfectly as possible. After lowering the first part of the casting into place, I poured and poked perlite into the gap around it, then repeated with the second part of the casting. Once this was done, I could proceed with building up the brick wall of the bell. I set these bricks (salvaged old soft red brick) on edge to use fewer bricks, have a thinner inner wall, and give fewer joints to seal.

I started the bell with a 26" x 26" inside dimension. When I got to the level of the cooktop insert, I expanded it a bit to give more clearance to the sides of the 24" wide cooktop, and stepped out the wall below the cooktop to expose the floor to as much heat as possible. After reaching the top of this opening, I went back to the 26" square, and corbeled bricks inward to close off the top. Each course was stable in itself and with higher courses on. I had mocked up the closure on the floor first with no mortar, so I knew it would work. The photo looking up inside the dome should show the sequence. Larger openings may be possible to close with a similar method, and certainly smaller ones would be. I used mostly full firebricks (2 1/2" thick), with one layer of splits (1 1/4" thick); you could use all splits if a flatter dome was important. I got the full bricks cheap, so they were more economical for me to use.

I considered casting a refractory slab for the top, but was not confident of spanning the 26", and didn't have enough refractory on hand (at $50+ a bag) to make a thick enough slab.

I had a steel angle frame of suitable size for the access panel, and added a couple of anchors to corners to allow the cob to hold it solidly. I then filled the gap to the brick shell with refractory and brick scraps. I drilled the frame in ten places and attached stainless steel screws for mounting the 1/8" steel access panel. Decorative brass capscrews hold it securely while being easy to remove for inspection.

I set two steel pipes into the brick to securely support the cooktop insert, which has a 3/16" floor and 1/8" sides and top. I attached flanges to all sides to lock into the brick & cob opening, and added fiberglass rope stove gaskets around all joints. After sliding the insert into place, compressing the rope, I cobbed all around the edges to anchor it from coming out and further seal it.

I built up the cob shell in lifts of 6" to a foot depending on the help available, essentially full thickness so that there are no layers that might peel off. The joints show some contraction cracks, but are jagged and toothy enough to be well connected and are held by gravity. Allowing the cob to dry in lifts means that no big stresses can build up between cob and brick at the edges and steps of the brick.

The brick inner shell is 2 1/2" thick, and the cob is 6-7" thick, giving a total thickness with future plaster of about 10". This does take a long time to let the heat through; the steel panels give all the instant heat needed. In fact, I think that once the mass bench is added to absorb the rest of the heat generated, I will add several inches of cob on the roof of the cooktop insert. That currently gets far hotter than the cooking surface itself, and reducing the instant radiation should help retain heat in the bell for longer. As it is, it needs all the radiation it can make. The exhaust stovepipe after a couple of hours gets too hot to comfortably touch, and some heat is being wasted up the chimney. This indicates that the system can comfortably support the second bell.

In its current state, the brick inner shell which will back up to the block & stone chimney gets too hot to touch at the top after an hour or so, and after three or more hours of burning, the bricks at the floor are too hot for comfort.

The capstones at the top of the bell are not mortared to anything, but sit in a bed of cob. As most of them weigh in the neighborhood of 100 pounds, and have irregular bases, I think they will stay where they are.

This is such an insanely well documented and informative build, thank you! I noticed earlier in the thread you mentioned your exhaust exit is the same CSA as the system size, 8"x8". Is this because in a bell system there is much less of a risk of bottlenecking the gasses? I know in a duct bench you typically want a larger opening for the exhaust to enter to flow to the properly sized stovepipe to avoid constrictions, but I also know bell systems are quite different.

A bell has less internal friction than ducting, so can stand a bit of constriction. Also, there is more space than between a barrel and riser, so the gases don't need to make sharp changes in direction. But really, I made the hood around the exit slightly larger, so there would be less constriction where the gases have to converge on the exit. It currently goes to a 6" diameter stovepipe; the future chimney will be 8 x 8 flue tile, which is about 6 7/8" square inside. The feed is 7" x 7" (plus P-channel), the burn tunnel is 7" x 7 1/4" high, and the heat riser is 7 3/4" internal diameter. The 23' height of the chimney inside the house should give even better draft than the short temporary stovepipe which works fine.

That makes a lot of sense, I hadn't even considered the reduced friction in a bell, but I'm still very much a newbie. Thank you! Great job on this beast, it is a thing of beauty. I'm loving every update.

Glenn,

Is it time for an update???  I've been studying your build pretty hard in preparation for my own.  I'd love to see where your's is now.

Yes it is. I took complete photos of my inspection last fall, and have intended to add a description to this thread. I'll work on it in the next couple of days. It has kept me warm all winter (including the long cold March) on about 7/8 of a cord of wood.

Thanks for posting me a link to this thread.
I particularly like the 'hood' over the cooktop.
I'm thinking this could be adapted to become a bottom heated oven, but I have a long way to go before I can try it!

What did you use for the cooktop?

Hey, Thanks for your post, really cool to see yours being built. We want to build one this year in our wooden cabin but not workout plans, can anyone send me to a link where we can download these plans? Summer is still hot, hot, hot but as soon as it cools we want to get this project underway before the clap of winter descends.

Any help from you lovely permies would be greatly received

🙏😘

Hi Simone;
Matt Walker has plans for sale.
Here is a link to his site
http://walkerstoves.com/

Or we can give you the dimensions and help guide you thru building your own.

thomas rubino wrote:Hi Simone;
Matt Walker has plans for sale.
Here is a link to his site
http://walkerstoves.com/

Or we can give you the dimensions and help guide you thru building your own.

Thanks so much, I will go do some measuring and come back to you, your help and guidance would be a dream come true - we plan to start building it when the summer heat has gone, our expected temperaturas in the coming days are 35 degrees C and I can't think straight when we tip over 30 to be honest 😳

Speak soon and thanks again for your reply

Simone (Málaga, Spain)

Hey

We did some measuring and the size hole we have is 2 cubic metres and we could build a fire the size of 1 5 metres high, 1 metre wide and 1 metre long.

Any guidance would be amazing, we have a log burner if we can't achieve this but we would rather have a more efficient heating system.

The cabin is made from wood, insulated using hard foam boards.

Thanks again

Simone

Simone Icough wrote:

thomas rubino wrote:Hi Simone;
Matt Walker has plans for sale.
Here is a link to his site
http://walkerstoves.com/

Or we can give you the dimensions and help guide you thru building your own.

Thanks so much, I will go do some measuring and come back to you, your help and guidance would be a dream come true - we plan to start building it when the summer heat has gone, our expected temperaturas in the coming days are 35 degrees C and I can't think straight when we tip over 30 to be honest 😳

Speak soon and thanks again for your reply

Simone (Málaga, Spain)

With the dimensions I have given in various posts and image descriptions, and the knowledge that US standard firebricks are 9" x 4 1/2" x 2 1/2", you should be able to count and measure from the pictures to get a reasonable handle on the dimensions. The bell internal dimensions are totally flexible anyway; all you need to know is that for a 6" batch box or an 8" J-tube system, a bell internal surface area (sides and ceiling, not floor) of about 5.7 square meters is optimum for absorbing the right amount of heat from the fire. (Check that figure with the charts at batchrocket.eu.)

Thanks for your help and I will go read more of your posts also

Simone

Glenn Herbert wrote:With the dimensions I have given in various posts and image descriptions, and the knowledge that US standard firebricks are 9" x 4 1/2" x 2 1/2", you should be able to count and measure from the pictures to get a reasonable handle on the dimensions. The bell internal dimensions are totally flexible anyway; all you need to know is that for a 6" batch box or an 8" J-tube system, a bell internal surface area (sides and ceiling, not floor) of about 5.7 square meters is optimum for absorbing the right amount of heat from the fire. (Check that figure with the charts at batchrocket.eu.)

Hi Glenn,
  Beautiful build!! I live in lower Herkimer county & built a RMH in 2020 for my tiny pyramid house. It worked great the first year & less so each year after. Are you available for consultation or in house visit? I need help to figure out why it is not working properly now. Thank you.
    Sincerely,
       Khara

Hi  Khara;
It would be perfect if Glenn is available to help you out.
If he is not, then I might be able to help you long-distance.
You can contact me at  dragontech@blackfoot.net or here on Permies.

Hi Khara, I am certainly available for consultation. Let me know more particulars, and we can work something out.

Glenn Herbert wrote:Hi Khara, I am certainly available for consultation. Let me know more particulars, and we can work something out.

Thank you Glenn, would you mind sending me your email address?
I have already sent an email to Thomas & have a response, but would also take advice from you as well. Thank you for your time.