Help me understand the flaws: RMH, mass under the kitchen table

Hello, fellow permies!

I've been convinced about the rocket mass heaters for quite some time already, and since we moved to this old, old farmhouse it was clear; the heating will be with mass heaters.

A whole year passed before I started to build (since priorities included renewing a leaking roof, replacing all the wooden beams as well). In hindsight that was a good thing, during that year, spending the first winter under a blanket, next to eachother on the couch with an electric blower pointed at us, we dreamt up that the mass of the RMH would be under the future kitchen table, radiating into the lap of those sitting at it.

Materializing the dream
I combed through the permies rocket mass heater forum and read Ernie and Erica's 'The rocket mass heater Builder's Guide' again and again.
The decision was made to work with a diameter of 18cm ( 7 inches).
The floor here is tomettes ( = terracotta bricks) laid directly over the soil.
I made an isolation layer with perlite+clay, and laid a layer of fire bricks on it.
I then started to build my rocket mass heater, with the core - horizontal fire feed and burn tunnel, plus the riser - with fire brick, and the bell with old bricks salvaged from a broken bread oven that also stood in the way in the now woodshed.

In hindsight the decision to make a horizontal fire feed tube has one disadvantage, that is, one has to kneel to feed it, even though it stands on a little elevation ( = base layer of perlite+clay and the initial foundation layer of fire bricks).
The huge advantage of a horizontal fire feed, almost seamlessly going over to a burn tunnel is that one is very flexible in the length of wood and the point of burning.

Things changed during building - the cast iron plate to put over the burn tunnel, the legs of the table, the mass/no mass in the middle of the table as a heating pad to, in theory, keep food warm.
The biggest change was the amount of mass around the tubes. Since the bell of the system is made of salvaged bread oven brick instead of an oil drum, it absorbs and releases heat more slowly. Initially I built as much mass around the tubes as the YouTube video, based on Erica and Ernie's book suggests. But I had to take about 1.5 inches of mass off to 'release the heat', it got trapped/insulated under the mass ( ??? ). Now it's radiating beautifully. The mass here is local clay+about 20% building sand+a local funny grass sort that covers stuff,growing fast, instead of straw. And big and small stones to pump up the mass.

We love our RMH system and the neighbors, first in eye-brow-rising disbelief, now pat the warm mass approvingly and I can almost see them thinking "should I change my heating system...?"

It's all still new and the middle piece, with the 'pots and pans are kept warm here'-section is still in development. The initial massive mass didn't work for us there either, we're now considering stacking flat stones, connected with cob. We'll see.

Next heating build project will be, I think, a batch box system in the adjacent little house, livingroom-to-be. Rock(et) on!

Excellent Job Nina!
I love reading posts like yours!
I hear the same fascination and interest, that I had when I stumbled upon RMHs.
There was only one book and Permies to learn from back then and I dove right in!
You guys have done the same thing.

Stepping up to a batchbox build for your next stove is a sure sign that you are true Rocket Scientists, confident enough to Know you can do this!
I love it!
Keep up the good work and keep us posted on your progress.
Feel free to ask any questions about batchboxes we will be glad to guide you.
On my website (https://dragontechrmh.com/) I offer a book available in PDF or workshop hard copy, on how to build a 6" batchbox core.

Congratulations!  That table looks cozy and warm.

Thank you for sharing.

Bravo Nina! Great execution and perhaps the most innovative idea I've seen yet for a plate warmer to keep the dinner warm while enjoying the meal. We prewarm our plates in our proofing box but having the table act as the plate warmer is just the coolest execution ever for a plate warmer.

Enjoy your warm meals and toes and we anxiously await seeing your creative execution for the batch rocket. Cheers!

Oh my, what a great idea! All you need is a long table cloth and you have  a Kotatsu for your kitchen! I'm so impressed by your ingenuity!

Can you give more details on your heated table centre? It sounds like a neat feature but I worry slightly about combustables near heat sources.

Hello Nancy,

The table cloth is not needed, the warmth is held by the underside of our (wooden) table top and the lap and legs are warmed nicely. I even made a small ridge at the bottom of the mass, next to the ground, that - in theory - would keep toes warm! But the heat hasn't travelled so far (yet?).
Like I said, it's all new, we're happy to have this gentle heat, but the future will tell more about the all-round performance.

I made a photo for you of the center of the table.
Originally the mass was built to reach just above the table top where the hole is, with almost an inch clearing around the edges between the mass and the wood of the table top.
But it looked messy and it didn't warm up (or was I just being impatient?). I chiseled the centrepiece away and put slabs of slate (food presentation 'plates') on the table top to cover the hole. I had hoped that the rising heat would warm the slates, but as we know, air insulates as well and my hopes didn't come true.
So now the plan is to stack flat stones to make a heat bridge from the mass to the slates, possibly with cob in between for better conduction.

I need to do other renovation work first, the heat bridge has to wait for a bit, but I'll post my findings here later, when it's done.

Hey. Very cool project. Good idea about warming the legs/laps around the table. Don't worry about having to kneel to put in the wood. My firebox is below ground surface. I have to practically lie down to light it up 😁. Keeps you nice and fit to constantly crouch and stand up 👍. Waiting updates on user experience. DIY projects like this will evolve with time. New design elements added, new features added. Looking forward to updates!

Thank you!
The user experience so far is that the table heater works really well, but the bell isn't radiating as fast or as much as we'd like.
I re-used bricks from a broken, traditional French bread oven after having guessed they'd be suitable for the bell just by inspecting them.
They are porous, not dense like the firebrick that I used for the core.
But they insulate too much anyway, being flattish and broad; about 2cm in height and 10cm wide, lengths vary.
So we've decided to take the bell apart after the winter, at least 2/3 of it, and to re-build it from regular bricks.
The bread oven bricks will be used to make the core for the next rocket, if they're still good enough for the job.

At the moment quite some heat escapes through the chimney, the flue duct inside the house is about 6m (with 18cm diameter), and 7m up the chimney.
We're puzzling how to make the flue duct longer within the house, and still be able to walk around 'normally'.
It's an ongoing project, until it's "just right"

Yea. Seems like you are correct and bell is not giving away enough heat, gases remain hot till the end of system. My system has exhaust pipe - very last end - that will reach about +40 degrees when I have fired rocket for full 4 hour cycle - usually don't need more that this. And I have roughly the same size ducts as you have. Only I have it ~10m inside ground... Drawback is that I must have draft inducer to start things going. If it's up and running it works fine but to get it to work without draft inducer is a nightmare... In the other hand, as draft inducer is battery supplied (UPS) and draws roughly 60-100W/h I do not mind..

An update on the performance of our rocket mass heater.

We bought a wood splitter and it's making a world of a difference.
In the beginning we were searching for 'rocketable wood' in our classic heatingwood pile and did our best to split the logs that were too large with an axe (f'n scary). With the wood splitter - the one attached to a tree stump and operated with a heavy hammer - making rocket wood is easy, and we can heat the system more efficiently.

It seems the heat has penetrated the bricks of the bell and the system stays warm enough to release heat much faster.

Re-building the bell for 2/3 this coming summer is now moved from the 'to do-list' into 'let's watch this'-list

Nina, congrats on your new wood splitter, glad that is a game changer. I might be joining that club in a couple of years.

While I admit that splitting with ax, maul, and wedges can be an onerous chore, I have grown to love it over the decades. My dad had me crafting split rail fences from hickory at age 11 so I've had lots of practice.  Ofttimes my shoulders scream the day after a heavy session of oak and ash. But I can swing equally well with left or right hand; it helps my endurance at age 65.

Our family loves what y'all have created, both the rmh and the restoration of your farmhouse.  Joie de vivre!

I'm reading ALL the replies again and ... well, it might soud cheesy, but my Heart sings with joy to have found this lovely community at Permies.
The madness on the mainstream social media just isn't worth it anymore, and the warmth and sincere helpfulness of the folks here is super awesome!

Rico, good on you, swinging the axe so skillfully! I bet it's a technique that needs to be learnt, there are so many aspects to it I find; different sorts of wood, different bits of wood, the weight of the axe, the length of the handle and then the kind of wood one is aiming at achieving.
Personally, I'm a bit scared of wielding a big axe, and the one I'm feeling kind of comfortable with is no match to the aged bits of oak firewood that came with the house (about a decade old?). So the wood splitter is our friend

Maybe I'll start a thread with before & after photos of what we're doing here with the house. Hmmm...

I've changed the subject of this thread since we hit a major bump with our RMH.

One day I was busy in the kitchen when suddenly I heard a weird ticking noise. Looking up to where it came from, I saw the exhaust pipe that goes to the chimney glowing red at two spots. Calling hubby to come quick, I removed all the wood and coals from the burn tunnel as fast as I could (into a metal bucket that then went ouside) and the situation cooled down.

It has taken some time to gather the courage to open the bell top, take pipes apart and look what's going on. We just did that, and I'm dissapointed...mainly in myself, since something had gone totally wrong.

When building the RMH I had decided to make a horizontal wood feed. I was initially very happy with it, since it offers great flexibility in where the 'burn spot' is in the burn tunnel.

We noticed we were loosing a lot of heat (flying out the chimney?), or let's say; the mass wasn't heating up as much as we'd expected.
I thought it had to do with the thick-ish brick wall of the bell, which warmed up slowly, but also held heat.
And with the wide diameter exhaust pipe (18cm = 7 inches) and relatively short (?) exhaust pipe trajectory.

Here's a drawing with measurements:

all-around diameter: 18cm = 7 inches
wood feed opening: 18cm wide, 19cm high = 7" x 7.48"
burn tunnel, including wood feed: 91cm = 35.8"  , usually burning wood 33cm long = 13"
riser: 136cm = 53.5"
space between riser and top of bell (inside measurement): 5cm = 2"

the exhaust under the table is 2m + 2.5m + two times a bend of 90 degrees = 6 feet 6 47/64 inches + 8 feet 2 27/64 inches + two times a bend of 90 degrees
then it goes vertical for 1.5m with a 90 degrees bend = 4 feet 11 1/16 inches with a 90 degrees bend
then it goes horizontal for 1.5m with a 90 degrees bend = 4 feet 11 1/16 inches with a 90 degrees bend
and then it goes into the chimney which has a flexible pipe. The height of the chimney is 6meters = 19 feet 8 7/32 inches

The draft of the system always has been extremely good. Until that one day that it clogged up built up heat in the horizontal tube going into the chimney.

We opened the system today and found:
- a heap of ash around the edge of the heat riser
- ash and creosote in the tubes ( plummeting my mood )
- less ash in the manifold than I had expected

The creosote really bummed me out.
I'm guessing that:
- because of the horizontal wood feed, relatively much cool air entered the system above the fire?
- because of the relatively wide diameter of the system in relation to the relative shortness of the exhaust pipes the air/exhaust flew through the system at high speed, sucking ash with it and cooling the system to less than optimal.

Here are some photos from today's cleaning session. The 90 degree bends and the vertical+horizontal pipe leading to the chimney have been removed for cleaning.

Can any of you more experienced RMH builders give me some advice on how to repair the situation?
We've been thinking of:
- rebuilding the upper 2/3 of the bell with bricks to allow faster heat radiation (now built with flattish bricks from an old bread oven, 10cm = 4" wide + clay and sand finishing layer on the outside)
- making the tubing longer. Which is problematic since there is not really the space to make it longer without having to climb over the tubes&mass. Unless we sink the tubes in the floor, which is doable, the floor is 'original', meaning terracotta tiles on earth.
- changing the horizontal wood feed to vertical ( J-tube)...which I actually would rather not do unless I understand what a gamechanger that would be and why
- Can you help me understand how to fix the situation?

Thank you in advance.

Hi Nina;
Is your riser 7" dia (18cm)?
I know you do not want to hear this, but...
I'm thinking that your "L" tube is a big part of your problem.
I suspect you're not getting the riser temperatures you need to burn off the particulates.

One change I suggest is to increase the barrel-to-riser gap from 2" to 4"
An all-brick bell would release more heat in the room.

Here is why I believe you need a J-Tube rather than an L-Tube.
A properly built J-Tube has a maximum burn tunnel roof of 10-12",
If you build any longer, your fire is not burning hot enough, creating ash and creosote. (I know because I tried and had to do a Montana mid-winter emergency cleanout!)
On your 7" stove, the burn tunnel's maximum wall-to-wall length should be no more than 24"; 22"  would be better.
The vertical feed keeps the fire at full burn until the wood is all consumed.

We want a J-Tube to roar, not just burn. You always hear a J-Tube at full burn; that is how they got the name Dragons.
It is also essential that your riser is the same or larger in diameter than your feed system size.

To get the efficiency we want, you need to reach super-hot temperatures inside the riser; your L-Tube is not reaching those temperatures.

Another option is to eliminate the outdated pipe system and switch to a stratification bell.
Within the next month, I am completely removing my old piped system in our greenhouse/artist studio with a brick bell.

Hi Thomas,

Thank you for your thoughtful reply.

Yes, the riser is 7" (18 cm) in diameter.
We were burning the wood at different distances from the riser - with a horizontal feed one can do that.
The closer to the riser the burn point came, the more roar we would get.
BUT I'm pretty sure you're right about my system being a purring cat rather than a roaring dragon on average.

Increasing the gap between the riser and the top of the bell makes sense - sometimes I had the (intuitive) feeling the heat was hitting the bell top too hard and nearby.

I've been eying your Montana Masonry Dragon and it just might be we'll switch from the tube'n'mass to a batchbox bell heater as well.
Will talk with hubby about it.
The biggest hurdle/treshold I can foresee would be the door! I don't know a supplier hier in the EU (yet).
Hm hm hm.
The learning curve, building the RMH was huge. I don't mind learning again. We'll see.

Thanks again!!

Hi Nina;
You can have a J-Tube that feeds a stratification chamber; it does not have to be a batchbox.

Nina Surya wrote:and then it goes into the chimney which has a flexible pipe. The height of the chimney is 6meters = 19 feet 8 7/32 inches

Do you mean the standard rigid flexible pipe, with four angled sections turning against one another?

I would point out that with the L tube, although you can control the burn point, you need to be there constantly to move the wood every few minutes if you want to keep the wood burning at the optimal spot. I also would guess that lacking the back wall in the L tube, you are losing heat that the fire brick is storing there in a J tube to facilitate a higher initial temperature. Since it looks like you have a surface for cooking over the burn point, you now have two points of low heat around your fire rather than the normal single point at the top of the J tube.

Thanks guys, for brainstorming with me.
Coydon: the flexible pipe I'm talking about is a flexible tube with a kind of spiral, ribbed structure to it...I'll attach an image below.

After a night's sleep I'm still bummed about the faults in my build, but am a bit less drastically-minded (yesterday I wanted to tear the thing down and start again, building a batchbox).

I need to pace myself and think things through better before taking the next step.

One thing I'm still wondering about is: is this system too short in tubing and too wide in diameter, even if I'd modify the wood feed and burn tunnel, turning the L tube into a J tube ?
Initially I was worried it wouldn't draft properly, but the draft of this thing is mighty, and it was pulling fly ash even up into the flexible pipe in the chimney - which then landed, when the system cooled after use, gathering in the horizontal pipe that connects the RMH to the chimney.

I know, some things you just have to do and find out. But in the case of a more experienced someone having a clear idea about this draft thing, even if I'd modify L to J tube feed...please let me know.
Thanks again!

Nina Surya wrote:Thanks guys, for brainstorming with me.


After a night's sleep I'm still bummed about the faults in my build, but am a bit less drastically-minded (yesterday I wanted to tear the thing down and start again, building a batchbox).



One thing I'm still wondering about is: is this system too short in tubing and too wide in diameter, even if I'd modify the wood feed and burn tunnel, turning the L tube into a J tube ?
Initially I was worried it wouldn't draft properly, but the draft of this thing is mighty, and it was pulling fly ash even up into the flexible pipe in the chimney - which then landed, when the system cooled after use, gathering in the horizontal pipe that connects the RMH to the chimney.

I know, some things you just have to do and find out. But in the case of a more experienced someone having a clear idea about this draft thing, even if I'd modify L to J tube feed...please let me know.
Thanks again!

Nina, after reading your questions and a lot of the replies, I think there is a simple solution before you rebuild. ( if you decided to do that) and that is simply to fully design and put on paper exactly what you intend.  Go ahead and then post.  There are some great folks here with background of proven numbers that work.

A design first, and build to this DESIGN, (method)  seems to work much better than a "build and design as we go, practice"   While it can be done, it generally happens when a person has 4-6 stoves behind them.   Remember a house or even a garage is never built without a defined plan on paper first.  A good stove is no different.

It is pretty hard to design a stove on the forum in bits and pieces, nor does it have to be. Always present the whole picture and some of the sharp folks, can pick out potential problems if they see then fully in the design.

Your on the right track in asking questions.  We all have in the past.

Best of success!
Scott

I would agree with switching from L-tube to J-tube. A J-tube is still very easy and inexpensive to build. I keep my feed tube about 3/4 covered with a slab of cement board - others use bricks - and I get an inferno of combustion. My system has steady natural draft, and without the feed cover, it would be drawing so much air that it would cool the fire and throw away a lot of hot air while pulling a lot of cold air into the house.

The other big thing I would do is turn your "meatloaf" shaped piped mass into a rectangular hollow box (bell or stratification chamber). Sized correctly for your system, it can maximize your heat extraction without taking any more floor space. Based on my experience, an 8" J-tube needs about as much internal surface area as a 6" batch box (5.3 m2, 57 sq. ft.). The initial bell around the riser counts for this, so guessing around 2-2.5 m2/20-25 sf would leave for your 7" system probably 2.5-3 m2 or 25-30 sf. If your current mass footprint is 2m x 0.8m, and you make 10 cm/4" thick walls 0.8 m high overall, you could get 2 x 1.8 + 2 x 0.6 or 4.8 m x 0.4 m = 1.92 m2 walls plus 1.8 x 0.6 = 1.06 m2 ceiling = 3.0 m2 total. So an adequate ISA for full heat extraction is easy to get.

The user experience so far is that the table heater works really well, but the bell isn't radiating as fast or as much as we'd like.
I re-used bricks from a broken, traditional French bread oven after having guessed they'd be suitable for the bell just by inspecting them.
They are porous, not dense like the firebrick that I used for the core.
But they insulate too much anyway, being flattish and broad; about 2cm in height and 10cm wide, lengths vary.

For bricks in your bell, the shape doesn't matter as much as the density, how do the feel compare to the bricks used in the core? are they lighter?

ash and creosote in the tubes ( plummeting my mood )

How thick was it and what was the texture(greesy/oily)? Do you have pictures of the creosote build up in the bench pipes and in the chimney? a thin layer of black residue is common even it in well built RMHs (I am guessing from the start-up and ending portions of the burn).
How did the smoke look coming out of the chimney?

I saw the exhaust pipe that goes to the chimney glowing red at two spots.

How long were you burning when this happened?

I think there are two numbers to keep in mind when looking at how the heat of the RMH is used. The rate the heat is transferred into the mass and the rate the heat is transferred through the mass.

If the rate of heat transferred into the mass is too fast then the temperature of the air going up the chimney will be too low. That is why the ISA/pipe length numbers are important and why some systems have by-pass to help during start up.

If the rate of heat transferring through the mass is slower than the heat transferred into it (which I believe is the case with brick/cob), then the skin (first 0.5-1inch) of the mass will steadily increase in temperature until it gets close to the temperature of the hot flue air. At which point most of the heat is going up the chimney.

Nina Surya wrote:Coydon: the flexible pipe I'm talking about is a flexible tube with a kind of spiral, ribbed structure to it...I'll attach an image below.

Unless you are seeing creosote or ash build up particularly in that area, it might not be a factor with your current problem, but the conventional wisdom is that corrugated pipe like that adds significant drag to the overall system airflow. Using rigid preformed or 'turnable' angle sections should make your ducts flow better...

Since the OP mentions ferocious draft, I don't think that is a problem in this case.

Alex Ronan wrote:
For bricks in your bell, the shape doesn't matter as much as the density, how do the feel compare to the bricks used in the core? are they lighter?

Hi Alex, yes, the bricks in the bell are lighter, like ordinary building bricks.

How thick was it and what was the texture(greesy/oily)? Do you have pictures of the creosote build up in the bench pipes and in the chimney? a thin layer of black residue is common even it in well built RMHs (I am guessing from the start-up and ending portions of the burn).
How did the smoke look coming out of the chimney?

The creosote, dusted with some ash was dry, crisp and not thick. No photo's I'm afraid.
The smoke from the chimney was usually very difficult to detect, only when starting up the fire one could see white smoke, then nothing or vapor.

How long were you burning when this happened?

The whole morning, it was a chilly day. It happened in the afternoon.

If the rate of heat transferring through the mass is slower than the heat transferred into it (which I believe is the case with brick/cob), then the skin (first 0.5-1inch) of the mass will steadily increase in temperature until it gets close to the temperature of the hot flue air. At which point most of the heat is going up the chimney.

I believe this is the case with our system. A very good point! Thank you for your input!!

Glenn Herbert wrote:I would agree with switching from L-tube to J-tube. A J-tube is still very easy and inexpensive to build...

The other big thing I would do is turn your "meatloaf" shaped piped mass into a rectangular hollow box (bell or stratification chamber). Sized correctly for your system, it can maximize your heat extraction without taking any more floor space. Based on my experience, an 8" J-tube needs about as much internal surface area as a 6" batch box (5.3 m2, 57 sq. ft.). The initial bell around the riser counts for this, so guessing around 2-2.5 m2/20-25 sf would leave for your 7" system probably 2.5-3 m2 or 25-30 sf. If your current mass footprint is 2m x 0.8m, and you make 10 cm/4" thick walls 0.8 m high overall, you could get 2 x 1.8 + 2 x 0.6 or 4.8 m x 0.4 m = 1.92 m2 walls plus 1.8 x 0.6 = 1.06 m2 ceiling = 3.0 m2 total. So an adequate ISA for full heat extraction is easy to get.

Hi Glenn,
"meatloaf" haha! I was slapping cob on the tubes as if my life depended on it, we were well into December and were desperate to get our kitchen-livingroom happening.
Today I've dismantled the horizontal feedtube/burn tunnel and started converting it to a J-tube, making the whole much compacter as to the measurements mentioned by Thomas earlier.
Thank you for your input on the mass under the table. Interesting point, to convert it to a bell! So kind of you to give me the measurements to play with, very interesting!

It sure seems like there have been a lot of super ideas/adjustments/... in the building of RMHs over the years. So could someone tell me what the latest and bestest book is that details how to build the ideal RMH for a small, open floor plan house?

Sorry if this is the wrong thread to put this post in.

Hi Terry;
I offer the only available books on the newer RMH design.
One book completely covers Batchbox core construction.
The other is a how-to on constructing a spark-free door with a window.

You can find them at my website, dragontechrmh.com
You can order hard copies or PDF files by contacting me at dragontech@blackfoot.net

The creosote, dusted with some ash was dry, crisp and not thick. No photo's I'm afraid.
The smoke from the chimney was usually very difficult to detect, only when starting up the fire one could see white smoke, then nothing or vapor.

Then it sounds like you did a good job on version 1 of your RMH (more experienced can correct me if I am wrong). I look forward to hearing how your improvements work with version 2.

The whole morning, it was a chilly day. It happened in the afternoon.

If the rate of heat transferring through the mass is slower than the heat transferred into it (which I believe is the case with brick/cob), then the skin (first 0.5-1inch) of the mass will steadily increase in temperature until it gets close to the temperature of the hot flue air. At which point most of the heat is going up the chimney.

I believe this is the case with our system. A very good point! Thank you for your input!!

If you can put a thermometer on/in the exhaust after the bench, it would be helpful to watch the temperature of the air exiting the system. If the temperature starts getting high, it may be helpful to pause the burning and let the heat travel into the mass deeper.

Maybe try burning 1-2 hrs  then pausing for 1-2 hrs then burn again for 1-2 hrs (for example burn during breakfast then go do other tasks and then burn again during lunch) and see how the temperature change compares to when you burn for that whole time.

It sure seems like there have been a lot of super ideas/adjustments/... in the building of RMHs over the years. So could someone tell me what the latest and bestest book is that details how to build the ideal RMH for a small, open floor plan house?

On my next batchbox I would go with the new Shorty Core, unfortunately there isn't a book on it yet.

Here are some permies post about the Shorty Core:
Peter's development thread:
https://permies.com/t/234638/Development-compact-batchrocket-core
And Thomas's build in MT:
https://permies.com/t/254283/Shorty-Core-Montana-Version-burn
https://permies.com/t/254292/Airframe-Construction-Shorty-Core
https://permies.com/t/267527/Shorty-Core-bell

I also made a rocket oven with a horizontal channel, and faced a similar problem: the heat was trapped under the mass and left slowly, especially if the mass is large and poorly insulated. I tried to reduce the layer around the pipes, add sand and straw to speed up the heat transfer, but the result was average. I think it is important not to overdo it with the mass and immediately foresee how it will heat the room.

Ok, we finally had the guts and the weather (it's been too warm to fire it up, now a cooler day) to test fire the New Version of the RMH, mass under the kitchen table

There are two alterations at this point:
1. the horizontal feed tube is gone and replaced by a J-tube
and
2. there's more space between the top edge of the riser and the 'roof' of the bell.

The temperature outside was a bit higher than indoors, but after a little bit of initial smoke the draft pulled smoke into the system and...ROCKET!!! It was almost scary to have a roaring dragon in the house. It's working!


I'll give the convertion from 'meat loaf mass' to a bell under the table some more thought... Time flies, so many things to do.
Two options:
1. I leave as-is for now and run it another cold season
or
2. inspiration hits and the meatloaf gets replaced by a bell.

We'll see. Again: thank you all for participating!

Congratulations on a successful operation!
I suspected that those few modifications would make a world of difference!
If inspiration hits, then turn it into a bell before winter, but otherwise, it might be beneficial to see how well your improved dragon does with the piped mass this winter and next summer, improve it to have a bell.
The following summer, you remove the J-Tube and build your first Batchbox in its place!

Welcome to the wonderful world of Rocket Scientists!
Your Lab coat, pocket protector, and certificate are currently lost in shipping...
Perhaps they will arrive, maybe not.

As soon as it turns cold, start inviting the neighbors over so they can become apprentice rocket scientists under your guidance.
You will become the Dragon Lady of Middle-earth!

Gosh Thomas,
You do know how to put a glee on a fellow rocketeers face

I'll be posting any updates right here in this thread.

Thank you!

Well, hello there!

The autumnal weather has had us feeding the rocket dragon and it's roaring contently.
The much higher temperatures are also warming up the mass of the bell and the 'meatloaf' under the table, we're happy!
BUT - why's there almost always a but?

The much higher temperatures also make the plate on top of the bell crack.
It's a slab of metal wire reinforced concrete, repurposed from old (old!) rabbit cages. We're at slab number 3 at the moment...and running out of them.

We were searching for cast iron plates, but they're difficult to find at the required measurement ( minimum 60x60cm). Also, I'm not sure they'll last?
What would be a good, durable option to cover our bell with?

Here's a photo from today for illustration purposes - nothing new on the rocket, but the kitchen is undergoing a renovation!

Hi Nina, I personally had thought about using a cast iron 'cistern cover' (not sure how to translate to English). They are quite readily available here, also second-hand, but it may not be the case for you.

Nina,

Were you considering purchasing two plates?
For example this one:

Hubos cooking stove plate 600x310

They have a lot of other sizes, but not 600x600. Smaller elements handle expansion better than a big one.
I doubt that there is a better selection of cast iron masonry heater/stoves accessories anywhere on this planet.

Hi Julian,
That might be a good idea. I'll look for them here!

And Cristobal,
Somehow I'm convinced the bell should be covered with one single plate.
Have you tried two plates next to each other? If so, how did it work out for you?

I see you mention tomettes. Are you French(-speaking)? If so, maybe try looking for 'plaque d'égout en fonte ductile'. Based on the fact that the usual barrel over the J-tube can last for many years, I would think this would work well, assuming you put some stove rope / seal in the groove.