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...