Lightweight Rocket Mass Heater Mediterranean Climate

Day 3 started with yet another dry stack on the now insulated foundation


The foundation was then expanded and cobbed in place


At the same time the brick wall for the bench was continued


The first and second course of the core, held together with clay slip



I had the hands full, so the next photo is of the core built, with barrel and bypass in place

Also note the baffle to allow the hot exhaust to stay in the stratification chamber.

This is how we left it day 3.


While day 1 and 2 seemed like slow progress, day 3 was super fast and rewarding.

Benjamin Dinkel wrote:We built on the existing concrete floor

Hard to tell from the pics- did you put down a layer of something on the concrete before building up the bench? The "expanded clay" under the fire box, is that with perlite or something added?

Hi Coydon,
under the core there is a 10 cm (4") layer of expanded clay. The expanded clay is just out down, without a binder. It has pretty good insulation properties.

The stratification chamber is built right on the existing concrete floor. No extras.

Benjamin Dinkel wrote:under the core there is a 10 cm (4") layer of expanded clay. The expanded clay is just out down, without a binder. It has pretty good insulation properties.

For insulative cob, perlite is the basic ingredient and clay slip is essentially the binder.

Is your clay bentonite or are there other forms of expanding clay? From what i've found at wikipedia, sounds like hydrated bentonite changes form and cracks as it is heated. Not familiar with anything like that myself, interested to find out more on the subject...?

Hey Coydon,
I don't understand what you're asking or saying.
I used a similar product. It's like little balls of cereal, but made of clay.

Day 4 was probably my favorite day.

We raised all the brick walls to their finished height and solved a bunch of tricky situations with cob and brick cuts.





The exhaust path is closed off with a marble slab. On the bottom right you can see the bypass installed just after the manifold leads into the stratification chamber / bench.


Manifold installed and cobbed into place


Then the whole space around the core was filled with expanded clay and the top barrel put in place.



Baffle and bypass both in place and sealed with super wool


The first of the marble slabs covering the bench


Almost ready for the first burn

Benjamin Dinkel wrote:Hey Coydon,
I don't understand what you're asking or saying.
I used a similar product. It's like little balls of cereal, but made of clay.

My background in natural building is very hands on so I don't have a broad exposure to the terminology and don't recall hearing the term "expanded clay" before. What I have heard of is Bentonite clay that partially absorbs water where regular clay sheds it, thereby expanding. It would have many interesting uses for natural building, but the main constituent of a foundation would not be one of them.

Your product looks to have similar qualities to the perlite based thermal cob I've used, but sounds like it is much more robust structurally. I'd imagine much more expensive also? I'll have to look to see if it or a similar product is available to us in the USA...

I think we are talking about Molar clay, it is readily availible throughout Europe and the UK.
It is a form of expanded clay and used extensively in the growing industry as it hold water well but aids drainage.
It is used as cat litter due to the high absorption rate and also for growing bonsai trees.
It is also  key ingredient for lightweight, insulating building blocks.
Because the clay is baked at over 1000c it also has refractory uses.

Benjamin Dinkel wrote:It's on a first floor and the architect calculated a maximal weight of 300 kg/m2 or 62 pounds per square foot.

The calculated loads on concrete floors have always raise an eyebrow for me. I deal with it somewhat regularly with the equipment we install at different locations. Sometimes we have an engineer run calcs to tell us the psf an existing floor can take.

A monolithic house slab 4" thick typically has a 40 psf rating. Most driveways and garages are 4". An average weight car/light truck is 3000 to 5000 pounds. Because a light truck footprint is 80"x240" (134 sqft) and 134 sqft x 40 psf = 5360 pounds, you can park it on the same slab even though the tires are making contact on less than 4sqft.

In one sense, it is too bad an architect was consulted. That house slab must be fairly thick or have some good reinforcement to have 62 psf rating. You probably could have parked a large masonry heater in a smaller footprint and still not have loading issues (actual in the field, not in calculations and on paper).

There are risks associated with consulting a design professional and risks associated with not contacting a design professional.

Hey Jackson. The problem here was that we’re not in a first floor, but a second floor of an old house. It was reinforced not long ago. So the owner was obviously concerned about weight and had it checked.
Of course the people with the calculations stick to the safe side of life. But they also have the responsibility of something goes wrong.

Benjamin Dinkel wrote:Hey Jackson. The problem here was that we’re not in a first floor, but a second floor of an old house. It was reinforced not long ago. So the owner was obviously concerned about weight and had it checked.
Of course the people with the calculations stick to the safe side of life. But they also have the responsibility of something goes wrong.

Benjamin, that makes sense. I was thinking it was on the first floor. That would definitely be a situation where you may want to know for sure what the bearing capacity would be.

I also wanted to mention that I am not singling out/disparaging the design professionals themselves. The design criteria is very rigid, and the design professionals have to follow the design criteria.

On a positive side, reading the updates/posts, it sounded like you had to really adapt to the limitations which could be some priceless experience. It turned out great!

Benjamin Dinkel wrote:Hey Jackson. The problem here was that we’re not in a first floor, but a second floor of an old house. It was reinforced not long ago. So the owner was obviously concerned about weight and had it checked.
Of course the people with the calculations stick to the safe side of life. But they also have the responsibility of something goes wrong.

This seems a bit confusing- first post you said it was the first floor.

Also, the second floor of an old house has a concrete slab? Was that part of the reinforcing?

Hey Coydon,
yes, it's confusing. In Spain and Germany we count differently, the first floor is your second floor.

I don't know when they "upgraded" to concrete floors. Probably around the 70s. The new owner had to put steel beams under the "old" concrete floor.

On day 5 we closed the stratification bench and added mass on top in form of bricks bedded in very liquid clay mortar.


The top of the core was also closed with marble, we had more than enough and the material was free.


We then brushed a 1:1 mix of sand and NHL (natural hydraulic lime) on all of the surfaces to make a good connection and base for the plaster. It's like a thick paint, called "jabelga" in Spanish.


After everything was coated it was time for the test burn.


he rocket sprang to life and we couldn't identify any smoke outside. Success!

Day 6 was all about plastering. I used a lime based plaster. The first layer has no fiber and is reinforced with a mesh.


Here you can see the mesh on the left side (core).


We also added some tiles





On day 7 I applied the second thin finish plaster with horse manure and lime.


Then it was time to wait.
After 1 week the first burn worked alright, some smoke backs. I was really worried. But it turns out it was just weird weather.

The owner now operates the RMH with proficiency and after it was fully dried, she applied a coat of silicate paint.
Here is the end result:



and some details:

What does the 'T' in the exit flue provide for?

I decided to put it there for ease of cleaning and in case needed to be able to prime the exhaust

Awesome Ben!   Looks great!
The start of your lifetime career!
In twenty years, your name will be as well-known as Peter's!

Thank you Thomas!
If in 20 years I will have provided a sustainable heating solution to 20 homes I'll be content.
I'm grateful for all the work the scientists have put in and shared so freely.