Lightweight Rocket Mass Heater Mediterranean Climate

Dear Rocket Scientists,
in November I hopefully get to build the first traditional inside J-tube Rocket Mass Heater.
I will be using the barrel and the design from my other project that got cancelled.

So the system will be a 6" J-tube made from dense 1,5" thick firebrick, with a "5 minute"-riser .
The barrel is already made, manifold will be cut on site.

The design will be influenced by this RMH by Adi Segal


Which leaves the design of the bench, where we also encounter one of the limitations of the project.
It's on a first floor and the architect calculated a maximal weight of 300 kg/m2 or 62 pounds per square foot.

So a cob mass is definitely out of question, and I have warmed up to the idea of a hollow bench as a bell. Also for ease and speed of construction.
So I was planning on raising a 2 brick walls of about 3 inches thickness to a height of 14". Massive brick and a lime plaster. About 30" (minus the width of the bricks) apart from each other.
The bench would then need to be covered by some kind of slab that could span almost 30". I calculated with a thickness of maximum 3,5" including mortar and plaster to stay within the weight limit.

I can probably find some pre made concrete elements that fit my needs. Are concrete slabs an option?
Are there other good options that I'm not seeing?
Can metal profiles and heavy tiles be used too?

The bench will roughly be 3m long. With a width of 80cm that makes 2,4 m2 of footprint for the bench. Which makes roughly 720 kg or 1600 lbs and a bit more than half of that weight is the cover of the bench, which would be the warmest.

I'll make a SketchUp soon and share some pictures.

As always I'm grateful for answers and open for suggestions.

Benjamin Dinkel wrote:The bench would then need to be covered by some kind of slab that could span almost 30". I calculated with a thickness of maximum 3,5" including mortar and plaster to stay within the weight limit.

Check out facebook marketplace and OLX (do you have that in Spain?) to see if anyone is getting rid of old marble or granite worktops. My son works on house renovations and was given a set of kitchen cabinets complete with enough granite to make our bench top, which he kindly donated in our direction as he already had a load of reclaimed gneiss for his kitchen. Prices are all over the place but if you keep looking you might find someone who just want to clear some out.

Thanks Burra, good idea.
We have Wallapop and Milanuncios. Most people use Wallapop, but I can't get used to their gimmicky app.

I'll tell my client to keep her eyes peeled for any offers like that.

Benjamin Dinkel wrote:

I can probably find some pre made concrete elements that fit my needs. Are concrete slabs an option?
Are there other good options that I'm not seeing?
Can metal profiles and heavy tiles be used too?

Any other ideas for how to bridge the gap?

I imagine the top of the bench will be the main thermal mass, as it gets hottest. Am I right about that?

You could try to use the ceiling bricks. You could fill their holes with cob:

Long bricks in Leroy Merlin

Thanks Cristobal, I don't know how well that will work. Sounds like a lot of work also.


To all of you who heat with brick, how thick do you think the outer wall of the bell (in form of a bench) should be?
How thick the ceiling of the bell?
To keep the weight down I was going for a 16" finished height, probably topped by some kind of cushion at least some of the time.
That would make the walls of the bell 12" and the slab on top 4".

It's a Mediterranean climate, so some storage is good, but I'm not going for 3 days of inertia.


If you build a bench, does anyone have true and tested height and depth?
I'll consult the Wisners book, but would be nice to hear from y'all.

Instead of using  longer slab you can also build the bench bell of desired width and in the middle build column from bricks standing on their heads. Then on the columns you lay bricks on stretchers (the corners that are close to the column can be cut so it tapers down towards the column). This way you can use 2 bricks to span the wider bench. So the final width would be 460 mm if using 230 mm bricks.  If you make it 40 cm high it would be 6 layers of bricks.

Thanks Cristobal,
I was thinking about that. But there are weight restrictions in place. Calculating the weight I am reaching the limit with 16" total height and a depth of around 30". So if I go for 460 mm (18") the same weight will be distributed on less surface.

The structure that I suggested would use 78 bricks per 1 meter of bench, so assuming 3.5 kg per brick it would be 273 kg, not counting mortar and assuming no bottom layer of bricks. So close to your maximum. In case of wider bench it would be better distributed. However if using the bricks for the ceiling of the bench you would probably want to use to use two layers to make it more gas tight so it would add another 60 kg/linear m to the total weight.
In case of using the long ceiling bricks from the link I provided you could fill them with some insulating material (vermiculite or perlite with clay or clay with straw) to make the second layer lighter.

I will definitely go wider than 460 mm.



So the slab on top, for the moment I'm thinking marble or a thick tile, will need to be supported on some spots. Your column idea sounds good.
I made a calculation and lowering the thickness of mortar/cob I got the whole thing down to 250 kg/m2.

Adding insulation to the top slab doesn't sound good to me. This is where the main heat storage will come from.

If using bricks with holes it would be partial insulation that could be controlled by the mix of the filler. I don't know your design and I assumed that the bench only bell may have more hot gases than bell + bench.
I'm attaching the image of the columns, so if you go this route you will feel better that it's already being used in other builds.

Thank you, I'll upload a sketch soon. Then it might get more clear.

For now I still need some insight on minimum thickness of the front wall and top. So as to not burn yourself. I think I remember reading 4" (10cm) as a minimum. Weight wise I would consider that my maximum

Could you try adding some perlite in the gap for your front wall? If used sparingly, it could reduce but not stop heat transfer, and not add weight in that 4 inch gap.  As always your craft is practical and handsome, and I invariably learn something from your efforts.  

It doesn't appear that there is any gap in/around the front wall of this bell. To reduce weight, I would suggest the lower course(s) of brick be set on edge, so 2 1/2" thick or so, with standard bricklaying above supporting the top slab. This would give 4" of brick where the wall is hotter and less mass where it is cooler. The layout shown will be plenty stable with lower bricks on edge. You can either center the upper bricks on the lower ones giving a slight overhang on the face, or align the exterior faces for a flush surface. Do you plan on an exposed brick surface, or a finish coat covering them?

Doh
Yes Glenn.  Thanks, I misread that as a 4 " gap but it's a 4" thick brick front

Thanks Glenn.
I’ll do another little design drawing tomorrow.

I was planning on brick with a lime plaster on top. But I’m not sure yet how to reduce cracking in the plaster. Probably some plaster mesh.

Here's the latest iteration. The weight is still a bit too much, but I'll talk to the architect.

So I visited the site last week.
The architect was also present and we have the green light for the construction!!! We'll be building in mid November.

Most of the design is clear by now. But I have two questions that I have mulled over quite a bit.

1) The bell (bench) will only be 40cm high (30 cm internal height), 80 cm wide (50 internal) and roughly 3 m long. Will I need a dividing wall (in red in the sketch) so that the gases don't shortcut into the exhaust pipe? (The exhaust pipe will not go through the wall anymore but be located right behind the barrel, not very well visible here). If I take the bell thinking (water flowing up) serious I wouldn't have to. But I would like someone to confirm.



And 2) the bypass. I was trying to come up with solutions and am thinking of a valve in a T that exits almost on the top of the barrel. Are there any easier options?

Hi Ben;
To alleviate fears of the hot exhaust finding your chimney too soon, you can run an 8" or larger pipe at least halfway down your bench.
This will confirm that the hot air reaches the end of the bench.

For a bypass, A tee will work.
I have started using a traditional cast-iron stovepipe damper as a bypass.
Using small squares of superwool on the rod to seal the hole in the pipe, I then use stainless tie wire to attach a piece of superwool over the whole damper and a second piece where the hole goes through the other side of the pipe.
Mark the handle on the outside so the superwool side faces the highest heat.

Thank you Thomas.
Both seem good solutions, I'll follow your advice.

To clarify:
Bench: I connect the 8" pipe to the manifold and lead it (with a bit of an inclination?) into the middle of the bench?

Bypass: I attach the T towards the top of the barrel and use a damper with superwool?

Correct, leave the manifold with a larger pipe. Inclination should not matter; heat will rise.
Each bypass install is different. But they are generally mounted horizontally from the hottest point.