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Making the rocket mass heater a bit lighter for upstairs ?

I am really enthousiastic about building a rocket mass heater.
Waiting for arrival book of Erica and Ernie !

There is just one obstacle,
It can not get very heavy, because my house is in the attic.

Now I might have found a solution and mix the sand and clay with 'hydro-korrels' (-= in dutch)= baked clay-granules.
40 liter weigh around 16 kilograms (sand would weigh around 60 kg ! for the same amount of liters)

Would it be ok to make a mixture?

Other tips and tricks to make the RM-heater a bit less heavy are very welcome!

Thank you!!

The important part of rocket mass heaters is MASS. Without Mass, you have no heat battery. Rocket mass heaters are high-temp wood stoves with a huge mass battery wrapped around them and around the chimney to extract and store the maximum amount of heat from an efficient combustion. I think you are better off looking for a better solution for heating.
Maybe look into a "bell" instead of a bench.
Here's a diagram showing a system that has a vertically orient section and two sections that are bench shaped.
[Thumbnail for 8inchworkshop1b.jpg]
Lenny,   Assuming you don't have the permission or skill to beef up your floor, if you were to spread the weight out as much as possible, keeping to wall edges or places where floor beams are supported underneath by walls or posts, would be your safest bet..... You may not be able to extract as much heat with less thermal mass but if its your only option and your heart is set on it, than its a good place to start.
I would however look into what the load bearing capacity of the floor is based on how thick the beams are and spans.
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A bell rather than ducted bench mass is probably a good idea, with limited mass. Making the bell long and wide will spread the load over the most supporting beams and walls below; keep to edges and well-supported areas as Gerry says. You do need to raise the mass off the floor a few inches to keep heat away from wood and allow cooling air to flow underneath it.  

The internal surface area of a bell in relation to the combustion core size is critical; you can find information at batchrocket.eu, under "Sizing a batchrocket" and "Bell sizing".
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