Rocket mass heater on a wood floor construction

Dear Permies!
My partner and I just bought an old stone house. So exciting!
We are starting to plan renovation, and we would like to build a rocket mass heater.
We checked many designs, constructions, materials and informations.
During this search I came across an comment to never build a mass heater on a wooden construted floor because of the weight.
That it should be done only on concrete floors? Are there posibilities for an efficient, light weight heater? What do you think?
I attatch you a picture of our basement, so the heater would be on the floor above.
I would be thankfull for any directions of research, informations or shared experience!
Greets from France
Anja

As with many questions, the answer is "it depends." A traditional RMH with bench-shaped mass, if spread over a large number of floor joists, can be built on a sturdy wood-framed floor. From the picture, I would think you could have a bench mass running along one of the long walls (left or right in the picture) and be fine. If you need to put it in the middle of the house, you would need to add some structure in the basement to support it, either posts and beams to brace the joists, or a cutout in the floor and a masonry pier to support a mass independently.

Hi Anja;
Big Welcome to Permies!
You will be able to have a Rocket Mass heater in your house.
Yes, a solid concrete floor is ideal but most are built on a wood floor.
You simply need to spread the weight over many floor joists and maybe add some extra support  in the basement.
Depending on the style and size RMH (piped bench or brick bell) your RMH will weigh in between 2000-5000 lbs.
That sounds like a lot, but a waterbed weighs about 2000lbs. People have been putting them in bedrooms without ever thinking about supporting the floor.

Your issue after floor support is protecting that wood floor from the high heat.
A bell design RMH has very few issues with floor heating as the highest heat is at the top.
A floor mounted core and a piped bench must be elevated apx. 2" above the floor.  Generally  bricks laid flat with cement board on top is suitable  to elevate the core and mass.
Do not attempt to attach you rmh to the stone walls of your house. It must be isolated from them or your heat will transfer to the great outdoors.

Thank you for the welcome!
I am really glad to find this forum and so many interesting topics and active users!
So good to hear, that having a wood floor is not a total "no go" for a rmh project in our house.
We were thinking abour placing the rmh close to the "middle" wall of the house. (it's the one on the left on the picture) It is a big 50cm thick stone wall, there is the chimney running. Would you then also recomment to keep a distance between the wall and the heater if it is a "inside" wall of the house? My first impulse was to put the heater "attached" to the wall to help to support the weight. I came across such recommendations, but it seems resonable to keep a distance for not "loose" the heat in the wall?

thomas rubino wrote:Hi Anja;
Big Welcome to Permies!
You will be able to have a Rocket Mass heater in your house.
Yes, a solid concrete floor is ideal but most are built on a wood floor.
You simply need to spread the weight over many floor joists and maybe add some extra support  in the basement.
Depending on the style and size RMH (piped bench or brick bell) your RMH will weigh in between 2000-5000 lbs.
That sounds like a lot, but a waterbed weighs about 2000lbs. People have been putting them in bedrooms without ever thinking about supporting the floor.

Your issue after floor support is protecting that wood floor from the high heat.
A bell design RMH has very few issues with floor heating as the highest heat is at the top.
A floor mounted core and a piped bench must be elevated apx. 2" above the floor.  Generally  bricks laid flat with cement board on top is suitable  to elevate the core and mass.
Do not attempt to attach you rmh to the stone walls of your house. It must be isolated from them or your heat will transfer to the great outdoors.

It may not be a problem loosing heat to the wall.
Its a mass stone structure that will hold heat and radiate it back.
If it is an outside wall you will loose some to the outside, how much I dont know.
But others may.

Hi Anja;
I would not attach the rmh to the inside wall, but placing your rnh next to it and letting radiant heat warm it will work well.
What style of RMH were you thinking of?  A traditional J tube with a piped mass ?  Or perhaps a Batchbox style with bell?
Both are great designs.  
We are here to help you as you build. Keep us posted with lots of pictures and ask all the questions you can think of!

On an inside wall, you can have the RMH mass touching the existing wall, with only an inner liner skin on that side. The wall will act as part of the mass. Because the inner skin of a bell or the area right next to a tube will get very hot and expand a bit, there should be a tiny separation so the liner can move without causing cracks or leaks. A sheet of corrugated cardboard is sometimes used for this, and allowed to slowly char away in use and create an isolation plane.

Use that perfectly placed internal mass!

The rmh will be placed in the kitchen/ livingroom, and it would be great to have an opportunity to use it as a cooking surface also. We were considering more a batchbox design, but nothing is set yet, we are open for the solution that will be the best. Thank you for all your support, as soon as we will get the keys and start drawing more precise plans I will with pleasure share it and take any ideas and advice.

The choice between batch box and J-tube is independent of mass style choice (upright bell, horizontal bench bell, piped bench mass). You can combine whichever of these fits your situation best.

An upright bell with its concentrated load
will require some independent support in the basement, while others will likely be able to sit entirely on existing floor joists.