Hi Helene;
What size is your chimney ? 6" or 8" Block or metal pipe ? Is it all indoors or outside the wall ?
-25 C is -13 F .
A standard rmh as described in the builders guide, uses a barrel as a bell, a
transition area where the exhaust starts moving horizontally thru the pipes surrounded by
cob and buried under large rocks. This bench is commonly used as a couch or day bed. Some folks use
cob on the whole bench , some like me surrounded the
cob mass with brick or other hard material to avoid repairing chipped cob off the bench, and as additional mass.
Your
concrete slab is too small for this style of mass.
There are options here though. To start, we know you have no basement. What is your floor ?
Wood ? Is it or can it be supported to hold the weight of a solid mass ? Simply adding a few blocks under the floor joists is sufficient. Your RMH does not "HAVE" to be on your slab...
People routinely use bricks laid horizontally on the floor with air gaps between , then place concrete board on top of the brick. You build your entire rmh suspended 2" off the floor. You protect the floor from excessive heat, you gain another side of the mass to radiate warmth into your home. And in your case you might find that you have room to make a traditional bench! That should give you some options to concider.
Now lets talk about brick bells or another name is a stratifacation chamber. I looked and this is not in the builders guide that I could find. Plenty of info here at permies about bells.
Here is a link to a post descibing a ladys build of a small brick bell using a batch box design rather than a J tube design. She had limmited space (much smaller than your slab) and she built a superb little rmh to heat her home... You will also no matter what style or type of RMH you proceed with. (permies.com/t/43809/Masonry-stove-diy-build-feasible) its a long post but worth reading.
A brick bell RMH has a core unit the same as a bench style , risers are the same. The difference is that there is no pipes buried in a mass. This allows your RMH to go vertical rather than horizontal. Saving space on the
footprint .The hot air from the riser enters your brick bell and rises to the top. This causes the cooler air to sink towards the floor. Your chimney attatchment is located at floor level. Only the coolest air leaves . The entire brick box heats up and radiates warmth thru your home. This is similar to a Russian masonry stove but not built by a certifed Mason. Unfortunetly the insurance man likes Masonry stoves because they are universally accepted as a safe form of wood burning in a home. A RMH is also a safe form of wood burning... but your insurance man will not understand this...
This is enough food for thought for now. Come back and let us know what you are thinking. I've included 3 photos showing you my traditional rmh with bench and my brick bell rmh (that uses a barrel for instant heat) Lots of options including the choice of bulding materials for your core.