Thanks for your answer
Ok so :
- Regarding the weight, there's about 1 square meter of reinforced concrete that can withstand 200kg/m2 and the rest is an old wooden floor that can withstand (I guess) around 50-100kg /m2 maximum.
- yes the old fireplace has a chimney and that's what we want to utilize as exhaust.
- I am confident

So I'd like to know more about the batch box RMH.
- I figure everything is in the RMH builders guide. Can you point me to where to get a copy ?
Thank you !