Existing chimney and footing questions

I have 2 questions.
I have an existing chimney but it has a larger CSA than the exhaust pipes.How can i use the existing chimney to fit it with the rmh?

Also my floor is tile and under that there is concrete.Do i put a 4 inches/10 cm concrete slab on top of the tile and on top of that  i lay out the thermal mass?
Do i insulate under the 4 inch slab or above the slab?Do i have to put this slab under the combustion unit also or there all i put is just some inches of perlite?

Panagiotis, your big chimney is perfectly suitable. If real big, it could also be used as a bell, or a vertical slow heat exchanger.

For your floor, it all depends what you have below and what you want.

Basically, you can build directly on the tiles. Tho heat will travel downwards into the floor, and the ground underneath if it's built directly on it. Myself, i would lay a bed of air entrained concrete, or clay balls, to slow down the transfer of the heat downwards.

Satamax Antone wrote:Panagiotis, your big chimney is perfectly suitable. If real big, it could also be used as a bell, or a vertical slow heat exchanger.

For your floor, it all depends what you have below and what you want.

Basically, you can build directly on the tiles. Tho heat will travel downwards into the floor, and the ground underneath if it's built directly on it. Myself, i would lay a bed of air entrained concrete, or clay balls, to slow down the transfer of the heat downwards.

Thank you for  your response. i ve got some replies in a forum sayng that the big chmney could stall draft causing backdraft and smoke towards the room.
They said that i have to line it up to match the CSA of the system.Is this true?
So i could lay a couple inches of perlite and built the whole thing on top of the perlite?