Hi Panagiotis; I put a 4" layer of
straw filled
cob down first , then I laid my duct pipe on that and covered them with maybe 2 " of regular
cob , then I started laying my slate (similar to fieldstone) A layer of
cob then a layer of stone... then a layer of cob again , I called it cob
lasagna .I did carefully place my lowest stones to not put to much pressure on the ducting. Ultimately your ducting will rot out and you will be left with a smooth clay tunnel for your gases to travel. When I built mine I believe I used too thick a layer of cob in between the layers of stone ... less cob is better ... just
enough to cover the lower stone and enough to seat the new stone. I didn't understand that stone is a much better heat holder than cob... So less cob more fieldstone ! This is less work !