We have a 6x4 slab that our current wood stove sits on, that then vents into the black 6 inch stovepipe, up a 30 foot chimney. We are reading the book (Just bought it online), and googling and youtubing like mad. We really want to replace our wood burning stove with a rocket mass stove very soon. I have two questions. With the existing chimney, can I tie the rocket mass stove into it for exhaust? We can't really run a long bench for heat exchange, and would build on the 4x6 pad to make a sort of chunky heat exchange vs. a long heat exchange. I will begin drawing and pricing tomorrow, and will submit it to the forum for feedback. Main concern now is, "is using the current chimney viable?"