I actually have this stove connected to a 12" Chimney and yes the draft is weak. Still runs like a champ.
Mine spent the first year connected to a 7 1/4 x 7 1/4 masonry chimney. When I shoved the 6 inch liner down, insulated it with pour down, and hooked things up, I couldn't believe (and still can't) the difference!
I thought mine operated fine on that masonry chimney too
pen