doubling chimney diameter

[Charles2]

Suppose for architectural reasons on a 2-story house, a 6" chimney is doubled to 12" only in the last 6 feet visible above the roof. What effect, if any, would this have on the performance of the stove or chimney?

A related question that might actually be feasible: What if you ran a 6" diameter chimney inside a 12" diameter chimney for that last 6 feet?

 

[Grisu]

That's a big difference in surface area. Your chimney will essentially only have an effective height of the 6" portion meaning I would not count the last 6 ft in total stack height. You will get a lot of soot/creosote in that area since it is also at the very top and exposed to the elements.

A related question that might actually be feasible: What if you ran a 6" diameter chimney inside a 12" diameter chimney for that last 6 feet?

I guess some kind of rigid chimney liner should be ok. Not sure how to install that in your case, though.