I recently bought rods and a brush and used them on my 6" Duratech chimney this morning, and in the class A lengths of the chimney the brush fit loosely to the point I had to angle it to get it to contact the sides. Then, when I reached the single wall stovepipe coming off the stove, it was as snug as I thought it should be. Should the brush fit loosely in Class A double wall pipe?