I'm not sure of the physics or chemistry here, but is it possible that a longer chimney might mean less often cleaning is needed? I have a three-story metal insert going through my brick chimney. Is there any sense that the exhaust being distributed over a longer/greater surface means the deposits are similarly distributed less densely over any given area? By analogy--and I'm not sure it's a good one, if you have short hair, you need to shampoo more often than if you have long hair. Your scalp is always secreting the same amount of oil, which means short hair will have more oil per unit of strand length. There is no way I can clean the length of chimney I have without a professional--or professional equipment. But I also wonder if with a long chimney more of the bad stuff will concentrate closer to the stove, and dissipate in its concentration as it travels up the pipe--meaning I can be ok with cleaning just as far upwards as I could reach with a brush--and saving the full cleaning for a professional on a less frequent basis.