2. Chimney diameter (a)

Every chimney has an optimum diameter for the appliance it serves. If the diameter is too large, the flue gas temperature will drop owing to heat lost through the walls (A.) The symptom is a sluggish draught, weak resistance to 'blowback' in turbulent wind conditions – and a tendency for the flue to foul up and need almost constant maintenance.
XXConfusingly, a chimney that's below optimum diameter (B.) can exhibit a similar tendency – but it will also act as if it's being permanently constricted. The tell-tale sign is a regular tendency for smoke to dribble – or worse still belch into the room, coupled with an almost permanent smell of soot.
XXIn C. the chimney diameter is correct; heat is reasonably evenly distributed (except at the extreme top where the weather is encountered,) and draught is stable enough to extract all smoke and fumes to the atmosphere.