Ken Nagrod said:
We call it 'B' vent. That may be where it originated - "balanced"
More confusion: "B vent" is NOT direct vent or balanced flue. B vent does not supply combustion air via the annular space between the aluminum inner liner and outer galvanized skin. The annular space is simply intended to provide insulation, thereby allowing installation closer to combusibles. (1")
(I have no idea what our B vent is called in the UK.)