Huge thread recovery, however I am looking at either one of these cylinders or an alternative unvented tank.Its sounds unlikely but 6 months is normal, a few do 3 months and some never seem to need recharging.
Possibly the moving baffle is jammed and not covering the water/air interface.
Since an external EXV will cost about £100 installed then its less that a tenth of a new cylinder that seems th way to go! If you can fit it yourself then its only about £30 to buy!
Tony
Pros: Well insulated so low losses, well known and proven, now have a life/30yr warranty.
Cons: Larger size due to air gap, ability to service air gap.
Presumably it is sealed for death inside and non-serviceable? Is there some sort of seal, or the airgap baffle just free-floating to reduce surface area.
Daniel