Well, my first inclination was jointing compound but I'm not convinced that quantity would be in the cylinder. My second thought is foam insulation used during the tank manufacture which has somehow got inside the cylinder.
Maybe the most likely explanation is the compound used to help release the tank from the mould when it was pressed which wasn't cleaned off.
I would follow Proptec's advice and contact the cylinder manufacturer and ask their opinion.
Hi Barkinglama, I have something very similar if not identical to this happening with my hot water supply. Did you ever get to the bottom of what was causing it and how it was fixed? Thanks for any help, Alan
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