Selgas
From your desription I think the “Outlet Device” you are referring to is in fact a pressure relief valve fitted to the underneath of the water heater, if this is in fact the source of the water leak then it will be caused by a cold water inlet water pressure set too high and the heater is allowing the surplus pressure to be discharged through this valve. A pressure reducing valve should be fittd to the inlet cold water line to stop the cold water pressure creeping when the heater is not in use. If this fails then the only repair possible is to replace the dripping valve which will eventually get worse and worse until a full flow of hot water will be discharged as it will cause the same effect as a hot water tap being opened.
Hope this helps
