Vaillant Expansion Vessels
These parts are in the Vaillant boiler to ensure that the pressure within the boiler operates within safe parameters normally between 1 to 2.5 bar. The expansion vessel should be half full of air and half full of water both separated by a rubber diaphragm. The common problem customer's encounter when the Vaillant expansion vessel is faulty is the boiler pressure builds to 3 bar and hot water starts to dribble through the pressure relief valve and outside via the discharge pipe.
When the boiler then cools down the pressure in the boiler falls to 0 bar and won?t start. The customer then has to fill their Vaillant boiler back up by using the filling loop normally located below the boiler. Once the pressure in the boiler is back at 1 -1.5bar the boiler will start to work again.