Vaillant Expansion Vessel & PRV Kit 964102Kit contains: Expansion vessel Pressure relief valve GasPart Vessel Kits make boiler pressure issue repairs as easy as possible. Good practice suggests replacing a PRV at the same time as a faulty vessel. A faulty vessel can cause damage to the PRV resulting in the boiler still showing a low pressure fault. Buying both together is simple and will help eradicate further problems.