Boiler Expansion Vessel, expansion tanks and heating pressure vessels from Zilmet, Lowara, Flamco, Grundfos & FAM available from stock. These vessels are fitted with a Butyl/EPDM replaceable membrane which is suitable for drinking water (WRAS Approved). The benefit of a Expansion Vessel also known as an Pressure Vessel or Expansion Tank, is that it has a larger capacity that a storage tank, therefore the same performance can be achieved with a smaller volume which reduces the number of pump starts required - saving energy.