The Aquasystems ARB Potable Expansion Vessels are manufactured to comply with UK water regulations and are tested and certified by WRAS. The vessels are supplied with a replaceable membrane and feature an integrated wall bracket. They come complete with wall bracket accessories as pictured. The main purpose of the vessel is to compensate for the increase in volume of water due to the varying water temperatures in the system, e.g. Water being heated from 0°C – 100°C will increase by approx 4.5%.