The expansion vessel allows for this extra space, as when water temperature increases the membrane inside the vessel inflates to allow the water to fill the vessel. The membrane will continue to expand until the system reaches its maximum temperature. Once this has been reached the membrane will be fully inflated and takes up the capacity of the vessel shell. Gradually the temperature will drop, which will in turn decrease the volume of water. Due to the pressure from the pressurised air surrounding the membrane, water will start to exit the vessel until the membrane is deflated. The main purpose of an expansion vessel is to compensate for the increase in volume of water due to the varying water temperatures in hot water or heating systems. When water is heated it expands and as water is not compressible this increased volume will create a rise in pressure within the system. As an example, water being heated from 0°C-100°C will increase by approx 4.5%. All versions supplied with a replaceable membrane meaning the vessel can be serviced without being removed.