These expansion vessels for potable hot water are manufactured to comply with UK water regulations and are tested and certified by WRAS.
Supplied with a replaceable membrane which separates the water and air, to prevent contamination of the water system, corrosion of the pressure vessel or pressure loss in the water system.

Typically pressure vessels are used in pumped systems. When the pump starts, the pressure inside the vessel begins to increase. Once the vessel is full and the pump has reached its high pressure setting it will switch off. When water is then drawn off from the system the air pressure surrounding the membrane will force the water out of the pressure vessel and into the system. Once the membrane is fully deflated and the system has reached its minimum pressure, the pump will start on its low pressure setting and will begin the cycle again.

The purpose of this system is to prevent constant cycling of the pump and continuous running when the system is being used.

One of the main purposes of an expansion vessel is to compensate for the increase in volume of water due to the varying water temperatures in hot water 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%.
The expansion vessel allows for this extra space, as when water temperature increases the membrane inside the vessel expands 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 expanded 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 contracted.