An expansion vessel, also known as a pressure vessel, is a device that is used to maintain a constant pressure in a closed water heating system. These vessels are typically made of steel or plastic and are filled with air or an inert gas such as nitrogen. They are connected to the water heating system and are used to accommodate the expansion and contraction of the water as it is heated and cooled.

Expansion vessels help to prevent damage to the water heating system by absorbing the excess pressure caused by the expansion of the water. They also help to maintain the efficiency of the system by keeping the pressure within the recommended range. Expansion vessels are typically used in closed water heating systems, such as boilers and solar water heaters. They are also suitable for use in a variety of industrial applications, such as process cooling systems and air conditioning systems. They are relatively easy to install and maintain, and typically require very little maintenance.