The offered Expansion Tank is a small tank used to protect closed (not open to atmospheric pressure) water heating systems and domestic hot water systems from excessive pressure. The tank allows for the natural expansion of water as it heats up in your tank which is known as thermal expansion. The additional storage saves pipes, fittings and fixtures from any strain as the pressure increases when water gets hot. It is a small tank that is generally used to contain excess water pressure within domestic and closed water heating systems. The offered Expansion Tank is made for long term effective performance.