When water is heated, it expands in volume by 2 – 3%, depending upon the initial and final temperature. This can mean as much as three or four extra gallons of water in a closed system. If there is no demand for hot water, the expanded volume of water has no place to go. Since water is not compressible, the expanded water cannot be accommodated in the piping system, and therefore it creates hydraulic pressure, exerting a potentially hazardous stress on the system.A thermal expansion tank is designed to help protect potable (drinkable) hot water systems, helping to maintain them and keep them working efficiently. They are used to absorb excess water pressure, which can be caused by thermal expansion as water is heated. With a properly installed tank, the system pressure will be maintained, and will help prevent added stress to various components, saving you headaches – and money – down the road.