Expansion tanks or pressure tanks are vital to closed water heating systems as they help to alleviate pressure build up on the home water lines. These tanks are installed at the water heater and normally contain a divider inside of them that divides the water from the air. This allows the expansion tank to be able to control the pressure in the pipes and help the system last longer. Expansion tanks generally have their air pressure set at 12 psi and are controllable, but if you need one that is larger than you will need to consider an ASME expansion tank. Make sure if the expansion tank is to be installed in a home water line that it is a potable, meaning it is good for drinking water. Expansion tanks can also be helpful on boilers, such as the Amtrol BoilerMate. Consider expansion tanks from brands such as Amtrol and Watts.