The expansion tank is the primary device used for system pressure control. The excess water volume in the system, resulting from increased temperature, is stored in the expansion tank during periods of high operating temperatures and is returned to the system when the system water temperature is lower. The Expansion tank must be able to store the required volume of water during maximum design operating temperatures without exceeding the maximum allowable operating pressure and to maintain the required minimum pressure when the system is cold. The automatic fill valve is required to maintain the minimum system pressure by supplying water to make up for the leakage.
Expansion tank should be pressurized by compressing air into the tank. If air is in contact directly with water, the expansion tank is considered a Compression Type tank. In Bladder Type and Diaphragm Type there is no direct contact between the air and water.
There are three types of closed expansion tanks: