Expansion tank
![Expansion tank in central heating system.](/Images/godic/202501/12/Expansion_tank4721.jpg")
![Expansion tank for boilers.](/Images/godic/202501/12/Expansion_vessel_for_boilers4721.jpg")
![Expansion tank in a 1987 Saab 90(the brown plastic tank with white lid in top of picture).](/Images/godic/202501/12/SaabHengine4721.jpg")
An expansion tank or expansion vessel 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 is partially filled with air, whose compressibility cushions shock caused by water hammer and absorbs excess water pressure caused by thermal expansion.