Complex sentence: An expansion tank or expansion vessel is a small tank used in closed water heating systems and domestic hot water systems to absorb excess water pressure, which can be caused by thermal expansion as water is heated, or by water hammer.
Split into simpler sentences: An expansion tank is a small tank used in closed water heating systems and domestic hot water systems to absorb excess water pressure. Excess pressure can be caused by thermal expansion as water is heated, or by water hammer.