What is the purpose of an expansion tank in a plumbing system?

The purpose of an expansion tank in a plumbing system is to absorb excess pressure caused by thermal expansion in a closed water heating system. As water is heated, it expands, and if there is no space to accommodate this increase, it can lead to excessive pressure buildup. This pressure can damage pipes, fixtures, and appliances. An expansion tank provides a cushion of air that allows for the expanded water to flow into the tank, thus maintaining a safe pressure level within the system. This helps protect the components of the plumbing system from potential failure or damage due to high pressure, ensuring longevity and maintaining the integrity of the system.