An expansion tank on a water heater is a small, sealed container designed to absorb excess pressure caused by thermal expansion when water is heated. As water heats up, it expands, and without an expansion tank, the increased pressure can damage pipes, fittings, or the water heater itself. The expansion tank provides a space for the expanded water to flow into, helping to maintain safe pressure levels in the system. It’s a critical component for protecting the plumbing system, especially in closed-loop water systems where water cannot freely expand.