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

The purpose of an expansion tank in a hot water system is to absorb pressure changes. As water is heated, it expands, which can increase the pressure in the system. An expansion tank provides a designated space for this expanded water to go, thereby preventing excessive pressure buildup that could damage pipes, fittings, or the water heater itself. By accommodating the increase in volume caused by thermal expansion, the expansion tank helps maintain stable and safe pressure levels within the hot water system, ensuring functionality and extending the lifespan of the system components.

The other options do not accurately describe the primary role of an expansion tank; it is not primarily used to control water pressure directly or to store excess water in the sense of providing extra supply. It also does not serve as an indicator for water levels, which pertains more to tanks specifically designed for monitoring purposes rather than managing pressure variations.