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

The function of an expansion tank in a hot water system is to absorb excess pressure caused by thermal expansion of water. When water is heated, it expands, which increases the pressure in the system. If there is no mechanism to accommodate this increased pressure, it can lead to stress on the pipes and fixtures, potentially causing leaks or failures. An expansion tank provides a cushion for this increased pressure by allowing space for the expanding water to flow into, thereby maintaining safe and stable pressure levels within the plumbing system. This ensures the integrity of the plumbing infrastructure and helps prevent damage that could occur due to excessive pressure build-up.