What is the main function of a water heater expansion tank?

The main function of a water heater expansion tank is to provide a cushion for thermal expansion of heated water. When water is heated, it expands, which increases pressure within the plumbing system. The expansion tank absorbs this excess pressure and allows for the expansion of the heated water, preventing excessive pressure buildup that could lead to leaks or damage to the water heater and plumbing. By including an expansion tank, you ensure a safe and efficient system, as it mitigates the risks associated with thermal expansion.

The other options do not accurately describe the primary purpose of an expansion tank. Reducing energy consumption refers to methods like insulation or efficient heating systems, and filtering impurities pertains to water treatment systems, not expansion tanks. Preventing leaks in the system is a broader plumbing concern and is not the specific function of an expansion tank.