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

The purpose of the expansion tank in a hot water heating system is to provide storage space for excess water that results from thermal expansion. As water is heated, it expands, leading to an increase in volume. Without a proper outlet for this expanded water, pressure within the system could rise to dangerous levels, potentially damaging pipes, fittings, and the heating system itself.

By including an expansion tank, the system can maintain a safe and balanced pressure. The tank accommodates the increased volume, allowing the system to function efficiently without excessive pressure build-up. This is critical for maintaining the integrity and safety of the entire hot water heating system.

Other functions mentioned in the options, such as filtering impurities, regulating temperature, or increasing pressure, are not the primary roles of an expansion tank. While a hot water system may need to manage water quality or temperature, those responsibilities lie with different components, making option B the best answer.