What is the primary role of an expansion tank in a hot water heating system?

The primary role of an expansion tank in a hot water heating system is to allow for the expansion of water. When water is heated, it expands, and if there is no space for this expansion, it can lead to increased pressure in the system, which can cause damage or lead to failures in the pipes or heating equipment.

An expansion tank absorbs the excess pressure created by the expanding water, providing a buffer that accommodates the expansion without adversely affecting the system's integrity. This helps maintain a stable pressure level and prevents the system from experiencing potentially dangerous conditions.

While regulating steam pressure, increasing water temperature, and fire suppression are related to heating systems, they do not describe the primary function of an expansion tank, which is specifically designed to manage the physical properties of water under heat. Thus, the correct answer emphasizes the crucial role of accommodating thermal expansion in maintaining the system's safety and efficiency.