An expansion tank in a closed hydronic system serves four primary functions: (1) to accept changes in system water volume as water density changes with temperature, (2) to maintain a positive gauge pressure in all parts of the system to prevent air from leaking into the system, (3) to maintain sufficient pressures in all parts of the system to prevent boiling, including cavitation at control valves and similar constrictions, and (4) to maintain net positive suction head required (NPSHR) at the suction of pumps. These functions are crucial to ensure the safe and efficient operation of the system.