What can lead to the necessity of draining an expansion tank?

Draining an expansion tank is necessary primarily when there is high system pressure. In a heating or cooling system, expansion tanks are designed to accommodate the changes in water volume that occur with temperature fluctuations. If the system pressure rises beyond the designed threshold, it indicates that the expansion tank is either not functioning correctly or that the system itself may be under duress. High pressure can lead to potential damage to the tank and other components, thus necessitating a drainage to relieve that pressure and restore balance within the system.

Factors such as low water levels or temperature fluctuations typically influence the water's expansion and contraction but do not directly cause the tank to become over-pressurized. Similarly, old pump components can lead to inefficiencies or failures but do not directly correlate to an urgent need to drain the expansion tank unless they are causing system pressure to escalate. Thus, high system pressure is the most relevant factor indicating that immediate attention to the expansion tank may be required.