What safety feature must be included in hot water tanks?

Pressure relief valves are essential safety features in hot water tanks because they prevent hazardous situations that can arise from excessive pressure inside the tank. When water is heated, it expands, and in a closed system, this can lead to a significant increase in pressure. If pressure builds up beyond the tank's safe limits, it could potentially cause the tank to rupture or explode, posing serious risks to safety and property.

The pressure relief valve automatically opens to release excess pressure when it reaches a predetermined level, allowing steam or water to escape safely. This mechanism is crucial for ensuring that the hot water tank operates within safe pressure limits, thereby protecting both the equipment and individuals nearby.

In contrast, other choices may enhance functionality or efficiency but do not address the critical safety aspect that pressure relief valves provide. The digital thermostat can help regulate temperature, the ventilation system is necessary for certain types of heating systems but not specifically for hot water tanks, and the emergency shut-off switch may assist in quick action during urgent situations, yet it does not prevent pressure-related failures directly. Thus, the pressure relief valve stands as a fundamental requirement in ensuring the safe operation of hot water tanks.