Final answer:
A water heater with a separate storage tank and valves must have a temperature and pressure-relief valve for safety to prevent hazards due to over-pressurization.
Step-by-step explanation:
Any water heater connected to a separate storage tank and having valves between the heater and tank shall be provided with a temperature and pressure-relief valve. This is an essential safety device designed to prevent the excessive buildup of temperature and pressure which could lead to a rupture or explosion. On-demand electric hot water heaters, as described in the reference figures, heat water as it is needed, reducing the need for a large storage tank but still require safety mechanisms when storage tanks are used.
Furthermore, the cited figures and information about different water systems such as those in a power plant highlight the importance of maintaining system integrity. Specifically, the containment of radioactive water within the reactor to prevent it from mixing with nonradioactive water in other systems such as the steam generator and condenser is crucial.