Which of the following devices can help prevent thermal shock in hot water systems?

High limit controls play a crucial role in preventing thermal shock in hot water systems. Thermal shock occurs when there is a rapid change in temperature, which can lead to stress and potential damage to the system components, such as pipes and heat exchangers.

High limit controls monitor the temperature of the water and are designed to shut down the heating element or burner if the water temperature exceeds a predetermined level. By doing this, they help maintain a stable and safe operating temperature, preventing sudden spikes that could lead to thermal shock. This safety device ensures that the system operates within its designed temperature range, thereby protecting it from extreme thermal stresses.

In contrast, while a circuit breaker is essential for electrical safety by preventing overloads, it does not specifically address thermal shock. Similarly, a pressure regulator is important for managing pressure in the system, and a siphon pipe can help to prevent contamination but does not have a direct impact on thermal regulation. Hence, the high limit control is the most effective device for mitigating the risks associated with thermal shock in hot water systems.