Which of the following systems typically requires a backflow prevention device?

The requirement for backflow prevention devices is crucial in systems that have the potential for cross-connections between potable water and non-potable sources, which could lead to contamination.

Heating and air conditioning systems, for example, often have water circulating in closed loops, but they can also connect to domestic water supplies for various reasons, such as filling and maintenance. If a failure occurs, backflow could introduce contaminants into the drinking water supply.

Wastewater treatment plants are designed to handle sewage and wastewater; however, backflow prevention is essential to ensure that treated or untreated wastewater does not flow back into the public water supply, keeping the drinking water safe.

Domestic hot water systems also require backflow prevention. These systems can create conditions that allow backflow if there’s a change in pressure, such as when a water heater is drained or during maintenance. A backflow incident could allow stagnant water to mix with the potable water supply, posing a health risk.

Considering all these factors, it becomes clear that each of these systems can be vulnerable to backflow issues and, therefore, requires proper backflow prevention devices to safeguard against potential contamination of drinking water. Thus, it's understandable why the correct response encompasses all the options provided.