A backflow preventer used in a hydronic heating system is designed to:

The purpose of a backflow preventer in a hydronic heating system is primarily focused on protecting the drinking water supply from contamination. This is achieved by ensuring that any non-combustible or potentially contaminated water does not flow back into the potable water system. A backflow preventer serves as a one-way valve that allows water to flow in one direction only, thereby maintaining the integrity of the drinking water supply.

In the context of hydronic heating systems, which often involve the use of water for both heating and cooling purposes, the risk of contaminated water mixing with the potable supply is a significant concern. Therefore, installing a backflow preventer is critical to safeguard public health by preventing any harmful substances from entering the drinking water.

The other options provided relate to different functions that a backflow preventer does not fulfill. For example, while holding system pressure during a temporary shutdown is important for system operation, it is not the primary role of a backflow preventer. Similarly, preventing heating water from circulating in the opposite direction or preventing hot water from entering the cold water system are not actions that the backflow preventer is designed to perform. These functions are typically managed through other system components like check valves and proper system design.