What holds the Relief Valve closed?

The relief valve is designed to prevent backflow in a plumbing system, and it operates based on the balance of various forces acting on it. The correct understanding of what holds the relief valve closed centers around supply pressure.

In a backflow prevention assembly, when the supply pressure in the system is greater than the pressure exerted on the downstream side of the valve, this pressure differential keeps the valve in the closed position. Essentially, as long as the supply pressure is maintained, it pushes against the valve's disc, preventing backflow into the water supply system. The moment the supply pressure drops below a certain threshold, or if the pressure on the downstream side increases (such as during backflow conditions), the relief valve will open to allow for the correct flow and prevent contamination.

While gravity and external environmental factors can influence various components in plumbing systems, they do not play a central role in maintaining the position of the relief valve in the context of backflow prevention. The spring inside the relief valve (another component) typically provides a specific force that counteracts the supply pressure but is not the primary mechanism that holds it closed during normal operations.