What type of valve allows water to flow in only one direction?

A check valve is specifically designed to allow water to flow in only one direction. This function is critical in preventing backflow, which could potentially contaminate the water supply or cause damage to the system. Check valves operate based on fluid pressure; when the pressure from the inlet side is greater than the pressure on the outlet side, the valve opens and allows flow. Conversely, if the flow reverses, the valve closes to prevent backflow.

Isolation valves, on the other hand, are used to stop the flow of water entirely for maintenance or during emergencies, but they do not inherently prevent backflow. Control valves are designed to regulate the flow or pressure within the system rather than serve a one-way flow purpose. Flow valves, while they may control the rate of flow, are not exclusively designed to restrict flow to one direction. Thus, the primary and distinguishing feature of a check valve is its one-way flow capability, making it the correct choice in this scenario.