PRVs have spring-loaded diaphragms that resist the incoming pressure of the water supply to a home. They are typically installed just after the water meter on the system side (see top photo: This basement photo, starting from the bottom, shows the insulated incoming water main, the single-throw main shut-off (red handle), the water meter, and the pressure reducing valve (set screw stem sticking out to the right). PRVs usually come set at 45 pounds per square inch (PSI) but have a set-screw adjustment so that the PRV