Pressure reducing valves are mostly used in closed boiler systems. They are deigned as a labor saving device when filling the boiler piping with water. They take the pressure in the cold water supply (approx. 30-40PSI) and drop it to around 12PSI.  This is the typical pressure for a boiler system. It is a labor saving device because years ago, filling a system with water was a two man job. One man was in the basement opening the water valve to the system, and the other man was on the top floors of the building bleeding the radiators. It needed two men because the system could easily over pressurize. The water enterig the building typically was at a higher pressure than needed to reach the top floor. The excess pressure could over pressurize the system.

We no longer need two men because the pressure reducing valve allows a constant pressure in the system. The pressure reducing valve will prevent the system from over pressurizing.

Once the system is filled and set, the cold water valve should remain off.