Sterlco 1.25 Inch 511 Spring Packed Radiator Valve Straightway. Sterlco® Radiator Valve Model 511 is designed for steam heating systems with pressures of 25″ of vacuum to 150 PSI. It may also be applied to water systems with pressures up to 200 PSI. Like other Sterlco® Radiator valves, the 511 is equipped with positive ?on? and ?off? control for any type of heating element or radiator.