The 3-7 PSI range is factory-set based on the internal spring rate

it handles a range of plus/minus 0

The factory-set 4-12" w

If the vacuum is correct but the SPST switch shows an open circuit with a multimeter

Armstrong Fluid Technology 816305-058 Impeller 320819-301 The 3-7 PSI range isThe Armstrong Fluid Technology 816305 058 Impeller is a precision engineered 3. 875" bronze impeller designed for high performance fluid circulation in commercial and industrial HVACR systems. As a critical component for the Armstrong S 45 circulator pump, this centrifugal pump impeller is manufactured from high quality bronze to provide exceptional corrosion resistance and structural integrity, specifically in potable water or open loop systems where