The United Electric J21K-232 Differential Pressure Switch SPDT 0–25 PSI (NEMA-4) is a critical control component designed for HVAC air handlers, fans, and ventilation systems. It monitors pressure differences across filters, ducts, or blower compartments and triggers or interrupts electrical circuits when pressure reaches preset thresholds. Visit The Website: https://www.partshnc.c... https://www.partshnc.com/j21k-232-united-electric-spdt-0-25-nema-4-diff-sw