In normally open solenoid valves, when the coil is de-energized the plunger opens the valve seat, connecting the inlet to the outlet. When the coil is energized, the plunger closes the fluid flow. All the normally open solenoid valves are exclusively sold in the model without coil (S suffix).

Normally open solenoid valves used for refrigeration systems exploit the membrane/piston servo control operational principle. This type, like the normally closed solenoid valve, can be installed also on three main branches of a line (liquid line, intake line, and hot gas line).

The operation of normally open solenoid valves requires the presence of a minimum inlet pressure to open the membrane (or piston) and to keep it lifted off the main nozzle; and to close the membrane (or piston), ensuring the seal of the nozzle. These valves cannot work with a zero pressure differential.