Normally Open vs Normally Closed Solenoid Valves (NO vs NC)
Choosing between a normally open (NO) and normally closed (NC) solenoid valve is not about preference; it’s about what must happen when power is lost. This single decision affects safety, energy use, and system behavior. “Normal” refers to the valve’s state when the solenoid coil is not energized (no power applied).
Choose Normally Open (NO) if flow must continue during power loss, pressure buildup is a risk, or safety/cooling media must always be available; also more energy-efficient when the valve stays open most of the time.
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Choose Normally Closed (NC) if flow must stop immediately on power loss, the media is hazardous, flammable, or expensive, leaks/flooding are unacceptable, or the valve is mostly closed during normal operation.
What is a normally open (NO) solenoid valve
A normally open solenoid valve is open when de-energized and closes when energized. This means the fluid flows when power is OFF and stops only when power is applied.
A normally open solenoid valve stays open by default because a spring pushes the internal plunger upward. When power is applied, the solenoid coil generates a magnetic field that pulls the plunger down, forcing the valve to close. Once power is removed, the magnetic field disappears, and the spring pushes the plunger back up, reopening the valve.
What is a normally closed (NC) solenoid valve
When to choose a normally open solenoid valve
Use a normally open solenoid valve where the flow must continue on power loss, i.e, when stopping flow during a power failure would cause damage or risk.
Common examples of normally open solenoid valve applications are:
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Fire suppression systems: Water or extinguishing media must still flow if power fails.
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Industrial cooling systems: Continuous coolant flow prevents equipment overheating.
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Ventilation and HVAC bypass lines: Maintains airflow during electrical outages.
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Pressure relief or bypass circuits: Prevents pressure buildup when controls fail.
When to choose a normally closed solenoid valve
Use a normally closed valve when uncontrolled flow is dangerous or unacceptable.
Common examples of normally closed solenoid valves are:
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Water supply control (normally closed solenoid water valve): Prevents flooding when power or control fails.
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Fuel systems: Stops fuel flow automatically during shutdown.
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Pneumatic systems: Prevents unexpected actuator movement.
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Chemical or gas handling systems: Reduces leak and exposure risk.
Fail open vs fail close solenoid valves
These terms describe what happens during a power failure.
Many industrial and safety standards specify whether a system must fail open or fail close. Always check this first.
What are bi-stable (Latching) solenoid valves?
Bi-stable solenoid valves do not require continuous power, remain in their last position after a pulse, and are ideal for energy-saving or battery-operated systems. They are neither normally open nor normally closed and are used in specialized applications.
FAQs
Are solenoid valves normally open or normally closed?
Most solenoid valves are normally closed by default, but normally open versions are widely available for safety-critical applications.
How can I tell if a solenoid valve is normally open or closed?
Check the datasheet to see if the valve is NO or NC. If the valve allows flow when power is off, it is Normally Open; if it blocks flow when power is off, it is Normally Closed.
What does NC mean on a valve?
NC means Normally Closed. The valve is closed when not energized.
What is the difference between NC and NO solenoid valves?
NO (Normally Open) valves are open when power is off and close when energized. NC (Normally Closed) valves are closed when power is off and open when energized.
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