Normally Open (NO) vs. Normally Closed (NC) Solenoid Valve: Differences

The two most common options when selecting a solenoid valve are Normally Closed (NC) and Normally Open (NO) models.

So, what is the difference between NO and NC solenoid valves? Which type should be preferred for which application?

What Is a Normally Closed (NC) Solenoid Valve?

A Normally Closed (NC) solenoid valve is a type of valve that keeps the flow path closed when the coil is not energized.

When electrical power is applied to the coil, a magnetic field is generated, causing the valve to open and allowing the fluid to pass through. When the power is cut off, the valve returns to the closed position.

In short:

De-energized → Closed
Energized → Open

This operating principle can be used in systems where flow is required only when needed.

For example, if water or air is not supposed to flow under normal conditions and should only pass when commanded by the automation system, an NC solenoid valve can be considered.

Applications of NC Solenoid Valves

• Water control systems

• Compressed air lines

• Automatic filling systems

• Dosing applications

• Fuel and gas control systems

• Industrial automation systems

• Machine and equipment control

The NC operating principle can provide an important advantage, particularly in applications where the flow needs to stop when power is lost.

What Is a Normally Open (NO) Solenoid Valve?

A Normally Open (NO) solenoid valve is a type of valve that keeps the flow path open when the coil is not energized.

When electrical power is applied to the coil, the valve closes and the flow stops. When the power is cut off, the valve returns to the open position.

In short:

De-energized → Open
Energized → Closed

This operating principle can be considered in applications where the system normally requires continuous flow and the flow is expected to continue in the event of a power failure.

For example, if it is not desired for cooling water to stop completely when power is lost in a cooling system, an NO solenoid valve may be preferred depending on the design of the application.

What Is the Difference Between NO and NC Solenoid Valves?

The main difference between the two valves is the position of the valve when there is no electrical power.

Therefore, instead of asking “Is NO or NC better?”, it is necessary to determine how the system should behave in the event of a power failure.

Why Is Power Failure Important When Selecting a Solenoid Valve?

One of the important considerations when selecting a solenoid valve is how the system is expected to respond if electrical power is interrupted.

For example:

• If gas flow needs to stop when power is lost, an NC operating principle can be considered.

• If a specific cooling or circulation flow needs to continue during a power failure, an NO solenoid valve can be considered.

• If flow is required only when a command is given in an automatic filling system, an NC valve may be preferred.

• If the system needs to remain normally open and close only when a control signal is received, an NO valve can be considered.

The key criterion here is the operating and safety scenario of the application, rather than the product specifications alone.

Is It Enough to Consider Only the Open/Closed Position When Selecting NO and NC?

No. In addition to the NO or NC operating mode, other technical parameters should also be evaluated when selecting a solenoid valve.

1. Fluid Type

The fluid passing through the valve must be determined.

The same solenoid valve may not be suitable for water, air, steam, oil, fuel, or different chemical fluids. The body and sealing materials must be compatible with the fluid.

2. Operating Pressure

The minimum and maximum operating pressure of the system must be determined.

Especially in pilot-operated solenoid valves, the operating pressure and required differential pressure may be important factors in product selection.

3. Operating Temperature

The temperature of the fluid must be considered when selecting the valve body and sealing elements.

In high-temperature applications, a special product suitable for the operating temperature may be required instead of a standard solenoid valve.

4. DN and Connection Size

The connection size suitable for the required flow rate should be determined.

However, information such as “a DN25 valve is required” is not always sufficient for selecting the correct product. Flow rate, pressure, fluid type, and connection standard should be evaluated together.

5. Coil Voltage

The coil voltage of the solenoid valve must also be compatible with the electrical infrastructure of the system.

Different voltage options may be available depending on the application. Therefore, the existing electrical supply of the system should be checked during product selection.

What Problems Can Incorrect NO or NC Selection Cause?

Even if a solenoid valve has technically correct dimensions, selecting the wrong operating type can cause the system to operate in an undesired manner.

For example, if the system is designed to stop the flow when power is lost but an NO valve is used, the flow may continue during a power failure.

Similarly, using an NC valve in a system where flow needs to continue during a power failure may interrupt the flow and cause the process to stop.

Therefore, the following question should always be asked during selection:

“What position should the valve be in when electrical power is lost?”

The answer to this question is the starting point for selecting between NO and NC.

Solenoid Valve Selection Checklist

The following information should be determined to select the correct solenoid valve:

• Valve type: NO or NC

• Fluid: Water, air, steam, oil, etc.

• DN / connection size

• Operating pressure

• Operating temperature

• Flow rate

• Coil voltage

• Body material

• Gasket / sealing material

• Connection type

• Required valve position in case of power failure

Once these parameters have been determined, the solenoid valve model suitable for the application can be selected more accurately.

Conclusion: NO or NC?

The main difference between Normally Open (NO) and Normally Closed (NC) solenoid valves is the position of the valve when it is de-energized.

NC valve: Closed when there is no power and opens when energized.

NO valve: Open when there is no power and closes when energized.

Therefore, the selection should not be based on whether one valve is “better” or “worse,” but on the system's operating principle, power failure requirements, fluid properties, pressure, and temperature conditions.

Choosing the right product is possible not simply by determining the valve size, but by evaluating all of the system's operating conditions together.

Selecting the Right Solenoid Valve with İsimatik

For the correct solenoid valve selection in industrial systems, it is not enough to determine whether the product is NO or NC. The fluid, pressure, temperature, connection size, and electrical specifications must be evaluated together.

For solenoid valve selection suitable for your application, you can review İsimatik's product options and determine the right product according to the technical requirements of your application.