Normally closed vs normally open solenoid valve: which to choose?
A solenoid valve is an electrically operated valve that controls the flow of fluid (liquid or gas) by opening or closing an orifice. The two most common types are normally closed (NC) and normally open (NO) . The difference is simple: NC valves are closed when de‑energised and open when energised; NO valves are open when de‑energised and closed when energised.
Choosing the wrong type can lead to system failure, unnecessary energy consumption, or even safety hazards. This guide explains the difference, the pros and cons of each, and how to decide which is right for your application.
1. What does “normally closed” mean?
A normally closed (NC) solenoid valve remains closed when no power is applied. The spring or internal pressure keeps the valve seat sealed. When the coil is energised, the magnetic force lifts the plunger, opening the valve and allowing fluid to pass.
Think of it as a door that stays shut unless you actively hold it open.
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De‑energised state: closed (no flow)
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Energised state: open (flow)
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Default position: closed (safe, no flow)
2. What does “normally open” mean?
A normally open (NO) solenoid valve remains open when no power is applied. The spring holds the plunger away from the seat. When the coil is energised, the magnetic force pulls the plunger down, closing the valve and stopping flow.
Think of it as a door that stays open unless you actively close it.
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De‑energised state: open (flow)
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Energised state: closed (no flow)
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Default position: open (flow)
3. Key differences at a glance
Feature
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Normally closed (NC)
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Normally open (NO)
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State without power
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Closed (no flow)
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Open (flow)
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State with power
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Open (flow)
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Closed (no flow)
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Power consumption
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Consumes power while open
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Consumes power while closed
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Default safety
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Fails safe (no unintended flow)
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Fails safe (no unintended blockage)
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Typical applications
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Water dispensers, coffee machines, irrigation, medical dosing
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Cooling systems, emergency exhaust, fail‑open circuits, holding tanks
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4. When to choose a normally closed (NC) valve
NC valves are far more common because most applications require flow only occasionally, and it is safer to have the valve closed when power fails.
Choose NC when:
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Flow is needed only when the system is energised (e.g., filling a cup, dosing a chemical).
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Power loss should stop the flow to prevent flooding, overfilling, or spillage.
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The valve is open for a short time and closed for long periods – this saves energy because the coil is energised only during the brief open phase.
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You are designing a fail‑safe system where no flow is the safe condition.
Typical applications:
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Home water dispensers and coffee machines (water flows only when you press the button)
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Irrigation solenoids (valve opens to water plants, then closes)
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Medical dosing pumps (chemical added only when energised)
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Washing machines and dishwashers (water inlet control)
Example: A coffee machine must not drip water when idle. An NC valve ensures no water flows until the machine commands a brew cycle. If power fails, the valve stays closed – no leakage.
5. When to choose a normally open (NO) valve
NO valves are less common but essential in applications where flow must continue during a power outage, or where cooling or venting cannot stop.
Choose NO when:
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Flow is needed most of the time, and you only need to stop it occasionally.
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Power loss should not interrupt flow (e.g., cooling a laser, venting dangerous gases).
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The valve is closed for short periods and open for long periods – this saves energy because the coil is energised only during the brief closed phase.
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You need a fail‑open design where flow is the safe condition.
Typical applications:
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Cooling water circuits: if power fails, the valve should stay open to prevent overheating.
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Emergency exhaust systems: fumes must be vented even when power is lost.
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Compressed air holding tanks: keep air flowing to tools until intentionally shut off.
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Continuous chemical feed systems: shutting off power should not stop essential treatment.
Example: A laser cutter uses water cooling. If the power fails, the cooling water must still flow to prevent the laser from overheating. An NO valve remains open when power is lost, allowing passive flow.
6. Energy consumption considerations
Solenoid valves consume power only when the coil is energised. Therefore:
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NC valves consume power only when they are open. If your valve is open for short periods, an NC valve is more energy efficient.
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NO valves consume power only when they are closed. If your valve is closed for short periods, an NO valve is more energy efficient.
For battery‑powered devices (e.g., portable medical equipment, remote irrigation), choose the type that minimises energised time. If the valve must stay closed for 23 hours a day, an NC valve will use very little energy. If it must stay open almost continuously, an NO valve will use less energy.
Some modern solenoid valves are bistable (latching) – they consume power only during switching and hold their state without power. These are ideal for battery‑operated applications where both open and closed states are needed for long periods.
7. Safety and fail‑safe considerations
Safety is often the deciding factor.
Fail‑safe closed (NC) : When power is lost, the valve closes. Use this when flow during a power failure could cause:
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Flooding (water tanks, pipes)
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Overfilling (chemical tanks, fuel lines)
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Spillage of hazardous materials
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Uncontrolled release of gas or liquid
Fail‑safe open (NO) : When power is lost, the valve opens. Use this when lack of flow during a power failure could cause:
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Overheating (cooling circuits, engine lubrication)
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Pressure build‑up (venting systems)
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Starvation of essential supplies (medical oxygen, livestock water)
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Dangerous vacuum conditions
Always choose the option that leads to the safer failure mode for your specific system.
8. Common misconceptions
Misconception 1: “NC valves are better because they save energy”
Not always. If your valve must be open for hours, an NC valve will waste energy keeping it open. An NO valve would be more efficient.
Misconception 2: “All solenoid valves use power continuously”
Only standard direct‑acting and pilot‑operated valves do. Latching (bistable) valves use power only during switching.
Misconception 3: “You can change an NC valve to NO by reversing the wiring”
No. The internal mechanical design (spring position) is fixed. Reversing polarity on a DC valve will not change its normally closed/open state.
Misconception 4: “NO valves are more expensive”
Generally no. Manufacturing cost is similar. The price difference is usually negligible.
9. How to decide – a step‑by‑step checklist
Before choosing, answer these questions:
1. What should the valve do when power is completely off?
Stay closed → choose NC.
Stay open → choose NO.
2. How long will the valve be energised per cycle?
Open for short periods (seconds to minutes) → NC saves energy.
Closed for short periods → NO saves energy.
3. Is failure of power a rare or common event?
If common, fail‑safe behaviour is critical – follow the safety rule.
4. What is the media?
Some media (e.g., corrosive chemicals, steam) may influence valve material choice, but not NC/NO selection directly.
5. Is the valve for a battery‑powered device?
If yes, minimise energised time. Consider a latching valve.
10. Conclusion
There is no universally “better” type – it depends entirely on your application.
Use normally closed (NC) when you want flow only when power is applied, and when power loss should stop flow. This is the most common choice for intermittent use.
Use normally open (NO) when flow must continue when power is off, or when the valve is open most of the time and you want to save energy.
When in doubt, prioritise fail‑safe behaviour. Ask: “If power fails, which state is safer – open or closed?” Then select accordingly.
Sammy