Normally Closed vs Normally Open Solenoid Valves: How to Choose
One of the first decisions when specifying a solenoid valve is whether it should be normally closed (NC) or normally open (NO). The terms describe the valve's resting state — how it behaves when the coil is de-energised and no power is applied. Getting this choice right is fundamental to how your system fails safely, how much energy it consumes and how long the coil lasts. This guide explains the difference and helps you match the function to your application.
What "normally" actually means
The "normal" state always refers to the de-energised valve — power off. A normally closed valve blocks flow when de-energised and opens only when the coil is energised. A normally open valve allows flow when de-energised and closes when the coil is energised. In both cases the spring holds the valve in its rest position, and the magnetic force from the coil overcomes that spring to switch it.
This is why NC and NO are not interchangeable: they are physically different valves, plumbed and sprung for opposite rest positions. You cannot simply rewire one to behave like the other.
Normally closed (NC): the default choice
The vast majority of general-purpose two-way solenoid valves are normally closed, and for good reason. When power is lost — whether from a fault, an emergency stop or a planned shutdown — an NC valve returns to closed and stops the flow. For most fluid and gas control tasks, "no power means no flow" is exactly the safe, predictable behaviour you want.
NC valves suit applications where flow should only occur on demand: filling a tank, dosing a batch, dispensing water at a tap, or admitting compressed air to a tool. Because the valve spends most of its time closed and de-energised, coil heating and energy use are typically low.
Normally open (NO): flow by default
A normally open valve is the right choice when your process runs with flow present most of the time and you only need to interrupt it occasionally. Cooling-water lines, bypass circuits and certain drain or vent lines are common examples. If the valve should keep flowing during a power cut — for instance, to maintain cooling to equipment that must not overheat — an NO valve provides that fail-open behaviour.
The trade-off is duty: because an NO valve is energised whenever it is closed, holding it shut for long periods keeps the coil powered and warm. If your process needs the valve closed for hours at a time, consider the coil rating, ambient temperature and whether a continuous duty coil is required.
Quick comparison
| Characteristic | Normally Closed (NC) | Normally Open (NO) |
|---|---|---|
| State when de-energised | Closed — no flow | Open — flow passes |
| State when energised | Open — flow passes | Closed — no flow |
| Behaviour on power loss | Fails closed (shuts off) | Fails open (keeps flowing) |
| Coil energised when… | Flow is required | Flow must be stopped |
| Best when flow is… | Occasional / on demand | Present most of the time |
| Typical uses | Filling, dosing, dispensing, tool air | Cooling water, bypass, fail-open lines |
Energy and heat: an important practical factor
Because a solenoid coil only draws power while energised, the resting state directly affects running costs and coil life. A valve that is energised for long stretches generates heat, and sustained heat is a leading cause of premature coil failure. As a rule of thumb, pick the resting state that keeps the coil de-energised for the greater share of the duty cycle. If a valve would otherwise need to be held energised almost continuously, a latching (bistable) valve — which holds its position with a brief pulse rather than continuous power — may be a better and cooler solution.
Don't forget fail-safe logic
Before you commit, ask a simple question: if the power fails right now, what do I want this valve to do? If the answer is "shut off to make the system safe," choose normally closed. If the answer is "keep flowing to protect the process," choose normally open. This single question resolves most NC-versus-NO decisions and should be considered alongside the media, pressure and material factors covered in our how to choose a solenoid valve guide.
Remember too that pilot-operated valves rely on a minimum pressure differential to switch. In fail-open or fail-closed planning, confirm the valve will actually reach its intended state at your lowest working pressure, not just at full line pressure.
A note on three-way valves
With three-way (3/2) valves, NC and NO describe which port is connected to the common port in the rest state, allowing you to select, divert or vent a supply. The same fail-safe logic applies: decide which flow path you want when the coil is de-energised, then specify accordingly. If you are unsure how the porting maps to your circuit, our team can talk it through before you order.
Still unsure which to specify?
NC versus NO comes down to two things: how often the valve needs to be open, and what should happen on a loss of power. If you can answer those, you can specify with confidence. For help narrowing down a valve by media, size, pressure and function, try our Valve Search Wizard, or get in touch with our technical team before ordering — we are happy to confirm the right resting state for your application.