A normally closed solenoid valve blocks flow whenever it has no power and opens only while the coil is energized. A normally open solenoid valve does the reverse, passing flow without power and shutting only while energized. Choosing between a normally open vs normally closed solenoid valve comes down to one question with two layers: what should your system do the moment power disappears, and which position will the valve sit in most of the day? Answer those, and the right configuration usually picks itself. This guide answers two questions at once, how does a normally closed solenoid valve work through a power cycle, and when is its normally open counterpart the better choice.
This decision is easy to get wrong precisely because both valves look identical on the shelf and wire up the same way. The difference lives entirely in behavior, and a mismatch can hide inside a running system for months before it costs you. Below, we walk through the decision criteria engineers actually use and map the choice onto the kinds of systems these valves serve every day.
What Does “Normally” Actually Mean?
In valve terminology, “normally” refers to the valve’s resting state, the position it holds when the coil is de-energized. It has nothing to do with which position is more common in service or which state the valve spends more hours in. A valve installed in a system that keeps it powered around the clock still has a “normal” position, because that is where it will go the instant power is interrupted, whether by a switch, a controller, an outage, or a tripped breaker.
That resting position is set mechanically, by the return spring and the internal geometry, not by wiring or programming. No control setting can change what a valve does when its power is gone, which is exactly why the configuration has to be chosen correctly at ordering time rather than adjusted later in the field.
How Does a Normally Closed Solenoid Valve Work?
In a normally closed valve, the return spring holds the seal pressed against the seat whenever the coil is unpowered, so the flow path stays blocked by default. Applying voltage energizes the coil, and the magnetic pull of the energized winding, the same coil-and-plunger principle described by Encyclopaedia Britannica, overcomes the spring so the seal lifts and fluid passes. The valve then holds open for exactly as long as current keeps flowing, and the instant that current stops, for any reason, the spring reseats the seal and flow cuts off.
The defining trait is that closed is the automatic state and open is the commanded state, and that is why normally closed valves are treated as the fail-closed choice and why they sit as the industry’s default configuration: shutting off fluid when control is lost is the safe outcome in the bulk of water, air, and general-purpose applications, so this is the version engineers reach for unless the system gives them a reason not to.
If you want the component-level physics behind that action, the plunger, the coil, and the pilot diaphragm, we cover the full sequence in our post on how a solenoid valve works.
How Does a Normally Open Solenoid Valve Work?
A normally open valve inverts the arrangement. At rest, with the coil unpowered, the internal geometry holds the flow path clear and fluid passes freely. Energizing the coil drives the seal onto the seat and stops flow; kill the power and the valve springs back open.
Here, open takes care of itself and closing requires the coil’s continuous effort. The valve guarantees flow, not shutoff, in the absence of control, and that guarantee is valuable in a narrower set of systems, so a normally open valve is specified deliberately for a specific job rather than reached for by habit.
The First Filter: What Should Happen When Power Fails?
Start the decision here, because this filter is about safety and damage, and it overrules everything else. The position a valve takes on its own when its supply is cut is what engineers call its fail-safe position, and it should be a design choice, never an accident. Picture your system mid-operation, then remove electricity without warning.
If continued flow would cause the problem, flooding a space, overfilling a tank, draining a reservoir, dispensing chemicals with nobody in control, then flow must stop on power loss and you need normally closed. This is the logic behind fill valves, dosing valves, and shutoff duty generally.
If stopped flow would cause the problem, you need normally open. The classic example is cooling water that must keep circulating to protect equipment even during an outage, and the same logic covers any path that should fail open so pressure or trapped volume has somewhere safe to go. In these systems, a valve that fails shut is the hazard, not the safeguard.
If a power loss in your system creates real risk in both directions, that is worth a conversation before you order anything, because the answer may involve rethinking the plumbing rather than just picking a configuration. Call or text the Industrial Plastic Valves team at (775) 883-6108 and talk through the failure scenario with someone who has seen it before.
The Second Filter: Which Position Dominates the Duty Cycle?
Once the fail-state question is settled, the tiebreaker is efficiency. A solenoid coil consumes power and generates heat for every minute it stays energized, so the goal is simple: choose the configuration whose resting position matches the state your system occupies most of the time.
A valve that needs to pass flow for a few minutes per hour and stay shut the rest of the day should be normally closed, so the coil only works during those few minutes. A valve that stays open for hours on end and closes only briefly is a candidate for normally open, so the long open stretches cost nothing. Getting this backward means a coil that runs energized nearly continuously, which wastes energy, keeps the coil hot day and night, and adds thermal stress over the life of the valve for no operational benefit.
In practice, the two filters agree far more often than they conflict, because the configuration that is safe on power loss is usually also the one that is cheapest to run, so the second filter mostly confirms the first rather than overturning it.
Which Applications Call for Normally Open vs Normally Closed?
The systems our valves run in show the pattern clearly. Pool, spa, and hot tub fill and makeup valves are normally closed, because makeup water left running against a full pool has nowhere to go but the deck. Car and vehicle wash systems use normally closed valves to sequence water, soaps, and rinses, since every one of those flows should happen only on command. Water treatment and filtration shutoff valves are normally closed so an interruption isolates the system instead of letting untreated water keep moving. Fountains, splash pads, and bottle-fill stations follow the same rule, dispensing only when told to.
Normally open earns its place in the support roles around those systems: a cooling line protecting pump seals or heat-generating equipment, a drain valve that should release when the system shuts down, a recirculation path that must stay live unless deliberately closed. If your application sits in that second group, Industrial Plastic Valves carries normally open valves alongside its normally closed line, so tell us the series and size you have in mind and we will confirm the fit. You can explore the range through our specialty valve line and products page.
What a Mismatch Looks Like in the Field
Choosing the wrong configuration rarely announces itself at startup, because a miswired sense of “working” hides it: the system runs, the valve cycles, everything seems fine. The mismatch surfaces later, in one of two ways. The dramatic version is the power interruption that leaves flow running unattended, or seals off a line that needed to keep flowing, turning a routine outage into damage. The quiet version is the energy audit or the failed coil that reveals a valve which has been energized twenty-three hours a day to hold a position it could have held for free.
Both versions trace back to the same moment: the configuration was chosen by default instead of by walking through the two filters. The fix is equally unglamorous, which is to spend five minutes on the fail-state and duty-cycle questions before the order goes out.
Frequently Asked Questions
Can I convert a normally closed valve to normally open?
Not in the field. The normal position is a manufacturing configuration, not a setting, and the workaround people attempt, wiring the coil to stay permanently energized so the valve mimics the opposite behavior, buys the inverted logic at the price of a coil that never gets to rest. If you need the opposite behavior, that is a different valve, not a different hookup.
Is a normally closed valve open or closed when powered?
Open. Power always drives a valve away from the position in its name, so an energized normally closed valve is passing flow while an energized normally open valve is blocking it. That inversion matters most when reading control panels: a lit coil indicator tells you the valve is in its commanded state, which is the opposite of what its label says.
Do normally open valves cost more to run?
No, not inherently. A normally open valve and its normally closed counterpart in the same series typically use the same coil and draw the same power while energized, so neither configuration is more expensive by design. The running cost difference comes entirely from how many hours per day your particular system forces the coil to stay energized.
Get the Configuration Right Before It Ships
Normally open or normally closed is a single line item on a purchase order, and it quietly decides how your system behaves on its worst day. For more than 35 years, Industrial Plastic Valves has helped engineers, OEMs, and purchasing teams make exactly this call with confidence, and with no minimum order quantities, you can prove out a configuration with a single valve before committing a build to it.
Tell us what your system does and what should happen when the lights go out, and we will confirm the configuration, size, and series that fit. Call or text (775) 883-6108, or email your specs to [email protected], and get a straight answer from a real person who knows valves.