What Should Your Flow System Do When the Power Goes Out?

A normally open solenoid valve permits flow without electrical power and closes when its coil is energized. That definition answers an important question, but only one: the valve’s commanded position when power is absent.

Choosing the right valve also requires knowing the safest power-loss state, available pressure differential, required flow, exact medium, wetted materials, temperature, electrical supply, duty cycle, installation environment, leakage limits, and required approvals. Connection size and the letters “NO” do not establish any of those characteristics.

What “normally open” means

A normally open, or NO, solenoid valve permits flow in its de-energized state. Applying the correct electrical power to the coil commands the valve to close.

Electrical state	Valve and flow state
Power off	Valve open; flow permitted
Power on	Valve commanded closed; flow stopped or restricted according to the model’s leakage specification

In a conventional spring-return design, loss of electrical power releases the solenoid and allows the spring to return the valve to its open position. Some designs use gravity instead. The return mechanism is model-dependent, but the defining behavior remains the same: open when de-energized and closed when energized.

“Normally” means the unpowered default position. It does not describe the position in which the valve spends most of its working life. A normally open valve could remain energized and closed for much of each day, although doing so increases energized time and associated coil heat and energy use.

Normally open is also commonly described as fail open. That term identifies the intended response to electrical power loss; it does not guarantee that the valve will physically reopen after every fault. Contamination, damaged parts, a failed return mechanism, incompatible seals, or improper installation can prevent the mechanism from returning as intended.

Port count also matters:

• A two-way valve controls one path between an inlet and an outlet.
• A three-way valve switches connections among three ports.

Do not assume that a three-way valve routes fluid like a two-way shutoff valve merely because both are marked normally open.

How the coil closes the valve and the return mechanism reopens it

The principal components commonly include:

• A valve body
• Inlet, outlet, or working ports
• A solenoid coil
• A core or guide tube
• A movable armature or plunger
• A spring or another return mechanism
• A valve seat
• Body, diaphragm, stem, or seat seals as applicable
• Electrical terminals or a connector

In the de-energized state, the return mechanism holds the relevant closing element away from the main seat or otherwise establishes the valve’s open flow path.

When the coil receives its rated electrical supply, current creates a magnetic field that moves the armature. What happens next depends on the architecture:

• In a direct-acting valve, the armature or a coupled element acts directly on the main seat.
• In a pilot-operated valve, the armature operates a pilot passage, and process pressure helps move the main diaphragm or piston.
• In a direct-lift or assisted-lift valve, solenoid force and diaphragm action are combined.

Removing power collapses the magnetic field. A spring, gravity, or another model-specific return force then restores the de-energized flow path. The exact components and direction of movement vary, so parts identification and troubleshooting should follow the drawing for the selected model. A general overview of the components and operating sequence is available in Thomasnet’s normally open solenoid valve guide.

How one normally open three-port arrangement routes flow

In the normally open three-port arrangement described by CPV Manufacturing:

• De-energized: the pressure port connects to the cylinder port.
• Energized: the pressure port closes, while the exhaust port connects to the cylinder port.

That is one documented arrangement, not a universal port map for all three-way valves. The CPV Manufacturing comparison describes this specific pressure, cylinder, and exhaust routing.

For any unfamiliar three-way valve, consult its symbol and manufacturer diagram. The documentation must identify the ports and show both energized and de-energized flow paths.

Normally open vs. normally closed vs. bi-stable

No configuration is universally best. The correct choice depends first on what the process must do without power and then on how long the valve normally remains in each state.

Configuration	De-energized state	Energized or commanded state	After loss of power	When the coil consumes power
Normally open	Flow permitted	Flow stopped	A conventional return design is intended to reopen	While held closed
Normally closed	Flow stopped	Flow permitted	A conventional return design is intended to close	While held open
Bi-stable or latching	Retains its last commanded state	A pulse changes its state	Usually retains the last state	Primarily during switching pulses

A normally open valve can reduce energized time where flow is required most of the time. The same valve may use more energy than a normally closed alternative if it must remain closed for long periods.

A normally closed valve reverses that operating pattern: it uses no coil power while shut and normally requires power while held open.

A bi-stable or latching valve changes state in response to a brief electrical pulse and then retains that state without continuous coil power. This can reduce coil-on time, but it does not provide the same predictable spring-return response to an outage. The retained position and control arrangement therefore require evaluation. These operating distinctions are summarized in Tameson’s comparison of normally open, normally closed, and bi-stable valves.

Configuration is a system decision, not a contest over which type is generally better. Required outage behavior comes first; energy use and operating pattern follow.

Choose the power-loss state before choosing a valve

Begin with one question:

Which fluid state creates less harm if electrical control disappears?

Some cooling, lubrication, ventilation, HVAC, process-flow, and pressure-management systems may need flow to continue during an outage. These are possible applications, not universal prescriptions.

Consider a cooling loop. Continued flow may be desirable if losing circulation would allow retained process heat to damage equipment. But opening a valve is beneficial only if the rest of the system can still provide useful circulation and continued flow does not introduce another hazard. The analysis must account for the pump, stored pressure, gravity flow, bypasses, check valves, and downstream equipment.

Systems carrying hazardous media often favor a normally closed arrangement so that electrical power loss commands the valve to stop flow. That remains a qualified design tendency rather than an automatic rule: stored pressure, gravity or siphon flow, shutdown sequencing, redundant isolation, and applicable requirements can change the correct choice. General valve guidance similarly presents normally closed operation as common for hazardous media while emphasizing the need to consider the actual application.

Neither default position is inherently safer:

• Continued flow may help avoid overheating or loss of lubrication.
• Stopped flow may help limit flooding, fuel release, or unwanted chemical discharge.
• Opening after power loss can protect one process and endanger another.
• Closing after power loss can isolate a hazard or remove essential cooling.

Fire suppression, safety showers, fuel systems, and other regulated or life-safety applications are configuration-specific. A normally open label or generic application list does not establish code compliance, certification, or suitability. Thomasnet identifies several such application categories but also notes that selection depends on the actual valve and operating conditions.

Use this decision sequence:

1. Determine the required outage state. Must flow continue, stop, or switch to another route?
2. Choose the required state behavior. Is a spring-return position required, or may the valve retain its previous state?
3. Confirm process conditions. Establish pressure, differential pressure, vacuum conditions, required flow, and allowable pressure drop.
4. Verify the full specification. Check the exact medium, wetted materials, temperature, electrical supply, duty, environment, leakage requirements, and approvals.

Hazardous, regulated, and safety-critical systems require review against manufacturer documentation and applicable engineering and regulatory requirements. A catalog label alone cannot validate the complete system response.

Direct-acting, zero-differential, and pilot-operated designs

“Normally open” describes the default position. It does not explain how the valve develops enough force to change state.

A direct-acting valve applies solenoid force directly to the main closing mechanism. It generally does not rely on process-pressure differential for basic actuation. That can make direct action useful at low pressure, but it does not mean every direct-acting valve is suitable for vacuum, every medium, or every required flow.

A pilot-operated valve uses a pilot passage and process pressure to move a larger diaphragm or piston. This can support higher flow or pressure in suitable conditions. Many pressure-assisted designs need a minimum pressure differential. If the available differential is too low, the main element may not move correctly.

A direct-lift or assisted-lift valve combines direct mechanical movement with diaphragm operation. Some models are intended for zero-differential service, but the category name does not prove that capability. Check the stated minimum and maximum operating-pressure differential for the exact model.

Zero-differential capability can matter in:

• Vacuum service
• Gravity-fed systems
• Startup before pressure develops
• Drain-down conditions
• Low-head circulation loops
• Systems in which upstream and downstream pressures can equalize

Reading a zero-differential product listing carefully

As a model-specific example, the seller describes the WIC 2POG-1-D as a two-way, normally open, direct-lift diaphragm valve that operates from vacuum or 0 PSI through 90 PSI without a minimum pressure differential. The listing specifies one-inch female NPT ports, a 25 mm orifice, stated Cv 12, a PA66 body, NBR seals, and an operating range of 0–50 °C. These are seller-provided specifications for that product, not general characteristics of normally open valves; see the WIC 2POG-1-D product listing.

Those specifications do not establish potable-water approval, compatibility with a particular chemical, unrestricted mounting in every installation, or equivalent performance from another one-inch valve. Before buying, find both the minimum operating-pressure differential and maximum operating-pressure differential in the current manufacturer documentation.

A specification worksheet for selecting the valve

Complete the worksheet in this order. If an early answer is unknown, choosing a larger connection or a higher maximum-pressure rating will not compensate for it.

1. Required outage state

Record what must happen when electrical power disappears:

• Open and continue flow
• Close and stop flow
• Switch flow to another port
• Retain the last commanded state

Also establish whether the design requires a spring-return state or permits retained-state operation.

2. Exact medium

Do not specify only “water,” “gas,” “oil,” “fuel,” or “chemical.” Record:

• The exact fluid or gas
• Concentration or formulation
• Additives or cleaning agents
• Expected contamination
• Normal and abnormal temperature
• Any tendency to leave particles or deposits

Broad media labels do not establish compatibility for a particular formulation and temperature.

3. Flow requirement

Record the minimum, normal, and peak required flow. Also establish the maximum acceptable pressure drop across the valve.

If startup conditions differ from normal operation, document them separately. A valve that performs correctly at steady state may still be unsuitable where startup pressure or differential is insufficient.

4. Pressure conditions

Record:

• Static pressure
• Maximum inlet pressure
• Minimum operating-pressure differential
• Maximum operating-pressure differential
• Outlet pressure or backpressure
• Vacuum conditions
• Expected pressure excursions
• Allowable pressure drop

Pressure rating and operating differential are not interchangeable. A valve may withstand a stated maximum pressure yet still require a minimum differential to actuate.

5. Temperature

Record medium and ambient temperature separately, including expected operating extremes.

Temperature affects material and seal selection as well as coil operating conditions. Verify the permitted range for the complete valve and coil configuration rather than relying on a broad material description.

6. Wetted materials

Identify every material that contacts the medium, including the body, seat, diaphragm, plunger tip, O-rings, and internal metal or plastic components.

Possible body materials include brass, stainless steel, aluminum, nylon, and other plastics. Possible sealing materials include NBR, FKM or Viton, EPDM, and PTFE. These are available options, not universal recommendations. Compatibility depends on the exact medium, concentration, temperature, pressure, and exposure.

7. Valve architecture

Identify whether the selected valve is:

• Direct-acting
• Pilot-operated
• Direct-lift or assisted-lift
• Diaphragm, poppet, plunger, or another construction
• Two-way or three-way
• Spring-return or bi-stable

Then confirm that the architecture works at the system’s lowest available differential pressure.

8. Connection, orifice, and flow capacity

Keep these three specifications separate:

• Port or connection size describes the pipe or tube interface.
• Orifice diameter describes an internal restriction.
• Cv is a model-specific flow-capacity value.

A one-inch connection does not guarantee a one-inch internal opening or a particular flow rate. Required capacity depends on the desired flow, fluid properties, and allowable pressure drop. Use model-specific sizing information rather than selecting by connection size alone.

Also confirm the thread, flange, or tube standard and the required flow direction.

9. Electrical supply

Record voltage and current type exactly, such as:

• 12 V DC
• 24 V DC
• 24 V AC
• 110 V AC

AC and DC coils are not interchangeable simply because their nominal voltage numbers are similar. Verify coil wattage or VA, stated voltage tolerance, connector type, cycling requirements, and duty rating using the exact model code.

10. Duty and operating pattern

Record:

• Hours per day energized
• Longest expected energized period
• Expected switching frequency
• Required opening and closing behavior
• Whether the selected coil has the necessary duty rating
• Whether a latching valve is permissible

Where response time or cycle life matters, request model-specific data. General descriptions of solenoid valves do not establish universal values.

11. Environment and installation

Verify the manufacturer’s requirements for:

• Ambient temperature
• Moisture, dust, or corrosive exposure
• Enclosure or ingress-protection rating
• Vibration
• Permitted mounting orientation
• Required flow direction
• Upstream contamination control
• Electrical connector configuration
• Inspection and service access

Do not transfer a mounting or environmental claim from one product to another.

12. Leakage and approvals

If shutoff leakage matters, obtain the exact model’s internal-leakage specification and test conditions. Where relevant, also request external-leakage, response-time, and cycle-life information.

Regulated services require model-specific certification evidence. This includes potable water, food contact, hazardous locations, fuel gas, fire protection, and other controlled applications. A material name or generic use-case list is not an approval.

Finally, compare the retailer listing with the current manufacturer datasheet and model code. Product listings can omit limitations or contain inconsistent option information.

Coil duty, heat, and operating energy

A conventional normally open solenoid valve consumes no coil power while open in its de-energized state. It consumes electrical power while commanded closed.

A continuous-duty rating means the coil is intended for its specified energized service under the stated operating conditions. It does not mean the coil produces no heat, consumes negligible energy, or can ignore the specified ambient and installation limits.

The basic energy calculation is:

Energy (kWh) = coil watts × energized hours ÷ 1,000

For a model-specific example, the WIC listing gives a standard DC coil rating of 30 W. If that coil were energized for 12 hours per day:

• 30 W × 12 hours = 360 Wh per day
• 360 Wh × 365 days = 131,400 Wh per year
• 131,400 Wh ÷ 1,000 = 131.4 kWh per year

The 30 W input specification is seller-provided for the cited WIC model, and the resulting 131.4 kWh per year is an arithmetic example before power-supply losses. The same listing also gives lower-power options, but those ratings remain model-specific.

Do not turn this example into a universal cost estimate. Actual consumption depends on coil draw, supply efficiency, voltage, and how long the valve remains closed.

If the required outage state permits it, a normally closed valve may reduce energized time in a process that is usually shut. A bi-stable valve may reduce steady coil power by using switching pulses. Energy savings must not override the required power-loss behavior.

Troubleshooting failure to close, reopen, or seal

Troubleshooting must follow the exact valve manufacturer’s instructions and the site’s applicable electrical and pressure-system procedures. Hazardous fluids, high-pressure equipment, and regulated systems require appropriately qualified personnel. This article does not provide a universal lockout, energized-testing, or disassembly procedure.

Contamination, pressure or flow conditions, electrical faults, vibration, and improper installation are recognized contributors to solenoid-valve malfunction in The Lee Company’s failure-mode overview.

Symptom	Plausible causes or checks
Fails to close when energized	Incorrect voltage or AC/DC type; loose electrical connection; failed or incorrect coil; contamination at moving or sealing parts; pressure outside the rating; insufficient differential for a pressure-assisted design; blocked pilot flow; incorrect installation direction
Fails to reopen when de-energized	Mechanical sticking; contamination; damaged return mechanism; swollen or incompatible seals; blocked flow path; improper installation
Internal or external leakage	Debris or damage at the seat; worn or incompatible seals; excessive pressure; damaged body or connection; installation damage
Intermittent actuation	Loose connections; vibration; unstable electrical supply; contamination; marginal pressure or flow conditions
Restricted flow	A valve whose rated capacity is too low for the required flow and allowable pressure drop; clogged flow path; contamination; incomplete opening; incorrect flow direction
Overheated or failed coil	Excessive voltage; incorrect coil; voltage spikes; high environmental temperature; operation outside the stated duty rating

For a valve that will not close, first determine whether the electrical supply at the valve matches the coil’s rated voltage and current type. Manufacturer-approved checks should then address the connections, coil condition, pressure limits, required differential, installation direction, and contamination.

If a valve remains closed after power is removed, possible causes include a sticking armature, damaged return mechanism, blocked passages, deposits, incompatible seals, or improper installation. These are investigative possibilities, not diagnoses. An unpowered coil does not prove that the internal mechanism has returned to its open position.

Contamination can obstruct moving components, clog pilot or main flow paths, damage sealing surfaces, restrict flow, and contribute to leakage or failure to change state.

Keep maintenance within manufacturer-defined limits:

• Perform visual inspection at appropriate intervals.
• Keep electrical connections secure.
• Use upstream contamination control appropriate to the system.
• Use only cleaning methods and agents permitted for the valve materials.
• Conduct functional testing under an approved procedure.
• Replace seals or other parts only where the manufacturer supports service.

Do not assume that every valve can be rebuilt. The exact parts documentation should determine whether a coil, diaphragm, plunger, seal, or complete valve is replaceable.

Frequently asked questions

Is a normally open solenoid valve open when the power is off?

Yes. It is designed to permit flow while de-energized and to close when the correct electrical power is applied to its coil. “Normally” refers to the unpowered default state, not the position used for most operating hours.

Is a normally open valve always the same as a guaranteed fail-open valve?

No. “Fail open” commonly describes the intended response to electrical power loss. It does not guarantee reopening after contamination, seal damage, mechanical sticking, return-mechanism failure, or improper installation. Safety-critical designs must account for physical failure modes rather than relying on the NO label alone.

Can a normally open solenoid valve operate at zero pressure or under vacuum?

Some can, but not all. Direct-acting and certain direct-lift models may be designed for zero-differential or vacuum service. Many pilot-operated designs depend on adequate pressure differential or flow. Verify the selected model’s vacuum rating and minimum and maximum operating-pressure differential.

Does a normally open valve use electricity while it is open?

A conventional normally open valve uses no coil power while open in its de-energized state. It consumes power while commanded closed. Controllers, sensors, and other system components may consume electricity separately.

Why would a normally open solenoid valve stay closed after power is removed?

Possible causes include contamination, a sticking armature or plunger, a damaged return mechanism, blocked passages, swollen or incompatible seals, or improper installation. Follow the manufacturer’s troubleshooting procedure rather than assuming the electrical supply is the only possible cause.

The selection sequence to remember

Decide what must happen to flow when power disappears. Choose a valve architecture that can operate at the available pressure differential. Size it using verified flow data rather than port size alone. Confirm every wetted material, temperature limit, electrical rating, and duty requirement. Then check the current manufacturer documentation and any required approvals.

“Normally open” answers only the power-off position. It does not, by itself, establish safety, compatibility, capacity, or reliability.