Flow Switch Symbol
Definition: The Flow Switch symbol represents a pilot device whose contacts change state when fluid flow in a pipe or duct exceeds (or falls below) a set point, drawn per NEMA ICS / IEC 60947-5-1 conventions as a contact symbol with a paddle or flag actuator, here with Common and NO (normally open) terminals that close on flow.
Also known as: paddle flow switch, sail switch, waterflow switch, flow sensor switch, vane flow switch, air flow switch, proof-of-flow switch.
What the Flow Switch symbol means
The Flow Switch symbol denotes an automatic process switch — a contact operated not by a person but by the process variable of flow. A paddle, vane, or sail inserted into the pipe or duct deflects when fluid moves past it; at the set-point velocity the mechanism snaps the contact over. Drawn with Common and NO terminals, the contact is open at no-flow and closes when flow is established, which is the standard 'proof of flow' configuration: downstream equipment is only allowed to run once flow is confirmed.
Reading the symbol correctly requires the pilot-device convention: contact state is always drawn in the de-actuated condition, i.e. with no flow present. A 'normally open, closes on flow' switch therefore appears open on the diagram even though it spends most of its operating life closed. Flow switches appear in two big roles — permissive interlocks (chiller won't start its compressor until chilled-water flow proves) and alarm initiators (a fire-sprinkler waterflow switch signals the alarm panel when a head opens).
How to identify the Flow Switch symbol
Look for a standard contact symbol with a flag- or paddle-shaped actuator qualifier, often accompanied by a flow arrow. JIC/NEMA symbol charts draw the flow switch as a contact with a curved 'sail' mark; IEC 60617 composes it from the general contact plus a flow-actuation qualifier linked by the dashed mechanical line. Diagrams frequently annotate 'FS' with a device number (FS1, FS2) and sometimes the medium (air/water).
Distinguish it from its neighbours in the process-switch family by the actuator mark alone: float switch (float ball qualifier, level), pressure switch (diaphragm qualifier), temperature switch (thermal qualifier). The contact halves are identical — only the actuator symbol tells you which process variable operates it. As always, NO/NC state is drawn at no-flow.
Function in a circuit
In a paddle-type switch, flowing liquid deflects a paddle blade (often trimmable to pipe diameter) against a spring; past the set point, an over-centre mechanism or magnetic coupling snaps a SPDT micro-switch. When flow drops below the release point the spring returns the paddle and the contact reverts — the difference between actuate and release flow rates is the switch's built-in hysteresis (deadband), which prevents chattering near the set point. Air (sail) switches work identically with a larger, lighter vane suited to duct velocities.
Sprinkler waterflow switches add one crucial refinement: a pneumatic or electronic retard (adjustable 0–90 s, typically set 20–30 s) that delays contact closure so momentary surges from pressure fluctuations don't cause false fire alarms. Shuttle/piston and thermal-dispersion flow switches serve small pipe sizes and low flows where a paddle is impractical, but they present the same Common/NO contact interface to the circuit.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60947-5-1 governs the control-circuit switching elements (utilization categories AC-15/DC-13); IEC 60617 provides the contact-plus-flow-actuator symbol. For sprinkler waterflow duty in IEC-aligned markets, EN 12259-5 covers water flow detectors. |
|---|---|
| ANSI/IEEE 315 | NEMA ICS 5 covers pilot devices; the JIC/NEMA symbol set includes flow switch NO and NC symbols (contact with sail/paddle mark). Fire-alarm waterflow switches are UL 346 listed and their application — retard timing, supervision, alarm within 90 s — is dictated by NFPA 72 and NFPA 13. |
| Key difference | Symbol grammar matches the foot/pressure/float switch family in both systems: NEMA uses a pictorial paddle mark, IEC a composed actuator qualifier. Application rules differ by domain — HVAC proof-of-flow interlocks follow general machinery practice, while sprinkler waterflow switches carry fire-code obligations (UL 346 / EN 12259-5 listings, supervised circuits, retard settings) that generic flow switches do not. |
Terminals / pins
| Pin | Name |
|---|---|
| com | Common |
| no | NO |
Typical values
Typical paddle flow switch contacts are SPDT, rated 10–15 A at 125/250 V AC (resistive) and around 0.5 A at 125 V DC — the classic McDonnell & Miller FS4-3 class carries roughly 10 A full-load at 120 V AC. Actuation set points for liquid paddles span roughly 0.15–3 m/s (0.5–10 ft/s) pipe velocity depending on paddle trim and pipe size (typically 1–8 in / DN25–DN200); air sail switches actuate around 2–10 m/s duct velocity. Pressure ratings run to 10–17 bar for brass/bronze bodies; sprinkler waterflow switch retards adjust 0–90 seconds; hysteresis between actuate and release is commonly 20–50% of set point.
Where the Flow Switch symbol is used
- Chiller and heat-pump interlocks proving chilled-water or condenser-water flow before the compressor is allowed to start
- Fire-sprinkler systems, where a UL 346 waterflow switch with retard initiates the alarm when a sprinkler head opens (NFPA 72/13)
- Pump protection circuits stopping or alarming a pump on loss of flow to prevent dry running and dead-heading
- Duct-mounted sail switches proving airflow before electric duct heaters are permitted to energize
- Cooling loops for welders, lasers, induction heaters, and plastic-moulding machines interlocking power with coolant flow
- Water-treatment and dosing systems enabling chemical injection pumps only while carrier flow is present
Example
In a chiller control diagram, the Flow Switch symbol's Common pin wires to the 24 V control-circuit supply and its NO pin feeds the compressor-enable input of the chiller controller: with the chilled-water pump running and flow above the 0.5 m/s set point, the paddle deflects, Common closes to NO, and the compressor start sequence is permitted; if flow is lost, the contact opens within its release deadband and the controller locks out the compressor before the evaporator can freeze.
Key facts
- A flow switch is a process-actuated pilot device: fluid flow, not an operator, changes the contact state — the paddle/sail qualifier on the symbol identifies it.
- Contact state is drawn at no-flow: an NO flow switch (this symbol's Common/NO pair) is open on the page but closed during normal running flow.
- Its primary industrial role is proof-of-flow interlocking — compressors, heaters, and lasers are only enabled after flow is confirmed.
- Built-in hysteresis (release flow lower than actuate flow, often 20–50%) prevents contact chatter near the set point.
- Sprinkler waterflow switches include an adjustable 0–90 s retard to ride through pressure surges, and NFPA 72 requires alarm initiation within 90 s of sustained flow.
- Paddle blades are usually trimmable/segmented to match pipe sizes from about DN25 to DN200 (1–8 in).
- Typical SPDT contacts handle 10–15 A at 125/250 V AC — enough to switch small pumps directly, though driving a relay or PLC input is better practice.
- Sail (air) versions use a large lightweight vane and actuate at duct velocities of roughly 2–10 m/s, commonly interlocking duct heaters per heater listing requirements.
Frequently asked questions
What does the flow switch symbol look like?
A standard NO or NC contact with a paddle, sail, or flag actuator mark — often with a small flow arrow. JIC/NEMA charts draw a curved sail attached to the contact; IEC 60617 links the general contact to a flow-actuator qualifier with a dashed mechanical line. Only the actuator mark distinguishes it from pressure, float, and temperature switches, whose contact halves are drawn identically.
Is a flow switch shown open or closed on the diagram?
It is drawn in its no-flow (de-actuated) state, per the universal pilot-device convention. So a 'normally open, close on flow' switch — the standard proof-of-flow type with Common and NO terminals — appears open on the schematic even though it is closed for most of the machine's running life. Misreading this convention is a classic cause of interlock logic errors.
What is a flow switch used for in a chiller or heat pump?
As a compressor permissive: the chilled-water (and often condenser-water) flow switch must prove flow before the controller allows the compressor to start, and a loss of flow during running shuts the compressor down. This prevents evaporator freeze-up and dry-running damage. Manufacturers typically specify the set point around 50–70% of design flow.
Why does a fire sprinkler flow switch have a time delay (retard)?
Municipal pressure surges and trapped-air movement can swirl water past the paddle for a few seconds without any sprinkler head having opened. The retard — adjustable 0–90 s and commonly set 20–30 s — requires sustained flow before the contacts close, filtering out false alarms. NFPA 72 still requires the alarm signal within 90 seconds of genuine sustained flow, so the retard cannot be set arbitrarily long.
What is the difference between a flow switch and a flow meter?
A flow switch is a binary device: contacts open or close at a set-point flow, suitable for interlocks and alarms. A flow meter measures flow continuously and outputs a proportional signal (4–20 mA, pulse, or fieldbus) for monitoring and control loops. If the diagram shows a contact symbol with a paddle qualifier, it is a switch; a transmitter would be drawn as an instrument bubble/block with an analog signal line.
Can a flow switch switch a pump motor directly?
Small fractional-horsepower loads can sit within the 10–15 A AC contact rating of many paddle switches, but good practice is to use the flow switch as a pilot device driving a contactor coil, relay, or PLC input. This protects the switch contacts from inrush, enables proper motor overload protection, and lets the controller add logic such as start delays and loss-of-flow lockouts.
Related symbols
- Float Switch symbol
- Limit Switch NO symbol
- Pressure Switch symbol
- Pump (Motor-driven) symbol
- Solenoid Valve symbol
- Temperature Switch symbol
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