The standard symbol for a flow switch on a Piping and Instrumentation Diagram (P&ID) is a circle with an 'FS' tag (often FSL for Low or FSH for High), connected to a process line with a paddle or propeller icon. In electrical control schematics, the symbol for a flow switch is drawn as a standard limit switch contact (Normally Open or Normally Closed) actuated by a mechanical linkage, typically represented by a wavy line or a small paddle icon adjacent to the contact arm.
Because fluid systems cross over between plumbing, HVAC, and electrical controls, reading these symbols correctly prevents catastrophic dry-firing of boilers or burned-out pump seals. Below is the definitive reference for identifying, interpreting, and wiring flow switches across major global standards.
Complete Flow Switch Symbol Reference Table
Use this table to decode the specific graphic you are looking at on a prints package or ladder diagram. The 'FS' prefix always denotes a Flow Switch, while the third letter dictates the specific trigger state.
| Symbol Type / Tag | Graphic Representation | Meaning in Practice | Standard / Context |
|---|---|---|---|
| FSL (Flow Switch Low) | Circle with 'FSL', wavy line inside, linked to pipe | Triggers or alarms when fluid flow drops below a set minimum threshold. | ISA-5.1 / P&ID |
| FSH (Flow Switch High) | Circle with 'FSH', wavy line inside, linked to pipe | Triggers when flow exceeds a maximum safe limit (rare in HVAC, common in chemical dosing). | ISA-5.1 / P&ID |
| Electrical NO Flow Switch | Two parallel lines (contacts) with a wavy line actuator pointing away | Normally Open. Contacts close to complete the circuit only when adequate flow is detected. | IEC 60617 / NEC Ladder |
| Electrical NC Flow Switch | Two overlapping lines (contacts) with a wavy line actuator pointing away | Normally Closed. Contacts open to break the circuit (safety interlock) when flow drops. | IEC 60617 / NEC Ladder |
| HVAC Ladder Flow Proof | Limit switch symbol with a 'paddle' icon and 'FS' label | Used in boiler interlocks; proves water is moving before the gas valve is allowed to open. | NFPA 85 / JIC |
Regional Standards & Variants (NEC vs. IEC vs. Old UK)
While the P&ID process symbols remain relatively consistent globally via the International Society of Automation (ISA-5.1 standards), the electrical schematic symbols vary heavily by region. Misreading the electrical symbol can lead to wiring a safety interlock backward.
| Standard | Region | Visual Style | Actuator Symbol |
|---|---|---|---|
| IEC 60617 | EU, UK, AU, Global | Rectangular outline for the sensor body; distinct NO/NC contact boxes. | Semi-circle or wavy line inside a small box attached to the contact. |
| NEC / NFPA 79 (JIC) | North America | Line-and-rung ladder logic; contacts drawn as literal mechanical arms. | A literal 'paddle' or 'propeller' icon drawn on the mechanical linkage arm. |
| Old UK (BS 3939) | Legacy UK Systems | Similar to IEC but uses older, hand-drawn style mechanical linkages. | A simple diagonal line with a loop at the end, representing a physical float or paddle. |
The 'Rows People Get Wrong' Notes
When troubleshooting or drafting, these are the specific symbol confusions that cause field rework or failed inspections.
- Flow Switch (FS) vs. Pressure Switch (PS): This is the most common error. A pressure switch symbol uses a semi-circle or dome shape (representing a diaphragm) as the actuator. A flow switch uses a wavy line (representing fluid turbulence) or a paddle. If you wire a pressure switch into a flow-proof circuit, the system will fire even if the pipe is blocked, as long as static pressure is present.
- Flow Switch (FS) vs. Flow Transmitter (FT): A switch (FS) is a simple dry contact—it is either open or closed. A transmitter (FT) has a circle with a 'T' and outputs a continuous 4-20mA analog signal or a digital pulse. Do not wire 24VDC into the dry contacts of an FS expecting a variable signal; you will short the power supply.
- The 'Normal' State Confusion: In electrical terms, 'Normal' means the state of the switch when the system is de-energized and at rest (no flow). A 'Normally Closed' (NC) flow switch used for boiler protection is closed when the pump is off. When the pump turns on and water flows, the paddle moves and the switch opens. If your ladder logic expects a closed contact to prove flow, you must use a Normally Open (NO) switch.
Decision Path: Selecting and Wiring the Right Flow Switch
Use this decision tree to terminate your design phase and pick the exact hardware for your panel. This path assumes a standard 120VAC or 24VAC hydronic heating or cooling control circuit.
| Application Scenario | Required Contact State | Required Body Material | Concrete Hardware Pick |
|---|---|---|---|
| Boiler dry-fire protection (safety interlock) | NO (Closes on flow to prove movement) | Brass (compatible with copper/iron hydronic loops) | Honeywell L404B (Paddle type, 1' NPT) |
| Chiller cooling water proof (high flow rate) | NO (Closes on flow) | Bronze or Stainless Steel (corrosion resistant) | Gems Sensors RFS-150 (Thermal dispersion, no moving parts) |
| Pool pump dry-run protection | NC (Opens on loss of flow to drop contactor) | PVC / CPVC (chemical resistance to chlorine) | Hayward Aquarite Flow Switch (Clear housing, visual paddle) |
Default Recommendation: If you are building a standard prosumer hydronic heating manifold or replacing a failed switch on a residential boiler and the original print is missing, default to the Honeywell L404B. It is the industry workhorse, features an adjustable brass paddle that can be trimmed to fit 1' to 8' pipe diameters, and its NO contact logic aligns with 95% of North American residential boiler control boards.
Safe Interpretation When Markings Are Faded or Missing
On legacy equipment, the physical tag on the flow switch may be painted over, corroded, or missing entirely. Do not guess the wiring based on wire color alone—field technicians frequently use whatever spool was left in the truck. Use this verification protocol to safely identify the switch type and state.
- Isolate and Verify Dead: Turn off the control circuit breaker. Use a non-contact voltage tester, followed by a multimeter set to AC Voltage, to confirm 0V across the switch terminals.
- Set Multimeter to Continuity/Ohms: Place your probes across the two control wires connected to the switch.
- Test the 'At Rest' State (Pump OFF):
- If the meter reads < 1 Ohm (beeps), the switch is currently closed. It is either a Normally Closed (NC) switch, or a Normally Open (NO) switch that is mechanically stuck in the 'flow' position due to debris.
- If the meter reads OL (Open Loop / Infinite), the switch is currently open. It is either a Normally Open (NO) switch at rest, or an NC switch stuck open.
- Test the 'Active' State (Pump ON): Safely restore power, start the pump, and listen/feel for the mechanical click of the paddle. Re-test continuity (using insulated probes and extreme caution if measuring live, or power down immediately after flow is established to measure safely). The state must invert. If it does not invert, the paddle is broken or the microswitch is fused.
- Rule out Transmitters: If you measure a steady resistance (e.g., 250 ohms) or read a milliamp value (4-20mA) instead of a simple 0 or OL, you are not looking at a flow switch. You have a flow transmitter. Refer to the PLC analog input manual, not the binary ladder logic.
By strictly adhering to the P&ID tags for system design and the electrical schematic symbols for panel wiring, you eliminate the ambiguity that leads to nuisance trips or, worse, silent safety failures. Always verify the physical hardware against the schematic with a meter before energizing a new fluid control loop.
