By Wu Peng, Senior Process Instrumentation Engineer · Last reviewed October 12, 2026
A paddle flow switch is a flow switch whose sensing element is a thin blade, the paddle or vane, inserted into the pipe through a tee. Flow pushes the blade downstream; at the set point a magnet or lever operates a contact, and a spring swings the blade back when flow falls. It is one of the most common ways to prove water flow in 1 to 8 in pipe.
The published set points look simple, but each one is a gpm figure for one pipe size, with a different value for rising and falling flow. Published tolerances run from ±15% (Kobold PSR) to ±25% (Dwyer FSW-25). This guide puts real vendor tables side by side and converts them to velocity. It then works one sizing example and lists the installation rules and failure modes that decide whether the switch trips where you set it.
Contents
- How it works
- Published set points
- Set points as velocity
- Sizing worked example
- Paddle trimming
- Installation rules
- Where paddles fail
- Paddle alternatives
- FAQ
How it works
In a paddle type flow switch, the blade hangs from a pivot in the switch body and reaches down into the stream. Drag on the blade rises with velocity, and once the force overcomes the return spring the paddle swings far enough to operate the switch. Two designs are common. In one, the paddle carries a magnet that pulls a sealed reed switch; in the other, a lever passes through a seal and trips a snap-action microswitch.
The contact is usually single-pole double-throw (SPDT), though some compact models ship with a single SPST reed. On an SPDT switch, one side closes on flow increase and the other on flow decrease. The same switch can therefore raise an alarm on loss of flow or prove flow for an interlock. The general flow switch guide covers fail-safe wiring of that contact and how paddles compare with other sensing principles.
The spring sets the switch point. Changing its tension with the adjustment screw moves the actuation point, and the falling-flow point moves with it, always lower. The difference is the deadband, and it changes across the adjustment range, which is why vendors publish four numbers per pipe size, not one.
Published set points
Johnson Controls publishes full water tables for its F61 paddle switch. The values below use the factory set of 1, 2 and 3 in paddles, trimmed to fit the pipe. “On” is flow increase (contact actuates) and “off” is flow decrease (contact resets).
| Pipe size | Minimum setting, gpm on / off | Maximum setting, gpm on / off |
|---|---|---|
| 1 in | 4.20 / 2.50 | 8.80 / 8.50 |
| 1-1/4 in | 5.80 / 3.70 | 13.3 / 12.5 |
| 1-1/2 in | 7.50 / 5.00 | 19.2 / 18.0 |
| 2 in | 13.7 / 9.50 | 29.0 / 27.0 |
| 2-1/2 in | 18.0 / 12.5 | 34.5 / 32.0 |
| 3 in | 27.5 / 19.0 | 53.0 / 50.0 |
| 4 in | 65.0 / 50.0 | 128 / 122 |
| 6 in | 190 / 158 | 375 / 360 |
| 8 in | 375 / 320 | 760 / 730 |
Water, horizontal pipe. Source: Johnson Controls F61 installation instructions, table for 1, 2 and 3 in paddles (5 in row omitted). Values for other models differ; confirm per datasheet.
Read the table in two directions. Down a column, the gpm rises steeply with pipe size: a larger bore carries more gpm at the same velocity, and above 3 in the paddle no longer grows with the pipe. Across a row, the deadband shrinks from the minimum to the maximum setting. In 1 in pipe the switch resets 40% below its actuation point at the minimum setting, but only 3% below it at the maximum setting.
Other vendors publish different numbers for the same pipe. The McDonnell and Miller FS4-3 actuates at 6 gpm and resets at 3.6 gpm in 1 in pipe at its minimum setting, against 4.20 and 2.50 gpm for the F61. Always size from the table of the switch you are buying.
Set points as velocity
The blade responds to velocity, so converting the table to velocity makes pipe sizes comparable. With v in ft/s, Q in gpm and the Schedule 40 bore d in inches:
v = 0.4085 × Q ÷ d²
| Pipe size | Bore, in | On / off at minimum setting, ft/s |
|---|---|---|
| 1 in | 1.049 | 1.56 / 0.93 |
| 2 in | 2.067 | 1.31 / 0.91 |
| 3 in | 3.068 | 1.19 / 0.82 |
| 4 in | 4.026 | 1.64 / 1.26 |
| 6 in | 6.065 | 2.11 / 1.75 |
| 8 in | 7.981 | 2.40 / 2.05 |
Calculated from the F61 table above with Schedule 40 bores.
From 1 to 3 in, the switch actuates at roughly 1.2 to 1.6 ft/s and resets at about 0.8 to 0.95 ft/s. Above 3 in the small paddles cover less of the bore and the required velocity climbs to 2.4 ft/s in 8 in pipe. A larger paddle fixes that, as the trimming section shows.
The same conversion explains the upper limit. McDonnell and Miller lists a maximum flow without paddle damage for each FS4-3 pipe size: 27 gpm in 1 in, 105 gpm in 2 in, 900 gpm in 6 in. Converted, every entry from 1 to 6 in works out to about 10 ft/s (10.5 ft/s for 5 in), which matches the vendor’s own warning not to use the switch above 10 ft/s. The pipe velocity calculator runs the numbers for other bores.
Sizing worked example
Take a centrifugal pump on 2 in Schedule 40 pipe with a normal flow of 55 gpm. The pump curve gives a minimum continuous flow of 20 gpm, and the switch must stop the pump below that. These figures are an illustration, not a field case.
1. Velocity check: 0.4085 × 55 ÷ 2.067² = 5.26 ft/s, under the 10 ft/s limit McDonnell and Miller sets for its switches.
2. Range check: the F61 2 in falling-flow point adjusts from 9.50 to 27.0 gpm, so 20 gpm is inside the range.
3. Margin check: the highest actuation point in 2 in is 29.0 gpm, and normal flow is 55 gpm, 90% above it.
4. Tolerance check: at ±25% (the FSW-25 figure), a 20 gpm setting can trip anywhere from 15 to 25 gpm.
The margin clears Dwyer’s rule of thumb, which calls for custom vanes when normal flow exceeds the actuation point by less than 10%. Tolerance is the risk here: a trip at 15 gpm would let the pump run below its minimum. The table gives only the ends of the adjustment, so set the switch on site against a reference, such as a clamp-on meter or the pump curve read from discharge pressure. Then lower the flow and confirm the switch trips before flow falls below 20 gpm.
If the liquid is not water, Dwyer approximates the actuation rate by dividing the water value by the square root of the specific gravity. For SG 1.2 the divisor is 1.095, so the 13.7 gpm minimum actuation point in 2 in pipe becomes about 12.5 gpm.
Paddle trimming
Most paddle switches ship with a stack of blades, or one blade marked with cut lines, and you trim it to the pipe. The paddle must never touch the pipe wall or any restriction. Johnson Controls says to trim its 6 in paddle along the arc marked for the pipe size and to remove any paddle too large to fit.
A longer blade catches more of the stream, so it switches at lower flow. In 6 in pipe the F61 actuates at 190 gpm with the 1, 2 and 3 in set, but at 74 gpm with the 6 in paddle trimmed to fit. In velocity terms that is 2.11 ft/s against 0.82 ft/s; with the trimmed paddle, 6 in pipe switches at a lower velocity than the small paddles manage in 3 in pipe.
Pipe material matters too. Dwyer restricts the 1 in trim of its FSW-25 to metal pipe and asks for at least 1-1/4 in line size in plastic pipe.
Installation rules
Give the paddle a settled velocity profile. Johnson Controls asks for at least five pipe diameters of straight pipe on each side of the switch, measured from the nearest elbow, valve or other restriction. Kobold gives the same five-diameter figure for its PSR series. The straight run requirements guide explains why a swirl or jet from a fitting moves the trip point.
Mount the switch on horizontal pipe with the body upright, or on vertical pipe with upward flow if the model allows it. Never mount it on a line with downward flow. Some models allow only the upright position: Dwyer wants its V4 within 5 degrees of vertical. Johnson Controls notes that its F61 trips at a slightly higher flow in upward flow, and Kobold publishes PSR values for horizontal installation only.
Point the arrow on the body with the flow. Johnson Controls says the switch should not see water hammer, and calls for an arrester when a fast-closing valve sits downstream. Finally, do not set the switch below its factory setting: Johnson Controls warns that the paddle may then fail to return to the no-flow position.
Where paddles fail
Most paddle switch problems start with what is in the water. RS Components describes paddles as very sensitive to foreign objects and residue buildup; fibers that catch on the blade can hold it in the flow position. Iron particles collect on magnet-coupled designs; Kobold recommends a magnetic filter for iron-bearing media. Heavy debris can bend a thin vane or jam the pivot.
Velocity and viscosity set hard limits. Above about 10 ft/s the blade can be damaged, as the FS4-3 data shows. Thick liquids drag harder on the blade at the same velocity and can shift the set point; Dwyer limits its FSW-25 to 43 cSt (200 SSU).
Fire sprinkler vane switches are a separate product. They are built for wet-pipe systems only; System Sensor warns that the sudden inrush of water in dry, deluge or preaction systems can break the vane. Do not substitute a process paddle switch for a listed waterflow switch, or the reverse. The water flow switch guide covers the sprinkler rules, from tamper signals to test intervals.
Application example
Power generation, South Pacific. A power plant asked for a flow switch on a DN50 PN16 process line with a flow range of 1,200 to 10,000 L/h and two low-flow alarms at 81 and 76 L/min. The two points sit only 6% apart, closer than the ±15% tolerance Kobold publishes for its PSR paddle switch. The request was quoted with a target-type flow switch in a DN50 body with 304 stainless flanges and a cast aluminum housing.
Paddle alternatives
A paddle switch is the right tool for clean, water-like liquids in 1 to 8 in pipe where flow or no flow is all you need. For other fluids, sizes or tighter set points, another principle usually holds the set point better.
| Option | Choose it when | Avoid it when |
|---|---|---|
| Paddle | Clean water or glycol, 1 to 8 in pipe, velocity under 10 ft/s | Fibers, iron fines, viscous liquids, tight set points |
| Piston | Small lines and low flows below a paddle’s range | Magnetic particles in the fluid |
| Thermal | Dirty water, gas, large pipe, no moving parts wanted | The trip must be faster than a few seconds |
| Meter with alarm | Two alarm levels, trending, or a precise low-flow limit | Budget allows only a contact |
Our thermal flow switch has no blade to foul and responds in 1 to 10 s (typically 2 s), up to 260 °C. When the control system should alarm on a measured value, a target flow meter sends 4-20 mA from lines up to 42 MPa. For a local reading with a built-in contact, the metal tube rotameter offers an alarm option on its indicator head.
If you were looking for a paddle that detects powder or grain in a silo, that is a rotary paddle level switch, a different instrument.
FAQ
What is a paddle flow switch?
It is a flow switch that senses flow with a thin blade inserted into the pipe. Moving liquid deflects the blade, and at the set point the blade operates a contact. It answers flow or no flow and is used mainly on water and glycol lines; tee-mounted types cover 1 to 8 in pipe.
How does a paddle type flow switch work?
The blade hangs from a pivot and is held back by a spring. As velocity rises, drag on the blade overcomes the spring and the blade swings far enough for a magnet or lever to operate an SPDT contact. When flow falls, the spring pulls it back and the contact resets at a lower flow.
What is a paddle switch used for?
In piping, a paddle switch proves liquid flow. It stops a pump on loss of flow, keeps a chiller or boiler from running without water, and confirms cooling water to equipment. In silos and bins, a rotary paddle switch is a different device that detects the level of powders and granules.
What is a flow paddle?
The flow paddle is the blade of a paddle flow switch: a thin metal vane, often stainless steel or brass, that sits across the stream. It is trimmed or selected to suit the pipe size, and its length sets how much flow it takes to operate the switch.
How to wire a flow switch?
Wire the common to the contact that stays made while flow holds the paddle deflected; a loss of flow or a broken wire then opens the circuit. Check the contact rating against the load, and drive a motor starter through an interposing relay if the switch rating is lower than the coil needs.
Request a quote
Send us the liquid, pipe size and material, normal and minimum flow, pressure, temperature and what the contact must switch. We will tell you whether a paddle switch will hold that set point or whether a thermal switch or a meter with an alarm is the better fit. Tell us the application and we configure one unit, not a shelf part. Reach our application engineers or use the form below.
Written and technically reviewed by Wu Peng and the Instranova engineering team.