The Complete Guide to Flow Switches

This guide is part of our Industrial Automation hub where you can discover more about AI, automation and control.

A flow switch is a device used to monitor the flow rate and pressure of liquids, air or other gaseous media through a duct, system or loop.

These switches or sensors can be used to monitor flow over a given period of time, or set up to continuously monitor total flow. Technically, a flow meter or indicator may not necessarily be a true flow ‘switch’ - if the device only registers and displays information, it’s more properly called a meter or indicator.

In most cases, though, they’ll be able to trigger actions or stop/start other components elsewhere in the system - pumps, for example - in order to adjust flow rate and pressure to the desired and programmed levels. This is then a true flow switch.

To understand how a flow switch works and what it does, it’s helpful to outline the core components that make up a typical switch or sensor of this kind.

Many types will include a paddle or magnetic trigger of some sort (the primary device), which is connected to a circuit and placed in the channel through which liquid or gas is passing. This paddle is displaced or rotated by whatever substance is flowing by, and sends a signal reading back to a secondary component known as the transducer.

The transducer takes this raw signal from the paddle and passes it on to a transmitter in a readable format. The transmitter, in turn, measures this reading against a predefined set of parameters and performs whatever signal or action is required to adjust the behaviour of components and mechanisms elsewhere.

In this way, flow switches are able to monitor, report and control the flow rate of liquid or gas through a specific part of a system, or through the entire system, ensuring flow stays within those pre-set parameters. If the rate exceeds or drops below what’s required, it can instantly trigger a series of actions such as activating an alarm, powering on a pump, diverting flow, shutting off parts of the system. Which actions a switch performs will depend entirely on the type of switch and what it’s been designed for.

Not all flow switches will have a physical paddle as such. Ultrasonic and other non-intrusive versions are also fairly commonplace, which work by bouncing a signal back off the medium being monitored. This is particularly useful in cases where the medium is contaminated, physically damaging or otherwise hazardous, or where it’s helpful to have no moving parts exposed to constant wear and tear.

Rotaflow or Variable Area Flow Sensors are more simple devices that use a tapered tube with a float (or sometimes an electromagnet) to measure, restrict or allow the free passage or gas or liquid through. Various types of magnetic flow switches are available, and they can be very effective in a range of applications as long as the medium is somewhat conductive.

Because there are so many different kinds of flow switch, as we’ll see in subsequent sections, there’s also a very wide range of flow switch installation methods and techniques. When looking for information on how to wire a flow switch, most important thing is to have a clear understanding of which type of switch you’re using and for what purpose.

There are some basic rules of thumb that apply to most flow switch installation methods, however. Broadly speaking, these include:

• Installing flow switches on a straight section of pipe or duct
• Ensuring there’s a good length of straight pipe both in front of and behind the switch - ideally, the equivalent of 10x the pipe’s diameter
• Avoid fitting or wiring a flow switch near to bends, other fittings, valves, drains, narrower or wider sections of pipe, and any other features that might cause obvious fluctuations in flow rate through this area

Just as the exact methodology for installing a flow switch will depend on what kind of switch you’re using and where, the same is true for testing the healthy function of any device already installed. To know how to test a flow switch, you’ll need to know precisely what it is you’re testing for.

In many cases, you’ll be able to confirm a flow switch is working properly simply by observing it in action and checking that the correct responses are triggered when flow starts or stops. In other cases, you may need to use an ohmmeter to measure the resistance of the switch's electronics and confirm that there’s a completed circuit (or ‘continuity’) being registered when the switch is activated.

If either of the above steps don’t produce the required result, it may be time to replace or repair the switch. For certain types and locations of flow switch - including those attached to safety equipment such as fire sprinklers in public places - it’s typically a legal obligation to have them tested regularly.

There are numerous different types of flow switches available, and each will take a subtly - but importantly - different approach to measuring linear, nonlinear, volumetric or mass flow rate of liquids or gasses. Before ordering or installing a flow switch, it’s extremely important to have a clear understanding of which specific type you need for the exact application you have in mind.

No single variety or brand of flow switch will perform any/all flow switch tasks to a suitable standard in any given scenario, so you’ll need to know what sort you need for the precise function it’s intended to perform.

There are numerous different types of water flow switches available. In the sections below, we’ll explore some of the more common ones.

Flow Switches for water pumps

A water pump flow switch is designed to make automatic, on-the-fly adjustments to the operating speed or volume transfer of an electric pump attached to a liquid system. By sensing the gravitational movement and overall flow rate of water in the system, it can send on/off signals to pumps that in turn raise or lower the system’s internal water pressure.

This can also prevent pumps from running dry in the event of a water supply failure, which would otherwise quickly start to damage the pump.

Flow Switches for sprinklers

A sprinkler flow switch is most often - and most critically - found in automatic fire sprinkler systems. In this scenario, its main function is to detect when water passing through the sprinkler pipework and valves exceeds a given flow rate, indicating that the sprinkler system has been triggered.

This will then send a signal back to the fire department that they need to come out and investigate. Most flow switches designed for this use also incorporate a delay timer that will only send out an emergency signal once continuous flow has been detected for a given period of time. This is to avoid false positive detections caused by unrelated or harmless changes in pressure elsewhere on the system.

Magnetic water Flow Switches

Magnetic flow switches function according to Faraday’s Law of Electromagnetic Induction, which essentially states that the voltage induced is proportional to the speed of movement when a conductor (water, in this case) is passing through a magnetic field.

In short, movement speed and volume is measured by generating a magnetic field for the water to pass through. This information is read by an electrical sensor, which in turn triggers an action based on the reading.

For a magnetic water flow switch to function properly, the liquid passing through it must be at least somewhat conductive. They’re often found in wastewater systems.

Inline water Flow Switches

An inline water flow switch is one that’s installed more permanently as an integral part of the overall system. This can be contrasted with an ‘insertion’ flow switch, which uses a probe or paddle introduced into the process pipe from externally.

Inline switches tend to cost more and are more complex to install and maintain, but they require far shorter straight run sections to work properly and can usually automatically condition the flow once installed.

Paddle water Flow Switches

Paddle water flow switches rely on a physical probe or paddle to hang in the pipe through which the water or other liquids will be passing. This is then turned or triggered at varying speeds and pressures depending on the flow rate of material through the channel, and these signals are converted into readings and actions.

Paddle flow switches for water are very good at consistently and reliably measuring flow rates, but they’re very sensitive to foreign objects and residue build-up, as well as needing periodic maintenance due to wear on their constantly moving parts.

Air flow switches can be used for air filtration and supply systems, duct heating, exhaust venting and much more. Like their water equivalents, they can be designed as either mechanical flow switches or non-intrusive varieties that sense rate of passage by other means.

In the following sections we’ll cover various examples of common air and gas flow switch uses.

Air Flow Switches for ducting and HVAC

Air flow switches are widely found in all manner of ducting and HVAC systems, often using a (mechanical) paddle-type operation to trigger a microswitch when flow increases or decreases beyond a set rate or volume.

In HVAC and ducting systems, many air flow switches can also be used to perform actions such as air handling, fan monitoring and filter monitoring.

Paddle air Flow Switches

Much like their water paddle flow switch equivalents, a paddle air flow switch - sometimes known as a vane-type switch - is a mechanical sensor that partially protrudes into the duct being monitored.

Gases and air being sent through the duct will physically move a paddle sitting directly in the air flow channel, and this, in turn, will trigger an action when flow increases or decreases beyond the desired rate.

Air Flow Switches for boilers

An air flow switch in a boiler will generally monitor the proper venting of waste gases from the system by fans via the flue. These gases must be safely expelled before the boiler fires up, and the boiler air flow switch is usually rigged to only allow ignition once it recognises the fans are already spinning at the required speed.

In the case of a blocked flue or fan not working properly, the air flow switch will detect that the pressure hasn’t dropped sufficiently, and won’t allow the boiler to fire on until the problem is addressed.

Inline Air Flow Switches

Again, much like an inline water flow switch, inline air flow switches are installed as an integral part of the duct through which gas or air is being fed. This makes them trickier to install and maintain in the first instance, but they also tend to be more multi-functional, better at reading continuous flow over longer periods of time, and generally require less upkeep than insertion or paddle flow switches.

Paddle type Flow Switches

Paddle flow switches are a type of mechanical switch that’s activated directly by pressure from the medium - usually liquid - passing through the duct or channel into which the switch is inserted. The paddle is often a small strip of metal or plastic that hangs inside the pipe and is attached to either a tensioned spring or a series of magnets.

In its resting position, it is considered open or closed, depending on the function of the switch and the larger system it’s part of. Any medium passing through the duct with sufficient (or insufficient) force to move the flow switch paddle to a second position will complete a circuit, and trigger an action or alarm response.

Thermal dispersion Flow Switches

A thermal dispersion flow switch is a ‘solid state’ piece of equipment, meaning it contains no moving parts (as opposed to a mechanical paddle - or vane-type product). A thermal flow switch involves a sensor probe being inserted into the centre of the flow in a duct or pipeline.

This probe is heated continuously through an input of electrical wattage, and the principle of thermal dispersion allows the rate of gas or liquid flow to be calculated according to how quickly that heat is ‘carried away’ by the molecules flowing past.

The amount of wattage having to be sent to the probe to maintain a consistent temperature gives the necessary numbers for flow rate to be calculated very accurately. As well as accuracy, thermal dispersion flow switches are prized for their flexibility (measuring both very low and very high flow rates), their ruggedness and their ease of insertion.

Oil Flow Switches

Fuel or oil flow switches work exactly like other liquid flow switches - and as such, they come in various configurations, with the most common being paddle or thermal variants.

Switches designed for use with fuels and oils tend to be somewhat more hard-wearing than versions intended for less aggressive media, and can usually operate at higher temperatures with more robust seals and housing.

Ultrasonic and non-intrusive Flow Switches

An ultrasonic flow switch is a popular variety of non-intrusive flow switch, meaning they don’t actually have to penetrate the wall of the pipe, duct or channel you’re attempting to monitor flow through.

Instead, an ultrasonic version can be clamped to the outside of the duct, where it reads and reacts to flow rate by pinging a signal back and forth from sensors. This signal is refracted and reflected by particulates or bubbles in the stream, before returning to the sensor, and the flow rate can then be calculated using the Doppler effect. (The Doppler effect is the reason an ambulance siren seems to change pitch as it passes you in the street!)

Because ultrasonic flow switches generally rely on particulates or aeration in the liquid, they tend not to be suitable for drinking or distilled water, and are much more commonly found monitoring wastewater and other ‘dirty’ media. Clamp-on varieties will only work with certain duct types due to risk of interference with the signal, and are seldom seen on lined pipes for that reason.

Mechanical Flow Switches

Mechanical flow switches are any type of switch that has one or more moving parts triggered by direct physical contact with the medium being monitored. They usually work by the movement of components causing a circuit to be completed, which in turn triggers the action or response needed.

Paddle-type flow switches are a common example of a mechanical flow switch. Mechanical switches need to be tested and periodically replaced, as a continual physical movement of parts eventually leads to wear and tear that can affect operation.

PVC Flow Switches

PVC flow switches are often among the most basic type of product for this application and usually favoured where a quick visual confirmation of flow/no flow status is required. A PVC flow switch is usually mechanical and can be either vane-type or magnetic with one or more moving parts.

They tend to be fairly economical to install, and are generally quite rugged in construction. They’re usually designed for easy cleaning and maintenance as well as offering good chemical compatibility and anti-corrosive properties.

Industrial Flow Switches

Industrial flow switches tend to be larger, more robust versions of many of the flow switch types outlined above. They’re usually able to withstand significantly higher volumes, pressures and flow rates, and are often designed to handle far more aggressive, heavily contaminated or hazardous materials.

Industrial flow switches often eschew magnetic operating parts, in order to function well with water containing rust and other metal particulates.

Calorimetric Flow Switches

Understanding how calorimetric flow switches work is fairly simple if you’ve already got to grips with the function of their closely-related thermal dispersion equivalents. A calorimetric switch uses two temperature sensors, one of which is heated and one of which monitors the ambient temperature of the medium in the channel.

When the difference between these two readings is compared, the flow rate can again be calculated - faster flow will result in a smaller difference, because heat is carried away more quickly from the warmed sensor under higher flow rates.

Pneumatic Flow Switches

A pneumatic flow switch generally substitutes an air valve for the mechanical or paddle-type switch and can be used to control the passage of air through relays or valves when the flow starts or stops.

An air valve will often be set up to bleed pressure from a system if the flow rate falls to too low a level. Pneumatic flow switches are also useful in applications where there’s a necessary pressure differential between different areas of ducting.

Adjustable Flow Switches

An adjustable flow switch is any type of device that allows the user to quickly and easily recalibrate the desired flow settings without disassembly of the unit. This is usually done with a flat-head screwdriver, opening or closing to adjust the sensitivity of a vane or paddle located inside the housing of the switch.

Rotary Flow Switches

A rotary flow switch functions as a basic turbine, generally in the form of a multi-bladed or paddled wheel mounted entirely within the duct’s stream on a free-spinning bearing. It registers flow rate as liquid (the most common medium for rotary flow switches) passes over it. They’re available in impeller, piston (shunt) and paddlewheel designs.

To know how a flow switch is used, you first need to know what task the switch is intended to perform in the specific system you’re talking about. The vast majority are intended to operate fully automatically unless damaged or degraded, so knowing how to use a flow switch is often more a case of knowing how to tell when it’s not functioning as it should. That will depend entirely on the role it plays in a given system.

Flow Switches on boilers

Flow switches on boilers can be found in both water and gas pipelines. For water channels, they’re generally installed to protect the boiler from operating in low/no flow states. Gas boiler flow switches are mainly used to prevent the boiler from firing before any build-up of gasses has been vented by fans via the flue.

Flow Switches for pumps

A pump flow switch is usually installed to prevent the pump from running dry in the event of a failure in the water supply. As water pumps can quickly overheat or become damaged by continued running in low/no flow situations, flow switches for pumps often function to shut down the power to the pump temporarily when it isn’t being supplied with enough liquid.

Flow Switches for shower pumps

A shower pump flow switch helps maintain water pressure being sent up through the piping and out of the shower head. They can be found on both hot and cold supply channels, and are generally mechanical.

Demand for water to the shower head causes the switch to activate and this, in turn, completes a circuit which tells the pump to begin operating. Over time, shower pump flow switches can degrade due to limescale, heat and other issues, and may eventually need replacing.

Flow Switches for heat pumps

A heat pump flow switch is often found in swimming pools and spas, where a fairly large heat pump is required to bring the water up to the desired temperature once it’s switched on, and recognises a demand for both warmth and water flow. The flow switch, in this case, works to stop the pump from operating when there’s an interruption in the water supply or when the pool is not in use.

Flow Switches for pool pumps

Pool pump flow switches perform very similarly to the function outlined above for heat pumps - the switch prevents the pool pump from continuing to run in low/no flow situations, which can cause serious damage to the component, and when the pool isn’t being used.

Flow Switches for sump pumps

Sump pump switches are typically installed to ensure that the water from a drainage system won’t overflow. Again, running a non-submersible pump dry will quickly damage it, so the switch will also help protect the workings of the pump by shutting it off if the water supply fails or the flow rate falls too low.

Flow Switches for hot tubs and jacuzzis

A hot tub flow switch also protects the pump components in spas, jacuzzis and hot tubs from overheating if the flow rate is too low. In the event of a water flow failure or an air pocket forming in the pump, the switch will open to prevent a meltdown of the pump or heating elements.

High temperature and pressure Flow Switches

High temperature flow switches are commonly used in industrial or manufacturing systems, where a continuous flow of liquids or gases sometimes needs to be maintained at temperatures up to 350 Celsius and above.

High pressure flow switches are designed and built ruggedly for use in similarly challenging applications where they’re likely to be subjected to far greater forces and flow rates from various media than would normally be found in household systems.

Sanitary and hygienic Flow Switches

Sanitary flow switches are often necessary in industries where hygiene is especially important - this might include food production, pharmaceuticals, medical and laboratory environments, or in industrial research and development where it’s important to keep the risk of contamination to a minimum.

They’re usually made from stainless steel and sometimes have completely sealed-off inner chambers housing turbine or magnetic switches.

Chiller Flow Switches

Chiller flow switches are usually installed to protect against low/no flow scenarios which can lead to freeze damage. They’re often found on industrial, commercial or institutional air-cooling systems, as well as chilling many different kinds of hot-running equipment including machine tools, medical imaging units, food and beverage production systems and more. Chiller flow switches can be either air or liquid types, depending on the compression system a given cooler is designed to use.

As well as identifying the correct type of flow switch for the position and application you need, it’s also important to select the right size and voltage of switch to work properly in the duct or system you’re installing to. You can usually filter online searches for flow switch sizes and by voltage, making it easier to find the product you need.

1 inch flow switches

1-inch flow switches of all kinds are intended for use in pipes or ducting of 1-inch diameter.

2 inch flow switches

2-inch flow switches of all kinds are intended for use in pipes or ducting of 2-inch diameter.

4 inch flow switches

4-inch flow switches of all kinds are intended for use in pipes or ducting of 2-inch diameter.

24 volt flow switches

24-volt flow switches are commonly found across a wide range of switch types and sizes, including paddle, rotary and electromagnetic flow switches for both gas and liquid applications.

There are various leading brands and flow switch manufacturers supplying the many different types of switches, accessories and systems they’re widely used in. Some of the best-known and most popular flow switch manufacturers include Honeywell and Dwyer.

Honeywell Flow Switches 

Honeywell flow switches are favoured in many industries due to their reliability, high accuracy, and precision operating characteristics. This inherent accuracy over life reduces the need for frequent recalibration and makes Honeywell flow switches a leading choice in applications such as heating, ventilation, and air conditioning systems, medical respirators and ventilators, gas metering and leak detection equipment.

Dwyer Flow Switches 

Dwyer Instruments flow switches are available in a wide range of configurations, including paddle or vane-type and thermal dispersion switches. They’re ideal for automatically protecting equipment and pipeline systems against damage from reduction or loss of flow, and Dwyer flow switches also have a leading reputation for long-term reliability and high life expectancy before requiring maintenance or replacement.

Selecting the correct type, size, voltage and function of flow switches for the precise application you’re installing them to is crucial if they’re to work properly in the role you need them to perform.

With such a wide range of products and brands available, it can be a confusing field to navigate, which is why we’re always happy to offer guidance and advice through our various support channels - drop us a line any time with questions or queries, and our expert team will be able to point you in the right direction.