A flow switch is an essential safety device that must be installed within any appliance that requires a flow of gas or liquid in order to operate, such as a boiler. Flow switches ensure that the gas or water that’s running to the appliance remains within set parameters. If the flow is not within these parameters, the switch can open or close to resolve the problem.

Liquid flow switches can be found in air conditioning units, pump systems or sprinkler systems in apartment buildings. Gas or air flow switches are used in clean room filter systems and air conditioning.

But where is the flow switch situated in your boiler and how do you know if it’s working correctly? We’ve answered those questions and more in this helpful article. 

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What is a flow switch on a boiler?

As described above, a flow switch works by allowing or stopping the flow of gas or liquid through a pipe. In a boiler, the flow switch is there to prevent the boiler from igniting if there is too much gas in the system that hasn’t been expelled. It will only allow the boiler to fire up if it detects that the fans are spinning at the required speed to get rid of excess gas. A boiler flow switch can also tell if the flow of water has been interrupted or has stopped completely, and will prevent the boiler from firing up in this case too.

There are a variety of flow switches available, depending on the type of system they’re installed in, and each one may work slightly differently. The main types are:

  • Paddle flow switch
  • Variable area flow switch
  • Magnetic flow switch


A paddle flow switch is the most common type of switch. It is a spring-return switch with a sensor that can monitor the flow. The switch will close when flow is present and open when there is no flow.

A variable area flow switch makes use of a tube with a device inside that is able to float. This device measures the amount of gas or liquid passing through and will restrict the flow accordingly.

A magnetic flow switch is best for water/liquid flow. It works by creating a magnetic field that the liquid can pass through. When the liquid goes through the magnetic field, it triggers an electrical sensor which will produce an action based on the flow rate.

How does a boiler flow switch work?

When a boiler flow switch first detects a problem with gas flow, there are a series of actions that need to take place to make the boiler turn off.

When the flow switch first detects a problem, it will alert another device - usually a transducer - that there is too much or too little flow. Depending on the type of switch you have, it could displace or rotate because of the gas flowing through the pipe. It is this movement that sends a signal to the transducer.

The transducer is able to interpret this signal and pass the message along to the transmitter. The transmitter takes the data and measures it against the preset parameters to see what the flow of gas is like. It is also able to perform an action to counteract the flow issue. The action could include the sounding of an alarm, turning on a pump or shutting the system off. 

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How to change a flow switch on a boiler

If your boiler isn’t turning on, it could be because the flow switch has sensed an issue and is sending a signal to the transmitter which stops your boiler from igniting. This is why a flow switch is an excellent safety device. You don’t want your boiler to turn on if there’s an issue as this could be dangerous for you and your family.

The most common problem involving flow switches is the accumulation of debris in the system. Debris and rust can clog the mechanism, which results in the flow switch being stuck in the on or off position. A suitably qualified engineer will be able to check this for you and remove the debris accordingly or they may suggest that they replace the part entirely.

You may also have a faulty flow switch that’s causing the problem. You should get a suitably qualified engineer to take a look at your system to determine if there are any major reasons why your boiler isn’t turning on. Below are some common reasons why your boiler may not be starting up:

  • A faulty fan
  • A broken printed circuit board (PCB)
  • A blocked flue

As mentioned above, a flow switch will prevent the boiler from igniting if the fan isn’t running correctly. A blocked flue is also a problem that can be detected by the flow switch. This may mean the gas is unable to go anywhere, which could result in carbon monoxide being released into your home. You may also have an electrical fault with the PCB. The PCB is the main board that runs and controls the boiler. If you can smell burning or there’s no power on the display panel, you may have a problem with the boiler’s circuit board.

If an engineer rules all of these options out but is still unable to find the problem, you may have a faulty flow switch. Most gas boilers come with a flow switch already installed inside the boiler. It will be fairly easy for a suitably qualified engineer to test the flow switch as it can be done without having to disconnect the electrical leads or dismantle the switch.

If the engineer determines that you need a new flow switch, they will replace it for you. You shouldn't take the casing off your boiler or try to fix the problem yourself. 

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