James Elston

Published April 9, 2024

A Glow-worm F75 error code indicates that there is a fault with the boiler’s pressure sensor – an electronic component. This sensor monitors the pressure that flows through the unit. 

Glow-worm boilers are known for their reliability and energy efficiency. As with most boilers, error codes show up as indicators of underlying issues. If you are suddenly faced with an F75 fault, do not ignore the problem.

Leaving the issue unattended could cause your boiler to stop working, resulting in several avoidable problems.

Not to worry, though! This article will explain what the F75 error is, what causes it, and what you can do to fix it.

Understanding The Glow-worm F75 Fault Code

As mentioned, an F75 error message on your Glow-worm boiler indicates a faulty pressure sensor.

The pressure sensor regulates the pressure of fluids like air, gas, or water within your boiler. The purpose of this sensor is to ensure the pressure remains level by allowing or blocking the flow of fluids.

An electrical fault could cause malfunctions, triggering the F75 error.

The error code could also come from the printed circuit board. When the heating process begins, the printed circuit board (PCB) tracks the volume of water flowing through the boiler. A faulty water pressure sensor could send readings to the PCB, causing it to shut down and display an F75 or F9 code, depending on your boiler model.

These fault codes provide crucial information for potential faults. This ensures that both you and a Gas Safe registered engineer can quickly identify and resolve the problem.

Causes of Glow-worm F75 fault

Let’s delve into the possible reasons for an F75 error code:

Pressure sensor blockage

One common cause of the F75 fault is when heating sludge or limescale buildup blocks the sensor. You can easily identify the blocked pressure sensor when the pressure gauge doesn’t increase once the heating is turned on.

Low water pressure

A low water pressure flow could mean that your boiler will have difficulty heating water. You must monitor the water pressure gauge on your Glow-worm boiler regularly and take the necessary steps if it falls below the recommended level.

An issue with heat pump

One possible cause of the F75 fault code is problems with the heat pump. Since this error is related to water circulation, if the heat pump isn’t functioning correctly, the PCB cannot accurately confirm if the water pressure is flowing as it should.

There are several reasons why a heat pump may not work effectively, including: 

  • Worn-down bearings, especially when the pump is older
  • Incorrect speed settings during installation
  • Sludge buildup, causing a blockage and airlocks 

To accurately identify and resolve the issue, it’s recommended to have a Gas Safe technician assess and diagnose the problem.

Fixing The F75 Fault In 5 Steps

You may be able to fix the F75 fault on your boiler by repressurising the system and resetting the boiler. 

Resetting your Glow-worm boiler is usually an easy process. However, it is crucial to prioritise your safety and consider seeking professional assistance when necessary. Before attempting any DIY repairs, ensure you have appropriate safety equipment, like gloves and goggles.

If you encounter an F75 fault, here are five steps you can take to address the problem:

Step 1: Identification of F75 fault

The first step is to identify the issue.

  • Check your boiler’s display panel for any error codes. If you see the F75 code, it indicates a problem with your boiler’s pressure sensor.
  • Next, check the pressure in your system. This is usually displayed on the boiler’s front interface, either via the digital or physical (bar or dial) pressure gauge. The ideal boiler pressure is between 1 and 1.5 bars. 
  • If the boiler pressure is too high (above 1.5 bars) or low (below 1 bar), it might point to a problem with a faulty pressure sensor.

Step 2: Safety precautions before DIY

  • Make sure you’re wearing safety gear like gloves and goggles.
  • Check that you understand how all the parts of your boiler function, including the pressure gauge and filling loop. Consult the manufacturer’s manual if needed.

If you feel unsure about how to fix your boiler, it’s best to seek professional assistance.

Step 3: Assessment of system pressure

If the pressure gauge indicates that your boiler pressure is too low, you will need to repressurise the system:

  • Turn off the boiler and allow it to cool down completely. 
  • Locate the filling loop under the boiler – a long pipe attached to one or two taps. 
  • Turn both the taps on the filling loop clockwise to allow water to flow into the boiler. 
  • If your pressure gauge is visible, allow it to rise to 1.5 bars before closing both taps anticlockwise. If your gauge is faulty or the reading is not visible, allow the water to flow for three seconds before closing the taps. Repeat this process until the boiler reaches the optimal pressure. 
  • Turn your boiler back on and proceed to reset your boiler (discussed next). 

Step 4: Boiler reset

Once you’ve checked the system pressure, follow the boiler reset procedure:

  • Locate the reset button on your boiler. It is usually a red button positioned either at the front or back of your unit or below the display screen. If you don’t have a reset button, you may have to use the power button. 
  • Hold down the reset or power button for approximately five seconds. 
  • Symbols will flash on the screen before the boiler restarts. 
  • The boiler should restart and display the pressure level.

If, after resetting your boiler you still encounter a fault code or if it fails to start up, contact a registered Gas Safe engineer who can identify and resolve the issue.

Step 5: Post-reset assessment

After resetting your boiler, monitor your Glow-worm’s performance closely to ensure it operates correctly and safely. Here are some recommendations for checking your boiler post-reset:

  • Look at your boiler’s display panel for any fault codes or messages that may appear.
  • Make sure to double-check the water pressure in your system again. Your Glow-worm boiler should register a pressure between 1 and 1.5 bars. If the pressure drops after you repressurise the unit, call in a boiler engineer rather than trying again. 
  • Examine the exterior of your boiler for corrosion or damage to any connections going into or out of the unit. If you detect an issue, call in a professional to have it repaired. 
  • Pay attention to any boiler noises or vibrations while running. This could indicate an issue with the unit’s heat pump.

Different models may have specific characteristics and requirements, so there may be slight variations in the troubleshooting steps. To determine if the F75 malfunction affects your model or to find instructions on resetting it, refer to your boiler manual or visit the manufacturer’s website.

When To Call A Professional

Contact a licensed Gas Safe engineer if none of these steps resolves the F75 error code (or any other error code). They possess the expertise to identify and resolve all kinds of boiler issues. 

Remember that beyond repressuring and resetting your boiler, it is unsafe and illegal to perform any boiler DIY fixes, like replacing the heat pump or repairing a broken pressure sensor. 

The engineer will inspect both the boiler pump and pressure sensor to determine what has caused the F75 fault code and provide you with steps to resolve it.

In some cases, the pressure sensor may just need to be cleaned by a boiler professional or Gas Safe engineer. However, in more extreme cases, like if the boiler is beyond economical repair or the costs of installing a new boiler will be less than the repairs, a replacement might be the best solution.


What is the F code on a Glow-worm boiler?

An F code on your Glow-worm boiler is a fault code indicating issues with the boiler’s performance or operation. There are various F codes, each with a specific meaning and cause. For example, the F28 boiler fault code means that the boiler failed to ignite. To find the specifics of an F code, visit Glow-worm’s website or consult your boiler manual.

What should the water pressure be on my Glow-worm boiler?

When your Glow-worm boiler is off, the pressure gauge should display a reading between 1 and 1.5 bar.

Does running hot water reduce boiler pressure?

Running water does not reduce the boiler pressure. It can, however, increase it. This is because water expands when heated, creating pressure in the system.


Addressing the F75 fault is crucial to ensure your boiler functions appropriately. Although DIY repairs are not recommended, the steps outlined in this article can assist in managing the Glow-worm fault before contacting a registered Gas Safe engineer, like Eco Happy.

Safety should be your top priority, so contact us at Eco Happy if you need professional assistance with your boiler error codes.

James Elston

Boiler Expert

James Elston is the top boiler replacement and heating expert at Eco Happy. He has over 20 years of experience in the industry, focusing on Gas Safe boiler installations and offering home-heating and energy-saving solutions to homeowners across the UK. From sourcing the most energy-efficient combi boiler to providing specialist heating advice, James ensures that Eco Happy maintains the highest standards and best customer service.

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