The Glow-worm boiler F75 fault code means the boiler cannot detect a rise in water pressure when the circulation pump starts. As a safety measure it locks out, leaving you without heating or hot water. This usually points to a problem with the pressure sensor, the pump itself, or restricted water movement caused by sludge, air or low system pressure.
If your Glow-worm is showing F75 and you live in the North West, Yorkshire or the Midlands, the guidance below explains the common causes in plain language and the practical steps you can take.
What Does the Glow-Worm Boiler F75 Fault Code Mean?
The Glow-worm boiler F75 fault code means the control board has not registered the expected pressure increase after the pump switches on. In a healthy system the pump moves water and the pressure sensor confirms that movement. When the sensor fails to report that change, the boiler shuts down to protect the heat exchanger from overheating.
You will see F75 on the display, hear the pump attempt to run, and find no heating or hot water. It is a circulation and pressure-detection fault rather than an ignition problem.
What Causes the F75 Fault Code On Your Glow-Worm Boiler?
The F75 fault code on a Glow-worm boiler is triggered when the system cannot confirm proper water circulation at start-up. Several everyday issues can produce this reading. Below we cover the main causes and the clear steps to resolve each one.
Faulty or Blocked Pressure Sensor
A faulty or blocked pressure sensor is the most common reason for an F75 lockout. The sensor contains a narrow channel that measures pressure changes as water moves. Over time magnetite sludge, limescale or general debris can clog that channel, especially in hard-water areas or systems that have not been flushed for years.
The sensor then fails to register the pressure rise even when the pump is working, so the boiler locks out. Electrical failure inside the sensor can produce the same result.
How to Fix
- Check the boiler pressure gauge when the system is cold. It should sit between 1.0 and 1.5 bar. Top it up if needed using the filling loop.
- Reset the boiler by holding the reset button for three to five seconds and watch whether the pressure gauge moves when the pump starts.
- If the gauge stays still or the F75 error code returns, switch the boiler off at the isolator.
- Call a Gas Safe registered engineer. In most cases they can remove, clean and refit the sensor. If the sensor is damaged they will replace it. A system power flush is often recommended at the same time to stop the problem returning.
- After the repair, ask the engineer to check that a magnetic filter is fitted or to install one if it is missing.
Malfunctioning or Seized Circulation Pump
The circulation pump pushes hot water around the radiators and back to the boiler. If the pump seizes, runs too slowly or becomes blocked with sludge, no meaningful pressure change occurs and the F75 code appears. Pumps typically last seven to ten years of normal use. You may hear humming, grinding or complete silence when the boiler tries to start.
How to Fix
- Listen carefully when the boiler attempts to fire. A healthy pump produces a steady hum; silence or a loud grinding noise suggests a problem.
- Bleed the radiators to remove any airlocks and then repressurise the system to 1.0–1.5 bar.
- Reset the boiler once and observe.
- If the code returns, contact a Gas Safe registered engineer. They will test pump speed and current draw, free a sticking impeller if possible, or fit a replacement pump. A power flush is usually carried out at the same time to protect the new component.
- After the repair, keep the system pressure within the recommended range and consider annual servicing to extend pump life.
Low System Pressure or Air in the System
If the central heating pressure has dropped below the normal operating range, or if air has entered the pipework, the pressure sensor may not register a clear rise when the pump runs. Low pressure can result from a small leak, recent radiator bleeding, or gradual loss over time. Airlocks restrict flow in the same way.
How to Fix
- Look at the pressure gauge on the front of the boiler. When the system is cold it should read between 1.0 and 1.5 bar.
- If the reading is low, open the filling loop carefully and bring the pressure up to the correct level. Close the loop tightly and wipe away any drips.
- Bleed each radiator starting from the lowest point in the house and working upwards, then top the pressure up again if it has dropped.
- Reset the boiler and check whether the F75 code clears.
- If the pressure falls again within a day or two, or the code returns, stop topping up and call a Gas Safe engineer. Repeated pressure loss usually means a leak that needs tracing and repairing.
Sludge or Debris Restricting Circulation
Black magnetite sludge builds up inside radiators, pipework and the boiler over years of use. When it restricts flow through the pump or the pressure sensor port, the boiler cannot detect proper circulation and displays F75. This is especially common in systems without a magnetic filter or those that have missed regular servicing.
How to Fix
- Carry out the pressure check and reset steps listed above.
- If the fault persists, arrange a visit from a Gas Safe registered engineer.
- The engineer will inspect the system, clean or replace the pressure sensor if needed, and carry out a full power flush to remove the sludge.
- Ask them to fit or check a magnetic filter so that future debris is captured before it can reach the pump and sensor.
- After the flush, the system should be refilled with inhibitor to protect the clean pipework.
Electrical or Wiring Fault Affecting the Sensor or Pump
Less often, a broken wire, poor connection or fault on the printed circuit board can stop the pressure signal reaching the control board. The boiler then treats the missing signal as a circulation failure and shows F75 error code.
How to Fix
- Confirm the boiler has a stable electrical supply and that the isolator switch is fully on.
- Reset the boiler once after checking system pressure.
- If the code returns, do not open the boiler casing yourself.
- Call a Gas Safe registered engineer. They will test the wiring continuity, sensor output and PCB signals with the correct equipment and replace any failed part.
- Once repaired, request a full functional test so you know the system is circulating correctly.
If you are based in the North West, Yorkshire or the Midlands and your Glow-worm keeps throwing the F75 fault code, Clever Energy Boilers can help. We provide expert boiler installation for homeowners and landlords across these areas and can advise whether a repair or a modern replacement makes better long-term sense.
When Repeated F75 Codes Signal It Is Time for a New Boiler
Persistent Glow-worm boiler F75 fault codes often mean the pressure sensor or pump is reaching the end of its reliable life. Average UK boiler repairs cost around £300, and pump or sensor work plus a power flush can climb higher.
Once a boiler is past ten to twelve years old, further faults become more likely and spare parts can grow harder to obtain. Older appliances also lose efficiency, adding roughly £200 a year to heating bills through wasted gas.
For many households the sensible next step is a new, efficient boiler that comes with a long manufacturer guarantee and modern controls. Clever Energy Boilers specialises in straightforward boiler installation throughout the North West, Yorkshire and the Midlands. We survey the property, recommend the right size and type of boiler, and handle the complete installation so you regain reliable heating and hot water without the worry of recurring F75 lockouts.
Request your free, no-obligation boiler quote today and see how simple the upgrade can be.
Frequently Asked Questions
Can I clear a Glow-worm F75 fault by simply resetting the boiler?
A single reset after checking and correcting the system pressure can clear a temporary glitch. If the F75 code returns within minutes or hours, the underlying cause is still present and a Gas Safe engineer is needed.
Does the F75 code mean the boiler is completely broken?
No. In the majority of cases the F75 points to a blocked or failed pressure sensor or a sticking pump. Both are standard repairs. Only when the boiler is already old and other components are also failing does full replacement become the more economical choice.
Will fitting a magnetic filter stop F75 codes returning?
Yes. A good magnetic filter captures magnetite sludge before it reaches the pump and pressure sensor. Most engineers recommend fitting one during an F75 repair or power flush to reduce the chance of the same fault reappearing.
How quickly should I get an F75 fault fixed?
Treat it as urgent once the basic pressure and reset checks have been tried. Leaving the boiler locked out for days means no heating or hot water, and continued attempts to run with restricted circulation can stress other components.
Is the F75 fault the same on Vaillant and Glow-worm boilers?
Yes. Glow-worm is part of the Vaillant Group and both brands use the same F75 code to indicate that no pressure rise has been detected when the pump starts. The causes and repair approach are identical.
