If your Glow-worm boiler is displaying the F9 error code, you’re not alone. Many UK homeowners and landlords run into this common fault, often at the most inconvenient times.
Whether you’re seeing the message “Glow-worm boiler F9 fault”, dealing with a pressure issue, or your boiler simply isn’t heating properly, this guide will help you diagnose the cause and find a fix.
You’ll get clear, practical advice on how to fix the F9 code, along with the confidence to know when to safely tackle it yourself and when to call a Gas Safe registered engineer.
What Is the Glow-worm F9 Fault Code?
The F9 error is your boiler’s way of warning you that something is wrong with water movement or system pressure. If water isn’t circulating correctly, or the pressure is outside the safe range, the boiler locks out to protect itself.
This fault can affect all types of Glow-worm boilers, combi, system, and regular, including models like Energy, Ultimate, and Betacom.
Why Does the F9 Error Code Appear?
Your boiler constantly monitors water pressure and flow. If either falls outside the recommended range, it displays the F9 fault rather than continuing to run, which protects internal components from damage and keeps your home safe.
Persistent F9 codes are rarely random. They usually trace back to one of a small set of causes, which is why working through them in order tends to find the answer quickly.
Causes of the Glow-worm Boiler F9 Fault
Several issues can lead to a Glow-worm boiler pressure issue or F9 code. Here are the most common.
Low Water Pressure (Below 1 Bar)
Low system pressure, usually below 1 bar, is the most frequent cause of the F9 error on Glow-worm boilers. Small leaks in radiators, pipes, or valves often reduce pressure slowly over time.
High Water Pressure (Above 2 Bar)
It isn’t just low pressure. High boiler pressure, well above the normal 1 to 1.5 bar (cold) range, can also cause problems. Excess pressure stresses internal components or forces relief valves to vent water, which can sometimes lead to a lockout.
Faulty Pump
A blocked, seized, or failed pump means water can’t circulate through your radiators. This triggers water flow problems and prompts an F9 warning.
Airlocks or Blockages
Airlocks, caused by trapped air in the system, can stop water flow completely, while a build-up of sludge or debris may block pipes or components like the heat exchanger and pump. Both restrict circulation and can result in error messages.
Faulty Pressure Sensor or Internal PCB Fault
Sometimes the sensors send false readings to the PCB, making the boiler think there’s a pressure issue when the real problem is with the sensor or internal electronics. Persistent or recurring F9 codes may point to these less obvious faults.
How to Fix the Glow-worm F9 Error: DIY Checks
You don’t need to be an engineer to perform a few basic safety checks that may resolve the F9 fault. These steps focus on external parts only and are safe for most confident homeowners.
Never open your boiler casing or attempt internal repairs. This is dangerous and should only be carried out by a Gas Safe registered engineer.
Step 1: Check the Glow-worm Boiler Pressure Gauge
Look at the gauge on your boiler. If it’s below 1 bar, the pressure is too low and you’ll need to re-pressurise. A healthy system should sit between 1 and 1.5 bar, usually shown in the green zone.
If it reads well above 2 bar with the heating off, the pressure is too high. In that case, bleeding radiators often helps lower it.
Step 2: How to Re-pressurise Your Glow-worm Boiler
Most modern Glow-worm models feature a filling loop, usually a flexible hose beneath the boiler. Slowly open the valves one by one and allow water in, keeping an eye on the gauge. Once you reach 1 to 1.5 bar, close both valves fully.
For a full walkthrough, see our step-by-step guide on how to repressurise a Glow-worm boiler.
If the pressure drops quickly afterwards, there’s likely a leak that needs professional attention. Finish by resetting your boiler to see if the F9 error clears.
Step 3: Bleed Radiators to Release Trapped Air or Reduce High Pressure
If radiators feel unevenly warm or the pressure remains high, turning off your heating and bleeding the radiators may solve the problem. Use a radiator key to open the bleed valve, air should hiss out first, followed by a few water drips, at which point you close the valve.
After bleeding, always double check your boiler pressure and top up if it’s dropped below 1 bar. Bleeding your radiators regularly also helps prevent airlocks, which are a common reason for a Glow-worm boiler not heating efficiently.
When to Call a Gas Safe Engineer About an F9 Fault
Many Glow-worm boiler pressure issues and F9 faults can be resolved with the steps above, but some situations call for an expert. If your F9 code persists or keeps coming back after basic checks, or you suspect the pump has failed or there’s an internal blockage, book a Gas Safe registered engineer.
If F9 keeps returning, check our Glow-worm boiler error codes guide in case a different fault is actually showing.
Some Glow-worm boilers also show an F75 fault linked to similar pressure and flow problems. Our Glow-worm F75 fault code guide covers its causes and fixes.
Internal leaks, faulty sensors, or PCB problems often present as recurring or persistent F9 codes and should always be professionally diagnosed. Any visible water leaking from inside the boiler should prompt you to turn off the unit and consult an engineer immediately.
Before booking anyone, check their credentials on the Gas Safe Register. A qualified engineer has the tools and training to test PCB signals, check sensors and wiring, and safely repair your boiler.
Repair or Replace? When an Ageing Boiler Is the Real Problem
If F9 keeps recurring on an older boiler, it may be a sign the unit is nearing the end of its working life rather than a single fixable fault. Repeated pump or PCB failures on a boiler past 10 to 12 years old often cost more to keep patching than they’re worth.
Our guide on whether to repair or replace your boiler can help you decide, particularly if you’re facing a second or third call-out within the same year.
Keeping Your Glow-worm Running Efficiently
Annual servicing does more than tick a box. An engineer checking your system every year catches worn pumps, failing sensors, and early signs of sludge build-up well before they cause a lockout.
If your boiler keeps playing up and you’re weighing up an upgrade, our Glow-worm boiler reviews cover current models, costs, pros and cons.
How to Prevent the Glow-worm F9 Error
Preventing F9 faults largely comes down to attentive care and regular checks. Arrange an annual service with a Gas Safe engineer, who will inspect and clean your boiler’s components, check the safety valves, and help spot developing issues before they become serious.
Make a habit of checking your pressure gauge every month, especially after cold snaps, and bleed your radiators regularly.
If you notice any leaks or a persistent drop in pressure, act quickly to resolve them. Fitting a magnetic filter can also reduce the build-up of sludge and debris, further protecting your system.
Glow-worm F9 Fault: FAQs
What Does the F9 Error Mean on a Glow-worm Boiler?
The F9 code is an alert that your boiler has detected a problem with water circulation or system pressure. This could relate to low or high pressure, a faulty pump, airlocks, or an issue with pressure sensors.
How Do I Fix the F9 Error on a Glow-worm Boiler?
Start by checking your pressure gauge and topping up if it’s low, or bleeding radiators if the pressure is too high or there’s trapped air. If these actions don’t resolve the problem, consult a Gas Safe engineer. Persistent or recurring F9 errors should always be checked by a professional to rule out hidden leaks or PCB faults.
Is It Safe to Fix a Glow-worm F9 Fault Myself?
It’s safe to carry out simple tasks like topping up pressure or bleeding radiators. Opening the boiler casing or attempting electrical repairs is both unsafe and illegal unless carried out by a Gas Safe registered professional.
Why Does My Glow-worm Boiler Keep Losing Pressure?
Ongoing pressure loss is typically due to leaks in the system, whether at radiators, pipe joints, or the pressure relief valve. If you find yourself constantly topping up, have an engineer check for internal leaks or faulty components.
Why Is My Glow-worm Boiler Not Heating After an F9 Error?
If your boiler still won’t heat after an F9 error has been reset, the underlying issue is likely a component fault, such as the PCB, circulation pump, or a blockage, and professional assessment is recommended.
How Much Does It Cost to Fix a Glow-worm F9 Fault?
Simple DIY fixes like topping up pressure or bleeding radiators are free. An engineer’s call-out typically ranges from £70 to £150, with replacement parts such as pumps costing between £200 and £400 depending on the model and the fault’s complexity.
How Can I Prevent the F9 Fault on My Glow-worm Boiler?
The best prevention is scheduling regular professional servicing, keeping an eye on pressure levels, bleeding radiators, and addressing any leaks as soon as they appear.
Are Glow-worm F9 Repairs Covered by Home Insurance?
It depends on your policy. Some home insurance plans include boiler breakdowns or leaks, while others require separate emergency cover. It’s wise to confirm exclusions and verify any requirements for repairs by accredited engineers.
Is It Safe to Keep Resetting My Glow-worm Boiler If F9 Keeps Coming Back?
Resetting once or twice to rule out a temporary blip is fine, but repeated resets within the same day without addressing the cause aren’t a safe long-term fix. Treat a same day repeat lockout as a sign to book an engineer rather than keep pressing reset.
Will a Magnetic Filter Help Prevent Glow-worm F9 Faults?
Yes, particularly in older or hard water systems. A magnetic filter catches the sludge and debris that would otherwise settle in the pump and heat exchanger, which are two of the more common underlying causes of F9 lockouts.
I’m Penny North, a home energy heating expert. My mission is to demystify new boilers and complex heating systems to help you achieve a warm, cosy home with lower energy bills and a smaller environmental footprint.