Glow Worm boiler error codes appear as an “F” followed by a number (e.g. F22, F75) on the digital display of modern models. Older boilers may show a red flashing light and a number of flashes. Each code is a built-in diagnostic message that tells the control board to lock the boiler out for safety until the problem is fixed.
In this guide, I’ll explain the most common Glow Worm fault codes, what they mean, why they happen, and the safe steps you can take before booking a Gas Safe engineer.
Most Common Glow Worm Fault Codes
What Does Glow Worm Error Code F22 Mean?
Glow Worm error code F22 means the system water pressure has dropped too low – usually below 0.5 bar.
Causes of F22 low pressure
- A small leak on radiators, pipework or the pressure relief valve
- Bleeding radiators without topping up afterwards
- Faulty expansion vessel or filling loop
How to fix F22
- Locate the filling loop (grey or blue handles/flexible silver hose under the boiler).
- Open both valves slowly until the gauge reaches 1–1.5 bar.
- Close tightly and wipe drips.
- The boiler usually resets itself within 30 seconds. If pressure drops again within days, call an engineer to find the leak.
What Does Glow Worm Error Code F75 Mean?
Glow Worm error code F75 is a famous (and often misunderstood) fault that means the boiler cannot detect a pressure rise when the pump runs – usually because of a stuck or failed pump or a faulty pressure sensor.
Causes of F75
- Seized or worn pump (very common after 7–10 years)
- Blocked pressure sensor pipe
- Airlocks or sludge stopping water movement
How to fix F75
- Bleed all radiators and top up pressure first – occasionally clears it.
- Listen for the pump – no humming = failed pump.
- Replacement Grundfos pump on most Glow Worm models costs £180–£280 fitted.
- Sensor pipe clean or sensor swap is cheaper (£90–£140).
What Does Glow Worm Error Code F28 Mean?
Glow Worm error code F28 means the boiler tried to light but failed to establish a flame (ignition lockout).
Causes of F28
- No gas supply or very low pressure
- Frozen condensate pipe (winter classic)
- Dirty ignition electrodes or flame probe
- Faulty gas valve
How to fix F28
- Check other gas appliances work.
- Thaw any frozen condensate pipe with warm water and insulate it.
- Reset once. Persistent F28 nearly always needs an engineer to clean electrodes or test gas pressure.
What Does Glow Worm Error Code F13 Mean?
Glow Worm error code F13 points to a problem with the main printed circuit board (PCB) or its wiring.
Causes of F13
- Water ingress onto the PCB (leaks from above)
- Power surge damage
- Age-related component failure
How to fix F13
- Power off for 10 minutes – sometimes clears a temporary glitch.
- Full PCB replacement is usually required (£220–£350 fitted).
What Does Glow Worm Error Code F14 Mean?
Glow Worm error code F14 is an overheat lockout.
Causes of F14
- Sludge or limescale blocking the heat exchanger
- Pump running too slowly
- Air in the system
How to fix F14
- Bleed radiators and top up pressure.
- Reset boiler after 20 minutes cooling.
- Recurring F14 = powerflush or pump replacement needed.
What Does Glow Worm Error Code F83 Mean?
Glow Worm error code F83 (newer models) means the boiler detects no temperature rise when heating – i.e. water isn’t circulating properly.
Causes of F83
- Airlocks
- Blocked system or heat exchanger
- Pump failure
How to fix F83
- Bleed system thoroughly.
- Engineer powerflush or pump replacement resolves 95% of cases.
What Do Glow Worm Error Codes F61, F62, F63, F64 Mean?
These codes all relate to the gas valve control circuit or PCB feedback.
How to fix F61–F64
These are engineer-only repairs – usually gas valve replacement or PCB swap.
FAQs
Can I still get hot water if a fault code is showing?
Most Glow Worm fault codes put the boiler into full lockout, so no heating and no hot water until the code is cleared.
How many times is it safe to reset my Glow Worm boiler?
Only once or twice after you’ve fixed the obvious cause (e.g. frozen pipe). Repeated resets can damage components and may affect warranty claims.
Do Glow Worm boilers store a history of fault codes?
Yes – press and hold the spanner/reset button for 5 seconds on most models to enter the fault history menu (last 10 codes).
Will a magnetic filter stop future error codes?
Yes – fitting a MagnaClean or similar filter dramatically reduces F75, F83 and overheat faults by catching sludge before it blocks the pump or sensor.
Are Glow Worm boilers still supported after Vaillant bought the brand?
Absolutely – parts and technical support remain excellent, and many engineers carry common spares for Energy, Betacom and Ultracom models on their vans.
The Boilers2Go Team
With years of experience in boilers and home improvements in general, Myles Robinson is a passionate advocate for making home heating simpler, safer, and more affordable for households across the UK. With years of experience in the heating industry, Myles combines hands-on knowledge with a focus on customer-first service, helping homeowners choose the right boiler, understand their systems, and keep their heating running efficiently all year round. His articles break down complex topics into clear, practical advice you can trust.