Glow-worm Boiler Fault Codes
Gas Safe registered fault diagnosis for Glow-worm Flexicom, Easimax, Energy, and Ultimate 3 models. F.28, F.22, F.9, F.75, F.1 — all explained with repair costs and safe homeowner checks.
Understanding Glow-worm boiler fault codes
Glow-worm is part of the Vaillant Group and shares components with Vaillant ecoTEC boilers — making parts relatively straightforward to source across London. Common across South London rental properties as a budget-friendly alternative to Vaillant, Glow-worm boilers use the same F-series fault code structure, making diagnosis familiar to any engineer trained on Vaillant Group products.
This guide covers the five most commonly searched Glow-worm fault codes, with technical explanations of what each fault means, what causes it, and what a Gas Safe engineer will do to fix it. Reion Miller (Gas Safe 919881) carries Glow-worm and compatible Vaillant Group parts and handles Glow-worm boiler repairs across South London.
Glow-worm fault codes explained
Ignition lockout
No flame detected after multiple ignition attempts. The most common Glow-worm fault, sharing the same ignition chain as Vaillant ecoTEC models due to Vaillant Group engineering.
Low water pressure
System pressure has fallen below the minimum operating threshold. Usually safe for a homeowner to resolve by repressurising — the most common Glow-worm self-fix.
Fan / APS fault
The PCB cannot confirm the fan is running correctly, or the air pressure switch has not triggered. Prevents ignition as a safety measure.
Pressure sensor failure
The pressure sensor is not reading correctly — the boiler cannot determine system pressure. Related to F.22 but a component failure rather than genuinely low pressure.
Flame loss after ignition
The boiler established a flame but lost it shortly after — the ionisation probe stopped detecting it. Different from F.28 (no ignition) because ignition was initially successful.
Glow-worm boiler fault code table
Extended fault code reference for common Glow-worm Flexicom, Easimax, Energy, and Ultimate 3 models.
| Code | Meaning | Common cause | Action |
|---|---|---|---|
| F.22 | Low water pressure | System pressure below 0.5 bar | Repressurise to 1.2–1.5 bar (homeowner safe if no leak) |
| F.28 | Ignition lockout | No flame detected after multiple attempts | Gas Safe engineer required |
| F.29 | Flame loss / gas valve | Flame lost after ignition — ionisation or gas valve | Gas Safe engineer required |
| F.9 | Fan / APS fault | Fan motor failure, blocked flue, APS failure | Gas Safe engineer required |
| F.75 | Pressure sensor failure | Pressure sensor failed — not genuinely low pressure | Gas Safe engineer required |
| F.62 | Gas valve fault | Gas valve not responding correctly | Gas Safe engineer — gas valve replacement likely |
| F.1 | Flame loss after ignition | Ionisation probe, intermittent gas valve | Gas Safe engineer required |
| F.71 | NTC sensor fault | Flow or return thermistor failure | Gas Safe engineer |
| F.72 | Flue NTC sensor fault | Flue temperature thermistor failure | Gas Safe engineer |
| F.73 | NTC short circuit | Thermistor shorted — wiring or sensor failure | Gas Safe engineer |
Why Glow-worm boilers fault more often in London
Glow-worm boilers are common throughout South London's rental housing stock — particularly across Wandsworth, Lewisham, Lambeth, Southwark, and Merton. As a Vaillant Group budget range, they offer reliable performance but South London's hard water shortens service intervals significantly.
Thames Water supplies water with some of the highest calcium concentrations in England. Inside a Glow-worm boiler, this deposits as limescale on the heat exchanger and around the pressure sensor. The result: F.28 ignition lockouts, F.75 pressure sensor failures, F.1 flame instability, and heat exchanger failures that annual servicing would have prevented. Because Glow-worm shares components with Vaillant, parts are readily available — but prevention remains far cheaper than repair.
Reion recommends fitting a Fernox TF1 or Adey MagnaClean magnetic filter at installation and running an annual service on every Glow-worm boiler in South London. Without this, F.28 and F.75 faults on 8–12 year old Glow-worm boilers in hard water areas are predictable.
Glow-worm boiler fault code questions
How do I reset a Glow-worm boiler after a fault code?
Why does my Glow-worm boiler keep showing fault codes in London?
Can I fix a Glow-worm boiler fault code myself?
How much does a Glow-worm boiler repair cost in London?
Are Glow-worm boiler parts easy to source in London?
Glow-worm fault code showing on your boiler?
Reion is Gas Safe registered (919881), carries Glow-worm and Vaillant Group parts, and serves all of South London. Fixed price after the £80 diagnostic — no obligation to proceed.
