Glow-worm Boiler Error Codes: Complete Diagnosis Guide

Quick Answer: Glow-worm is owned by the Vaillant Group and modern Glow-worm boilers (Energy, Ultimate, Betacom 4) share the same F-code fault system as Vaillant ecoTEC models -- the codes are functionally identical. The most common callout codes are F.22 (low water pressure), F.28 (ignition lockout), F.29 (flame loss), and F.75 (pump/pressure sensor fault). If you know your way around Vaillant F-codes, you already know Glow-worm F-codes; the diagnostic approach, likely causes, and parts are the same.

Summary

Glow-worm has been part of the Vaillant Group since 2001, and current Glow-worm boilers are built on the same platform as Vaillant ecoTEC models. The Energy, Ultimate 3, and Betacom 4 ranges all use the same PCB architecture, gas valves, pumps, and sensor components as their Vaillant equivalents, and display the same F.XX fault codes on the digital display. For engineers who primarily work on Vaillant, a Glow-worm callout should feel immediately familiar -- the diagnostic menu, reset procedure, and fault hierarchy are identical. Older Glow-worm models (Betacom 24c/30c, Flexicom, Easicom) use LED indicator lights rather than digital F-codes, which are covered separately below. This guide covers all commonly encountered codes with the Vaillant equivalence explicitly noted throughout.

Key Facts

Quick Reference Table

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Code Description Most Likely Cause Urgency
F.1 Ignition lockout (older models) Frozen condensate, gas supply off, electrode fault High
F.4 Ignition lockout (older models) Gas supply off, pre-pay meter empty, gas valve fault High
F.5 Overheat / safety cutout (older models) Pump failure, circulation blockage High -- do not reset repeatedly
F.9 Low system pressure (older models) System leak, PRV discharge, expansion vessel fault Medium
F.13 NTC temperature sensor fault Wiring fault, faulty sensor, PCB fault High
F.14 Flow/return NTC sensor fault Wiring fault, faulty sensor, PCB fault High
F.15 NTC thermistor / overheating risk Faulty thermistor, wiring fault High
F.16 Fan fault (older models) Fan motor failure, wiring, PCB High
F.20 Safety cutout -- overheating Pump failure, circulation blockage, stuck diverter valve High -- do not reset repeatedly
F.22 Low water pressure System leak, PRV discharge, recent radiator bleed Medium -- repressurise and monitor
F.23 Excessive temperature differential Pump not circulating, blockage, air lock High
F.24 Temperature rising too fast Pump failure, low water volume, circulation fault High
F.25 Flue gas temperature too high Flue thermistor fault, blocked heat exchanger High
F.27 Flame detected when none expected Moisture ingress, gas valve not sealing, electrode fault High -- safety critical
F.28 Ignition lockout -- failed to light Gas supply, frozen condensate, electrode, gas valve High
F.29 Flame loss during operation Gas pressure drop, flue blockage, gas valve intermittent High
F.32 Fan speed incorrect Fan motor failure, wiring, flue restriction High
F.33 Fan frost protection active Normal in freezing conditions -- monitor only Low
F.49 eBUS communication fault Short circuit on eBUS, control wiring fault Medium
F.61 Gas valve control fault Gas valve, wiring harness, PCB High -- do not reset repeatedly
F.62 Gas valve shutoff delay Gas valve not sealing promptly, PCB High -- safety critical
F.63 EEPROM fault PCB failure High -- likely PCB replacement
F.64 Electronics/sensor fault NTC sensor short, PCB fault High
F.75 No pressure change on pump start Pressure sensor, pump, system sludge High
F.76 Heat exchanger overheat protection Thermal fuse wire/connection, heat exchanger High -- do not reset repeatedly
F.83 Insufficient temperature rise on ignition NTC thermistor fault, dry fire, low water volume High

Detailed Guidance

What does F.22 mean on a Glow-worm boiler?

F.22 -- Low Water Pressure Detected

Identical to Vaillant F.22. The boiler has detected system water pressure below the minimum operating threshold (typically below 0.3-0.5 bar). The boiler locks out to prevent dry-fire damage.

Likely causes (ranked by probability):

  1. System leak -- radiator valve weep, pipe joint, automatic air vent dripping. Most common cause of recurring F.22.
  2. PRV (pressure relief valve) discharge -- check the copper discharge pipe externally. If wet/dripping, the PRV is lifting, usually due to expansion vessel failure or waterlogging.
  3. Recent radiator bleed -- customer has bled radiators and not topped up pressure.
  4. Expansion vessel fault -- pre-charge lost or diaphragm failed, causing PRV to lift on heat cycle.
  5. Faulty pressure sensor/transducer -- less common but possible on older units.

Diagnostic steps:

  1. Check the pressure gauge -- if genuinely low (below 0.5 bar), repressurise via the filling loop to 1.0-1.5 bar cold.
  2. Reset the boiler and fire. Monitor pressure during heat cycle -- it should rise slightly (0.5-1.0 bar above cold reading is normal).
  3. If pressure rises excessively (above 3 bar), the expansion vessel has likely failed. Check Schrader valve on EV for water (diaphragm gone) and pre-charge pressure (should be 0.75 bar on most models, check data plate).
  4. If pressure drops back within hours/days, investigate for leaks systematically: all radiator valves, pipe joints, boiler internals (heat exchanger, PRV outlet, pump connections).
  5. If the gauge reads 1.0+ bar but F.22 still shows, suspect the pressure sensor/transducer. Check wiring and resistance.

Parts commonly needed:

Vaillant equivalence: Identical to Vaillant F.22 in every respect. Same diagnostic approach, same likely causes, same parts.

What does F.28 mean on a Glow-worm boiler?

F.28 -- Ignition Lockout (Failed to Light)

Identical to Vaillant F.28. The boiler attempted ignition (typically 3 attempts) and failed to detect a flame. One of the most common callout codes with a wide range of possible causes.

Likely causes (ranked by probability):

  1. No gas supply -- pre-payment meter run out, gas cock turned off, other gas appliances also not working. Always check the obvious first.
  2. Frozen condensate pipe -- extremely common in winter. The 21.5mm condensate pipe freezes at an external section or elbow, backing up into the boiler and preventing ignition.
  3. Ignition electrode/lead fault -- electrode tip worn, cracked porcelain, or carbon build-up preventing spark. Spark lead resistance out of spec or cracked insulation.
  4. Gas valve fault -- valve coils failed, sticking, or intermittent. Check resistance across coils.
  5. Low gas pressure -- inlet gas pressure below 17 mbar (nat gas). Check meter pressure, then boiler inlet pressure.
  6. Flue blockage/terminal restriction -- birds nest, debris, or incorrect flue terminal causing recirculation of products of combustion.
  7. PCB fault -- if spark is present and gas is flowing but flame not detected, the ionisation sensing circuit on the PCB may be faulty.

Diagnostic steps:

  1. Check gas supply -- is the gas on? Do other gas appliances work? Check meter.
  2. Check condensate pipe -- especially in cold weather. Look for ice at bends, external runs, or the soak-away termination. Thaw with warm (not boiling) water.
  3. Attempt reset -- if the boiler fires on reset, it was likely a transient issue. Monitor.
  4. If no ignition on reset, open the boiler:
    • Check for spark at the electrode -- listen/look during ignition attempt. No spark = check electrode gap (3-4mm typical), lead condition, and lead connection at PCB.
    • Check gas pressure -- connect manometer to test nipple. Verify inlet pressure (20 mbar nominal, min 17 mbar). Check burner pressure (varies by model -- typically 10-15 mbar on max rate for nat gas).
    • Check gas valve operation -- you should hear the valve click/open during ignition sequence. If no click, check wiring to valve, then valve coil resistance.
  5. Check flue -- visual inspection of terminal for obstruction.

Parts commonly needed:

Vaillant equivalence: Identical to Vaillant F.28. Same causes, same diagnostic procedure. Gas valve, electrode, and ignition lead are often the same part numbers or direct cross-references.

What does F.29 mean on a Glow-worm boiler?

F.29 -- Flame Loss During Operation

Identical to Vaillant F.29. The boiler ignited successfully but the flame was lost during the burn cycle, and re-ignition attempts failed. The boiler can light -- it just cannot sustain the flame.

Likely causes (ranked by probability):

  1. Intermittent gas supply issue -- gas pressure dropping under load. Could be meter not keeping up (older U6 meters), shared supply with high-demand appliance, or partially blocked gas cock.
  2. Flue/air supply issue -- wind-related flue terminal problems, intermittent downdraught, or partial flue blockage allowing initial light but snuffing under full rate.
  3. Gas valve intermittent fault -- valve opening for ignition but not sustaining, or modulating incorrectly. Often worse on higher burner rates.
  4. Ionisation sensing issue -- flame present but ionisation signal too weak for PCB to recognise. Dirty/worn sensing electrode, poor earth, or low ionisation current.
  5. Condensate pipe partial blockage -- allowing initial operation but gradually blocking as condensate production increases.

Diagnostic steps:

  1. Check if F.29 is intermittent or consistent. Intermittent = likely external factor (wind, gas pressure fluctuation). Consistent = internal component.
  2. Monitor gas pressure at the boiler inlet during operation -- watch for drops. Check dynamic inlet pressure holds above 17 mbar at full rate.
  3. Check burner pressure stability during operation using the manometer.
  4. Check ionisation current via the diagnostic menu (should be >1 microamp, ideally 3-6 microamp).
  5. Inspect flue terminal for correct positioning, clearance from obstructions, and wind exposure. Consider a flue terminal guard if wind-related.

Parts commonly needed:

Vaillant equivalence: Identical to Vaillant F.29. Same diagnostic hierarchy.

What does F.75 mean on a Glow-worm boiler?

F.75 -- No Pressure Change Detected on Pump Start

Identical to Vaillant F.75. The PCB monitors the pressure sensor for an expected pressure spike when the pump starts. If no change is detected, it assumes either the pump has failed or the pressure sensor cannot detect the change.

Likely causes (ranked by probability):

  1. Pressure sensor blocked with magnetite/debris -- the most common cause. Magnetite (black iron oxide sludge) accumulates on the sensor diaphragm. Very common in systems without a magnetic filter or inhibitor.
  2. Pump seizure or weak pump -- the pump either does not start at all or runs too weakly to generate detectable pressure change.
  3. Pressure sensor electrical fault -- sensor itself failed rather than just blocked.
  4. Air lock -- large air pocket at the pump preventing effective circulation and pressure change.
  5. System blockage -- severe sludge throughout the system preventing any meaningful flow/pressure change.

Diagnostic steps:

  1. Listen and feel -- when the boiler attempts to start, can you hear/feel the pump running? Place your hand on the pump body.
    • No vibration/sound = likely pump seized or failed.
    • Pump runs but F.75 persists = likely pressure sensor issue.
  2. Check pump -- if seized, try gently freeing the shaft via the bleed screw (flat-blade screwdriver on the pump face). If it frees and runs, it may work temporarily but replacement is advisable.
  3. Check pressure sensor -- remove and inspect. If the diaphragm/port is clogged with black sludge, that is your cause. Clean or replace.
  4. Test pressure sensor -- disconnect and check resistance. Should show a value that changes when pressure is applied.
  5. Check system condition -- if heavily sludged, recommend a powerflush and fitting a magnetic system filter (MagnaClean, Adey, or similar). Add inhibitor (Sentinel X100 or Fernox F1).

Parts commonly needed:

Vaillant equivalence: Identical to Vaillant F.75. This fault is just as common on Glow-worm Energy/Ultimate models as it is on Vaillant ecoTEC Plus. Same pressure sensor, same pump, same sludge-related root cause.

Engineer tip: As with Vaillant, replacing the pressure sensor alone on a sludged system is a temporary fix. If you do not address the system sludge (powerflush + filter + inhibitor), the new sensor will fail again within 6-18 months. Always advise the customer and document the recommendation.

What does F.83 mean on a Glow-worm boiler?

F.83 -- Insufficient Temperature Change on Burner Ignition

Identical to Vaillant F.83. When the burner fires, the PCB expects to see the flow temperature rise within a set time. If the NTC flow/return thermistors do not register the expected temperature change, F.83 is displayed.

Likely causes (ranked by probability):

  1. Faulty NTC thermistor -- sensor not reading temperature changes accurately.
  2. Poor thermal contact -- NTC sensor loose in its pocket.
  3. Dry fire / low water -- insufficient water in the heat exchanger (air lock, low pressure).
  4. Limescale build-up -- insulating the sensor from the water.
  5. Wiring fault -- intermittent connection at the NTC sensor or PCB.

Diagnostic steps:

  1. Check NTC resistance values with a multimeter. At 20 degrees C, an NTC thermistor should read approximately 10k-12k ohms. At 50 degrees C approximately 3.5-4k ohms. Compare flow and return -- they should be similar at ambient.
  2. Check the sensors are seated correctly in their pockets.
  3. Check for air in the system at the boiler.
  4. Monitor live temperature readings in the diagnostic menu during a burn cycle.

Parts commonly needed:

Vaillant equivalence: Identical to Vaillant F.83. Same NTC components and diagnostic approach.

What does F.20 mean on a Glow-worm boiler?

F.20 -- Safety Temperature Limiter (STB) Activated -- Overheating

Identical to Vaillant F.20. The boiler has exceeded its safe operating temperature and the safety temperature limiter has tripped.

Likely causes:

  1. Pump failure -- pump not circulating water through the heat exchanger.
  2. System blockage -- severely sludged heat exchanger or system pipework.
  3. Air lock -- large air pocket preventing water flow across the heat exchanger.
  4. Diverter valve stuck -- on combi models, if the diverter is stuck mid-position, water flow is restricted.

Diagnostic steps:

  1. Allow the boiler to cool completely before investigating.
  2. Check pump operation -- is it running? Check for seizure.
  3. Inspect the heat exchanger for blockage (flow/return temperature differential under load should be 11-20 degrees C; much higher indicates restricted flow).
  4. Check NTC thermistor readings via diagnostic menu against actual temperature.

Warning: Do not repeatedly reset an F.20 fault. If the safety limiter is tripping genuinely, repeated resets risk damage to the heat exchanger.

How do Glow-worm codes relate to Vaillant codes?

Glow-worm has been a Vaillant Group brand since 2001. The relationship between the two brands' fault codes is as follows:

Current models (Energy, Ultimate 3, Betacom 4):

Older models (Betacom 24c/30c, Flexicom, Easicom):

Practical implication for engineers:

If you are comfortable diagnosing Vaillant ecoTEC faults, you can diagnose current Glow-worm faults with the same knowledge. The main difference is cosmetic -- the boiler casing and branding. Internally, the platform is shared.

Older Glow-worm Models: LED Indicator Diagnostics

Older Glow-worm Betacom (24c, 30c) and Flexicom models use LED lights rather than digital fault codes.

LED State Meaning Action
Green LED steady Normal operation No fault
Green LED off No power to boiler Check fuse, wiring, 230V supply
Yellow LED flashing DHW/CH knob position fault Adjust hot water and heating knobs to correct settings
Red LED steady Lockout -- low water pressure Check pressure gauge, repressurise to 1.5 bar via filling loop, reset
Red LED flashing Lockout -- overheat Allow boiler to cool, check pump operation and circulation. Likely requires engineer

For these older models, the installation and servicing manual for the specific model is essential. The LED codes provide less granularity than F-codes, so hands-on diagnosis is more reliant on systematic component testing.

Frequently Asked Questions

How do I reset a Glow-worm boiler after a fault code?

Press and hold the reset button for approximately 3 seconds until the display clears. On current Energy and Ultimate models, this is the button with the flame/reset symbol on the front panel -- identical to the Vaillant reset procedure. If the fault code returns immediately after reset, there is an active fault that must be diagnosed. Do not keep resetting. For codes F.27, F.61, and F.62, do not attempt to reset -- these relate to gas valve safety and require professional diagnosis.

Are Glow-worm parts interchangeable with Vaillant parts?

In many cases the internal components are physically identical or from the same supplier, but the part numbers differ between the two brands. The gas valve, pump, NTC sensors, pressure sensor, and electrode are frequently the same component with different packaging. However, always verify against the specific model's data plate and parts list before fitting. Fitting a Vaillant-branded part in a Glow-worm boiler may affect warranty status -- use Glow-worm branded parts where the boiler is under warranty.

Is Glow-worm a good brand or just a budget Vaillant?

Glow-worm is positioned as the Vaillant Group's value brand for the UK market. The internal components are largely shared with Vaillant, and reliability is comparable. The main differences are in casing design, warranty length (Glow-worm offers up to 15 years on Ultimate 3), and brand positioning. From an engineering standpoint, servicing a Glow-worm is functionally identical to servicing a Vaillant. The Energy range is made in Britain and carries Made in Britain accreditation.

Where can I find Glow-worm fault codes for my specific model?

The official Glow-worm trade website provides model-specific fault-finding guides for each range (Energy, Ultimate, Betacom). Visit glow-worm.co.uk/trade and navigate to Service and Support > Fault Finding Codes. You can also access the fault history on the boiler itself via the diagnostic menu (press and hold the info button for 5 seconds).

Regulations & Standards