Glow-worm boiler error codes
27 codes that Glow-worm publishes for its boilers, each with what it means, the likely cause, what you can safely check and when to call a Gas Safe registered engineer. Tap your code:
Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone. Anything else is for a Gas Safe registered engineer. If you smell gas, call the National Gas Emergency Service on 0800 111 999. Codes that need one are marked Gas Safe engineer.
Read this first
Glow-worm boilers, like the Energy 7 combi, show a fault as F. and a number. The operating instructions for the householder explain three — F.22 (not enough water), F.28 (the boiler did not light) and F.83 (dry fire) — and send everything else to a competent person. The rest of this page comes from the fault-code table in the installation and maintenance instructions, written for the engineer, in plain words. Glow-worm is part of the Vaillant group and many numbers match Vaillant’s, but not all mean the same thing: F.32 is a fan fault on both, yet Glow-worm words it as the fan frost-protection function being active with the fan speed outside tolerance.
F.22 Not enough water in the boiler
- What it means
- Safety shutdown: there is no water, or too little, in the boiler — the system pressure is too low.
- Likely cause
- Water lost from the heating system; the installation manual also lists a faulty water pressure sensor or a loose pump or sensor cable.
- What you can safely check
Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone.
- Read the heating system pressure on the display. Glow-worm asks you to check it once a month: it should be between 0.5 and 2.7 bar.
- If it is too low, fill the heating installation: open the radiator valves, slowly open the filling cock as your installer showed you, fill to the required pressure, check the display and close the filling cock.
- When to call a Gas Safe registered engineer
- Call a Gas Safe registered engineer if F.22 stays after filling or keeps coming back.
Source: Glow-worm, Energy 7 combi operating instructions (0020300878_00, 2020): appendix, troubleshooting Checked 2026-09-28.
F.28 Boiler did not light
- What it means
- After three unsuccessful attempts to light, the boiler stops in fault mode.
- Likely cause
- The installation manual lists a faulty gas meter or tripped gas pressure switch, air in the gas, low gas pressure, a blocked condensate siphon, and faults in the ignition parts, gas valve or electronics.
- What you can safely check
Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone.
- Press the reset button for one second. The boiler makes another attempt to light.
- Try this no more than three times.
- When to call a Gas Safe registered engineer
- After three reset attempts Glow-worm says to consult a competent person: for a gas boiler, a Gas Safe registered engineer.
Source: Glow-worm, Energy 7 combi operating instructions (0020300878_00, 2020): appendix, troubleshooting Checked 2026-09-28.
F.83 Dry fire
- What it means
- When the burner starts, the flow or return sensor sees little or no temperature change.
- Likely cause
- Too little water in the boiler, or a flow or return sensor not sitting in the right place on its pipe.
- What you can safely check
Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone.
- Press the reset button.
- If F.83 comes back and you press reset once more, the boiler waits 45 minutes before it tries again, and the display shows S.54.
- When to call a Gas Safe registered engineer
- If the fault persists after that, Glow-worm says to consult a competent person: a Gas Safe registered engineer.
Source: Glow-worm, Energy 7 combi operating instructions (0020300878_00, 2020): appendix, troubleshooting Checked 2026-09-28.
F.00 F.01 Temperature sensor not connected
- What it means
- The boiler has lost the signal from a water temperature sensor: F.00 the flow sensor, F.01 the return sensor.
- Likely cause
- A plug loose or not plugged in, a break in the wiring, or a faulty sensor.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: every check Glow-worm lists for this code is in the installation manual, inside the boiler.
Source: Glow-worm, Energy 7 combi installation and maintenance instructions (0020300875_01, 2020): appendix E, overview of fault codes Checked 2026-09-28.
F.10 F.11 Temperature sensor short circuit
- What it means
- A water temperature sensor has short-circuited: F.10 the flow sensor, F.11 the return sensor.
- Likely cause
- A faulty sensor or a short in the cable or housing.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: every check Glow-worm lists for this code is in the installation manual, inside the boiler.
Source: Glow-worm, Energy 7 combi installation and maintenance instructions (0020300875_01, 2020): appendix E, overview of fault codes Checked 2026-09-28.
F.20 Overheat: safety shutdown
- What it means
- The overheating temperature was reached and the boiler shut down.
- Likely cause
- Glow-worm lists a bad earth connection, a flow or return sensor with a loose connection, or a stray spark from the ignition lead or electrode.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: every check Glow-worm lists for this code is in the installation manual, inside the boiler.
Source: Glow-worm, Energy 7 combi installation and maintenance instructions (0020300875_01, 2020): appendix E, overview of fault codes Checked 2026-09-28.
F.23 Temperature difference too big
- What it means
- Safety shutdown: the difference between the flow and return temperatures is too large.
- Likely cause
- A blocked or weak pump, air in the boiler, or the flow and return sensors connected the wrong way round.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: every check Glow-worm lists for this code is in the installation manual, inside the boiler.
Source: Glow-worm, Energy 7 combi installation and maintenance instructions (0020300875_01, 2020): appendix E, overview of fault codes Checked 2026-09-28.
F.24 Temperature rising too fast
- What it means
- Safety shutdown: the water temperature rose too quickly.
- Likely cause
- A blocked or weak pump, air in the boiler, system pressure too low, or a blocked or wrongly fitted non-return valve.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: every check Glow-worm lists for this code is in the installation manual, inside the boiler.
Source: Glow-worm, Energy 7 combi installation and maintenance instructions (0020300875_01, 2020): appendix E, overview of fault codes Checked 2026-09-28.
F.25 Flue gas too hot
- What it means
- Safety shutdown on flue gas temperature.
- Likely cause
- Glow-worm lists a break in the plug or wiring of the optional flue gas safety cut-out.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: every check Glow-worm lists for this code is in the installation manual, inside the boiler.
Source: Glow-worm, Energy 7 combi installation and maintenance instructions (0020300875_01, 2020): appendix E, overview of fault codes Checked 2026-09-28.
F.27 Flame detection fault
- What it means
- Safety shutdown: the flame detection has failed.
- Likely cause
- Moisture on the electronics, a faulty flame monitor, or a leaking gas solenoid valve.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: every check Glow-worm lists for this code is in the installation manual, inside the boiler.
Source: Glow-worm, Energy 7 combi installation and maintenance instructions (0020300875_01, 2020): appendix E, overview of fault codes Checked 2026-09-28.
F.29 Flame lost
- What it means
- The flame went out while the boiler was running.
- Likely cause
- The gas supply stopped for a moment, flue gas being drawn back in, bad earthing, the ignition transformer, or a blocked condensate siphon.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: every check Glow-worm lists for this code is in the installation manual, inside the boiler.
Source: Glow-worm, Energy 7 combi installation and maintenance instructions (0020300875_01, 2020): appendix E, overview of fault codes Checked 2026-09-28.
F.32 Fan speed out of range
- What it means
- Fan frost-protection function active: the fan speed is outside its limits.
- Likely cause
- A loose fan or circuit-board plug, a break in the wiring, a blocked fan, its speed sensor or the electronics.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: every check Glow-worm lists for this code is in the installation manual, inside the boiler.
Source: Glow-worm, Energy 7 combi installation and maintenance instructions (0020300875_01, 2020): appendix E, overview of fault codes Checked 2026-09-28.
F.61 F.62 Gas valve control fault
- What it means
- F.61: fault controlling the gas valve. F.62: the gas valve switched off late.
- Likely cause
- A short in the gas valve wiring, a faulty or leaking gas valve, or faulty electronics.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: every check Glow-worm lists for this code is in the installation manual, inside the boiler.
Source: Glow-worm, Energy 7 combi installation and maintenance instructions (0020300875_01, 2020): appendix E, overview of fault codes Checked 2026-09-28.
F.63 F.65 Electronics fault
- What it means
- F.63: memory chip (EEPROM) fault. F.65: the electronics are too hot.
- Likely cause
- Faulty electronics; for F.65 also heat from something around the boiler.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: every check Glow-worm lists for this code is in the installation manual, inside the boiler.
Source: Glow-worm, Energy 7 combi installation and maintenance instructions (0020300875_01, 2020): appendix E, overview of fault codes Checked 2026-09-28.
F.68 Unstable flame
- What it means
- The flame signal is unstable.
- Likely cause
- Air in the gas, low gas pressure, the wrong air–gas ratio or gas injector, or a break in the flame-sensing electrode or its cable.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: every check Glow-worm lists for this code is in the installation manual, inside the boiler.
Source: Glow-worm, Energy 7 combi installation and maintenance instructions (0020300875_01, 2020): appendix E, overview of fault codes Checked 2026-09-28.
F.70 Invalid product code
- What it means
- The boiler’s Device Specific Number is not valid.
- Likely cause
- The display and circuit board were replaced together without resetting the number, or the output coding resistor is wrong or missing.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: every check Glow-worm lists for this code is in the installation manual, inside the boiler.
Source: Glow-worm, Energy 7 combi installation and maintenance instructions (0020300875_01, 2020): appendix E, overview of fault codes Checked 2026-09-28.
F.73 F.74 Water pressure sensor fault
- What it means
- F.73: the water pressure sensor is not connected or has short-circuited. F.74: an electrical problem in the water pressure sensor.
- Likely cause
- The sensor or its cable.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: every check Glow-worm lists for this code is in the installation manual, inside the boiler.
Source: Glow-worm, Energy 7 combi installation and maintenance instructions (0020300875_01, 2020): appendix E, overview of fault codes Checked 2026-09-28.
F.75 Pressure sensor
- What it means
- Glow-worm’s table: the pressure switch is faulty.
- Likely cause
- A faulty pressure switch.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: every check Glow-worm lists for this code is in the installation manual, inside the boiler.
Source: Glow-worm, Energy 7 combi installation and maintenance instructions (0020300875_01, 2020): appendix E, overview of fault codes Checked 2026-09-28.
F.77 Condensate or flue flap
- What it means
- No response from the flue non-return flap (or condensate device).
- Likely cause
- A faulty flue non-return flap.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: every check Glow-worm lists for this code is in the installation manual, inside the boiler.
Source: Glow-worm, Energy 7 combi installation and maintenance instructions (0020300875_01, 2020): appendix E, overview of fault codes Checked 2026-09-28.
F.84 F.85 Flow and return sensors fitted wrongly
- What it means
- F.84: the flow and return readings do not make sense (swapped, or not fitted correctly). F.85: both sensors are on the same pipe or the wrong pipe, or not connected properly.
- Likely cause
- How the temperature sensors were fitted.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: every check Glow-worm lists for this code is in the installation manual, inside the boiler.
Source: Glow-worm, Energy 7 combi installation and maintenance instructions (0020300875_01, 2020): appendix E, overview of fault codes Checked 2026-09-28.
F.89 Pump connection
- What it means
- The pump is not connected, connected wrongly, or the wrong pump is connected.
- Likely cause
- The pump connection or a short in the wiring.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer: every check Glow-worm lists for this code is in the installation manual, inside the boiler.
Source: Glow-worm, Energy 7 combi installation and maintenance instructions (0020300875_01, 2020): appendix E, overview of fault codes Checked 2026-09-28.
A code that is not listed here
Glow-worm’s table notes that it covers several products, so some codes may not appear on your boiler. For any code not here, Glow-worm’s instructions send you to a competent person. An S. code (S.54 and so on) is a status, not a fault.
We list only codes the source below states. If your code is not here, it may belong to a model the source does not cover: your model’s manual is the authority.
Related pages
Source
Every code on this page comes from these pages, published by Glow-worm and read on the date shown. Meanings are written in our own words; nothing is taken from other makes, other models or third-party code sites.
- Glow-worm: Energy 7 combi operating instructions (0020300878_00, 2020): appendix, troubleshooting Checked 2026-09-28.
- Glow-worm: Energy 7 combi installation and maintenance instructions (0020300875_01, 2020): appendix E, overview of fault codes Checked 2026-09-28.