Glow-worm F79 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F79 fault code mean?
F79 is not a fault code that appears in any publicly documented Glow-worm or Vaillant Group fault code reference. If your boiler is displaying something that looks like F79, it is most likely being misread — the most probable candidates are F78 (a fault with the domestic hot water outlet sensor or NTC at the external controller), F75 (pump start or pressure fault), or F73/F74 (heating circuit pressure sensor signal out of range). It is also possible the code is specific to an older or less common Glow-worm model range not covered in general published guides. Until the exact code is confirmed against your boiler's own installation and service manual — or by Glow-worm's support line — it should be treated as an unverified pressure or sensor-related fault that requires professional diagnosis. Do not ignore it: when a Glow-worm boiler enters a fault or lockout state, it has detected a condition outside safe operating parameters and has shut down to prevent damage or danger.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Misread display — actual code is F75 (pump or pressure fault) Common
F75 is one of the most commonly occurring faults in Glow-worm boilers and relates to the pump failing to raise system pressure on start-up, or a pressure sensor not detecting the expected response. It is easy to confuse the digit 5 with a 9 on certain LCD displays, particularly if the screen is aged or dirty. Check the display carefully in good light and compare against the F75 entry in your boiler manual.
- Misread display — actual code is F78 (DHW NTC or hot water sensor fault) Common
F78 on Glow-worm and Vaillant-family boilers typically indicates an interruption to the domestic hot water outlet sensor, often triggered when a UK link box or external controller is fitted but the hot water NTC sensor circuit is not correctly bridged. The digit 8 can occasionally be misread as a 9. If you have recently had a controller or smart thermostat installed, this is a strong possibility.
- Misread display — actual code is F73 or F74 (pressure sensor signal fault) Sometimes
F73 and F74 point to a heating circuit pressure sensor reading that is outside the expected range — either too low or indicating a short circuit in the sensor wiring. These are relatively common faults in Glow-worm boilers and sit numerically adjacent to F79, making misreading plausible on certain display types.
- Model-specific code not in general published lists Sometimes
Glow-worm produces several boiler ranges, and certain model-specific diagnostic codes are not included in publicly available fault code guides. If you have confirmed the display clearly shows F79 (not F75, F78, or F73/F74), it may be a code unique to your particular model or firmware version. Glow-worm's trade support line can cross-reference the code against your boiler's serial number.
- Faulty or damaged display causing incorrect code output Rare
A failing PCB or display panel can occasionally output garbled or incorrect fault codes, showing a digit or character that does not correspond to the actual underlying fault. This is uncommon but worth considering if the code appears intermittently or alongside other unusual boiler behaviour.
How to fix it
- Check the boiler display carefully in good light DIY safe
Look closely at the fault code shown. LCD segments can be partially lit, making an 8 look like a 9, or a 5 look like a 9. Compare what you see against the fault code table in your boiler's user manual. If you do not have the manual, Glow-worm's website hosts downloadable versions by model range at glow-worm.co.uk.
- Check your system pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler. For most Glow-worm boilers, the correct cold operating pressure is between 1.0 and 1.5 bar. If the needle is below 0.5 bar, low pressure may be contributing to the fault. You can top up pressure using the filling loop — your user manual shows exactly where this is and how to use it. Stop as soon as the gauge reads around 1.2–1.5 bar and do not overfill.
- Attempt a boiler reset DIY safe
On most Glow-worm models, the reset procedure involves turning the central heating control knob to the reset position (marked with a power or reset symbol), holding for a few seconds, then returning it to your desired temperature setting. Refer to your specific model's manual for the exact method. Only reset the boiler two or three times at most — if the fault code returns each time, resetting further will not fix the underlying problem and a Gas Safe engineer is needed.
- Check that the gas supply to your home is working DIY safe
Confirm other gas appliances in the property (such as the hob or gas fire) are working normally. If they are not, there may be a wider gas supply interruption — contact your gas supplier. Do not attempt to interfere with the gas supply pipe or any gas components on or around the boiler.
- Contact Glow-worm directly to verify the code DIY safe
Because F79 is not a standard published Glow-worm fault code, it is worth calling Glow-worm's customer and trade support line with your boiler's model number and serial number to hand. They can confirm whether F79 is a valid code for your specific appliance and advise on next steps. Contact details are at glow-worm.co.uk.
- Call a Gas Safe registered engineer for diagnosis and repair Gas Safe engineer
If the fault persists after checking pressure and attempting a reset, a Gas Safe registered engineer should inspect the boiler. They will use Glow-worm-compatible diagnostic equipment to read the fault history, confirm what the code actually relates to on your specific model, and carry out any necessary work on sensors, the pump, wiring, or other internal components. Only Gas Safe registered engineers are legally permitted to work on gas boilers in the UK.
Parts you may need
- Heating circuit water pressure sensor · from £55
- Circulation pump · from £85
- DHW NTC thermistor (hot water temperature sensor) · from £20
- Flow or return NTC thermistor (heating circuit) · from £18
- Boiler PCB (control board) · from £180
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£350, depending on the underlying cause.
Frequently asked questions
Is F79 a real Glow-worm fault code?
F79 does not appear in any publicly documented Glow-worm or Vaillant Group fault code list. It is most likely a misread of a nearby code — such as F75 (pump or pressure fault), F78 (hot water sensor or external controller wiring issue), or F73/F74 (pressure sensor signal fault). It could also be a model-specific code not included in general published guides. The best approach is to check the display carefully, consult your boiler's own manual, and if in doubt call Glow-worm's support line with your model and serial number.
What should I do if my Glow-worm boiler keeps showing the same fault code after a reset?
Resetting the boiler two or three times is reasonable, but if the same fault returns each time it means there is an underlying problem that a reset cannot fix. Continuing to reset without addressing the root cause can sometimes make diagnosis harder. At this point you should call a Gas Safe registered engineer who has experience with Glow-worm or Vaillant-family boilers. They will be able to read the boiler's fault history and carry out the correct repair.
Could the fault be related to low boiler pressure?
Quite possibly, especially if the actual underlying code is F75 (pump and pressure fault) or F73/F74 (pressure sensor fault). Check your pressure gauge — it should read between 1.0 and 1.5 bar when the system is cold. If it is below this, top up via the filling loop as described in your user manual. However, if pressure is repeatedly dropping, there is likely a leak somewhere in the system that a Gas Safe engineer needs to locate and fix. A pressure sensor replacement typically costs around £80–£150 including labour; a pump replacement usually falls in the £200–£350 range.
Can I fix a Glow-worm sensor or pump fault myself?
No. Beyond checking the system pressure, thawing a frozen condensate pipe, and resetting the boiler, all internal repairs must be carried out by a Gas Safe registered engineer. This is a legal requirement under the Gas Safety (Installation and Use) Regulations 1998. Attempting to replace sensors, the pump, wiring, or any other internal component yourself is unsafe, will void any remaining warranty, and could invalidate your home insurance. If a more significant component such as the PCB is at fault, costs can be higher — typically £300–£500 or more — and it is worth getting a quote to compare against a new boiler if the appliance is over ten years old.