Glow-worm F.78 fault codewhat it means and how to fix it
The boiler expects a hot water outlet sensor through Glow-worm's external control link box and cannot see one — the link was never bridged, or the sensor has failed. Wiring for the installer.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F.78
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What F.78 means
F.78 only appears on an installation using Glow-worm's external control link box, which is why most owners never meet it. The Energy7 and Ultimate3 manuals define it as "Interruption to DHW outlet sensor at external control" and give a single, specific cause: "UK link box is connected, but domestic hot water NTC not bridged". The Compact manual has "Interruption: domestic hot water outlet temperature sensor with external control — NTC sensor defective — replace the NTC sensor".
In plain terms, the link box has told the board a hot water outlet sensor is present, and either the terminals that should have been bridged when the box was installed were left open, or the sensor that was connected there has failed. It is not in the two-digit Ultracom2 or Betacom tables, nor in any earlier Glow-worm list.
The fault lives entirely in the control wiring, so no user guide offers a step for it; the person to call is the installer who fitted the link box, or any Gas Safe registered engineer with the link box instructions to hand. Fitting the bridge or replacing the sensor is a short job; finding which of the two it is takes longer than doing it.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Hot water NTC terminals never bridged | Glow-worm's stated cause. The link box instructions call for a bridge across the DHW NTC terminals when no outlet sensor is fitted; left open, the board reads an interrupted sensor from the day of installation. | Common |
| Outlet sensor connected through the link box has failed | The Compact manual's cause and measure: "NTC sensor defective — replace the NTC sensor". Only applies where an outlet sensor was actually fitted. | Occasional |
| Broken wire between link box and boiler | A cable pulled or a terminal loosened during other work on the controls gives the same reading as a missing bridge. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£160
Time on site
45 minutes
Usually a wiring correction within a call-out fee, £80–£130. An outlet thermistor replaced £100–£160.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Glow-worm flow or return temperature sensor (clip-on NTC)
£12–£35Authorised Glow-worm parts distributor
Price checked 28 September 2026
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Related Glow-worm fault codes
- F.00Get it checkedThe flow temperature sensor circuit is open — plug off, wire broken or thermistor failed — so the boiler has no flow temperature and will not run. Engineer's code on every range.Related: temperature sensors
- F.10Get it checkedF.10 on an Energy, Ultimate, Compact or two-digit Ultracom2 means the flow sensor has shorted. F10 on an Ultracom, Flexicom or Ultrapower names the return thermistor instead.Related: temperature sensors
- F.11Get it checkedOn Energy, Ultimate, Compact and the two-digit Ultracom2 and Betacom, F.11 is a shorted return sensor. On an Ultracom hxi or cxi, F11 is a main board connection fault. Engineer either way.Related: temperature sensors
- F.72Get it checkedThe flow and return sensors disagree by an amount the board will not accept — one sensor is wrong, or on Glow-worm's own account water is not circulating. An installer's call in the first instance.Related: temperature sensors
- F.71Get it checkedThe flow sensor is still connected but its reading has stopped changing while the burner runs — usually because it has come off the pipe. An engineer reseats or replaces it.Related: temperature sensors
- F.13Get it checkedOn a system or regular boiler with a cylinder sensor, F.13 means that sensor has shorted. F13 on an Ultracom, Flexicom or CI-range boiler points at the main board instead. Engineer's code.Related: temperature sensors
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Related guides
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.