Glow-worm F.85 fault codewhat it means and how to fix it
Flow and return sensors reading identically because both are clipped to one pipe, or one is on the wrong pipe or unplugged. An installation fault, corrected by refitting the sensors.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F.85
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.85 means
Where F.84 catches sensors that have been swapped, F.85 catches sensors that have been put in the same place. Glow-worm's Energy7 and Ultimate3 manuals define it as "Fault: Temperature sensor — the flow and/or return temperature sensors have been installed on the same pipe/incorrect pipe; temperature sensor not connected or is connected incorrectly". The Compact manual's heading is blunter, "flow and return temperature sensors have been installed incorrectly", with one measure: check that each is on the correct pipe.
The two-digit Ultracom2, Betacom 24a/28a and Betacom2 have F85 as "flow and return heating sensors fault — flow and return heating temperature sensor connected to same tube". Two readings that track each other exactly tell the board it has lost the information it uses to protect the heat exchanger, so it stops. Like F.84 this is a consequence of work on the boiler rather than a fault that develops in service, and Glow-worm give no householder step for it on any range; the homeowner page does not list it.
An engineer checks that each thermistor is on its own pipe in the position the installation manual shows, that both plugs are in the right positions on the board, and that neither was left off after a heat exchanger or pipe change. There is no 85 in the Ultracom, Flexicom, Ultrapower or CI tables.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Both thermistors on one pipe | Glow-worm's "installed on the same pipe". Two identical clip-on sensors and two adjacent pipes; after a heat exchanger or hydraulic block change one is clipped beside the other. | Common |
| One sensor unplugged or in the other's socket | "Not connected or is connected incorrectly". A plug left off after a repair, or the two plugs crossed at the board, gives readings the board cannot reconcile. | Occasional |
| Sensor on a pipe that is not the flow or the return | Glow-worm's "incorrect pipe" — a bypass or the hot water side, which follows neither the flow nor the return. Read the position from the installation manual rather than from where the clip happens to fit. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£130
Time on site
30 minutes
A call-out fee, £80–£130, to refit the sensors. A thermistor is seldom needed.
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.