Glow-worm F.84 fault codewhat it means and how to fix it
The return is reading hotter than the flow — the two sensors are on each other's pipes, or the pipework is crossed. Glow-worm send this one to the installer, because it is nearly always workmanship.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F.84
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.84 means
A boiler's flow should be warmer than its return whenever the burner is alight. F.84 is raised when the readings say otherwise by more than a few degrees: the Energy7 and Ultimate3 manuals give the threshold as "values not consistent, difference < -6 K" and the diagnosis as "flow and return temperature sensors have been inverted" or "have not been correctly installed". The Compact manual's three measures all begin "check whether … installed correctly".
Glow-worm's homeowner page is unusually candid for a consumer document — "flow and return pipe work crossed", "circulation fault, pump, valve, blockage", "flow and return temperature sensors incorrectly fitted or on the wrong pipe" — and asks for the installer first. The two-digit Ultracom2, Betacom 24a/28a and Betacom2 describe F84 as a permanent temperature difference caused by "inverted or disconnected flow and return heating temperature sensors" or "defective temperature sensors".
Almost every F.84 follows work on the boiler or the system: a heat exchanger change that left the sensors swapped, or a system piped to the boiler the wrong way round. It does not develop on its own after years of running correctly, so if it appears on a boiler nobody has touched, a return sensor that has failed high is the likelier reading. The Ultracom, Flexicom and Ultrapower tables carry no 84.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Sensors swapped after a repair | Glow-worm's "inverted". The flow and return thermistors are identical parts on adjacent pipes, and a heat exchanger or hydraulic block change is the usual moment they end up on each other's pipe. | Common |
| System flow and return crossed at installation | The homeowner page's "pipe work crossed". The boiler is right and the system is backwards. Rare on an established system; likely on a new or recently altered one. | Occasional |
| Return thermistor failed high or flow failed low | The two-digit manuals' "defective temperature sensors". A drifted sensor can make the pair read as though inverted, and this is the explanation when F.84 appears with no recent work. | Occasional |
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
Two sensors swapped back is a call-out fee, £80–£130. A failed thermistor £100–£160. Repiping a reversed system is outside these figures and a matter for the installer.
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
Own this appliance and had this code? . Nothing is published until a person has read it.
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
Own a Glow-worm boiler? Save it and its codes come first next time.
Related guides
Common questions about Glow-worm F.84
F.84 appeared the day after my new boiler was fitted. What does that mean?
Most often that the flow and return were connected the wrong way round, or the sensors were swapped on the bench. Glow-worm's own page says installer first, and this is why.
Can it be the pump?
The homeowner page allows a circulation fault. With no circulation at all the return can briefly appear warmer than the flow, but a wiring or pipework inversion is far more common.
Is it the same as F.85?
F.85 is both sensors reading the same because they are on one pipe; F.84 is the two reading backwards. Different symptom, same visit.
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
- Manufacturer support portal
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.